Why even mess with carburetors? Because it's fun and carbs are actually pretty simple, elegant devices of great flexibility. Technical Editor Kevin Cameron has seen his share of carburetors and spent a whole lot of time thinking about how they work. Editor-in-Chief Mark Hoyer recently got a 1995 Ducati 900SS running again with a pair of factory Mikunis. It can be done! Listen now to find out how.
Read Kevin Cameron's detailed carburetor tuning story: https://www.cycleworld.com/story/bikes/kevin-cameron-explains-how-to-tune-carburetors/
Listen on Spotify: https://open.spotify.com/show/6CLI74xvMBFLDOC1tQaCOQ
Read more from Cycle World: https://www.cycleworld.com/
Buy Cycle World Merch: https://teespring.com/stores/cycleworld
Read Kevin Cameron's detailed carburetor tuning story: https://www.cycleworld.com/story/bikes/kevin-cameron-explains-how-to-tune-carburetors/
Listen on Spotify: https://open.spotify.com/show/6CLI74xvMBFLDOC1tQaCOQ
Read more from Cycle World: https://www.cycleworld.com/
Buy Cycle World Merch: https://teespring.com/stores/cycleworld
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SportsTranscript
00:00 Welcome to the CycleWorld podcast. I'm Mark Hoyer, Editor-in-Chief. I'm with Kevin Cameron, our technical editor. This week,
00:06 carburetors. Why even screw with them?
00:09 Yeah, carburetors, you know, fueled our passion for a really long time. They started off with sort of soaked rags or wicks,
00:20 evaporative
00:24 stuff fluffing into a thing that maybe it's gonna, maybe it's gonna light or not,
00:28 and then we started figuring it out. And for the very long time, we had a beautiful life of
00:34 tuning and relying on signal and all the things that make a carburetor go.
00:39 You know, now we're into this world of EFI and it's like, why even screw with it? Well, there's plenty of them out there.
00:44 They're fun. They're flexible. You have signal. You have lots of things to play with. So I wondered about
00:53 starting off or helping people understand sort of kind of the basics of a carburetor, how they function, and then maybe the variety of
01:00 carburetors too, you know, that we have that we might find on our older bike.
01:05 The basic original carburetor is just an air pipe going from the outdoors
01:14 to the engine, which is connected on this side, and
01:20 moving up and down in this doorway is an obstruction called the throttle slide. That controls the air.
01:28 Dangling down from the throttle slide and moving with it is a tapered needle, a
01:36 metering needle, and it rides up and down inside of a needle jet, which is brass.
01:42 And
01:44 persons like myself who were up to their
01:49 chin in this technology have lots of
01:52 multi-compartment boxes filled with jets and needles and needle jets and idle jets and throttle slides and what-have-you.
02:01 And it was a kind of...
02:03 You open the Bible in search of truth and you do the same when you open these boxes,
02:09 because somewhere in there is something that will work.
02:12 And
02:16 the principle of the carburetor is quite simple. If you have air moving
02:21 through a passageway,
02:25 its pressure drops because
02:28 part of the energy of that air,
02:30 which was pressure molecules going in all different directions and banging on the walls trying to get out, which is what pressure is,
02:38 suddenly, "Oh, we can go this way!" And
02:42 that energy is subtracted from the pressure energy. So moving air has a lower pressure than
02:49 static air. And if you mount underneath the air passage,
02:54 what is the equivalent of a toilet bowl? It's a
02:58 device which has a float-controlled lever that maintains a constant level.
03:04 If you're going down the interstate at 85 miles an hour,
03:10 it's pulling more fuel and the float
03:12 sinks a little bit, opening the valve, and
03:16 it maintains the constant fuel level. That is the reference pressure for the fuel system.
03:22 And then you just have combinations of jets that allow
03:26 the correct amount of fuel to be mixed with the air going in.
03:31 Normally, a simple main jet system
03:35 gets richer and richer the faster the engine turns. So what do they do?
03:40 They say, "What if we bled a little air into that system?
03:45 Wouldn't the airflow increase just like the fuel flow does?" And the one would cancel the other and we would have nice,
03:52 constant fuel mixture. And that's the goal. That's our goal. Yeah, that's our goal. Yeah.
03:58 So,
04:01 you don't have to know about all of these
04:05 elevated principles and what-have-you. You can allow yourself to be drawn into it.
04:11 And that's the charm.
04:14 But on the other hand,
04:16 it can just be a
04:18 pain. You've got this
04:22 lovely old bike. The tank is not dented. It's actually got a decent paint on it. It won't run.
04:28 Why not? One, the idle jets are blocked
04:34 because it was put away wet. You don't do it with horses. You don't do it with motorcycles.
04:39 And that fuel that is inside there
04:43 contains what the fuel engineers call
04:47 "existent gum."
04:49 And that doesn't evaporate and it accumulates in small places, such as the idle jets.
04:56 And I have...
04:59 We encountered a motorcycle. We put the idle jets in an ultrasonic tank and left them there for a
05:05 long time. And it never came clear until we did what you're never supposed to do. We poked something through the hole.
05:13 Never do that unless you have to. Yeah, I was gonna say you got it sometimes. I mean,
05:20 I was a guitar player. I was perhaps more of a guitar player previously, but
05:28 guitar strings
05:30 are great. The solid steel ones are nickel. They don't rust.
05:35 They're really nice. And you just cut them with a little angle.
05:38 But don't scratch the sides. If you've got to do it, because some of the jets are really expensive now.
05:43 There's some old Honda 4 jets that are just astronomically expensive.
05:48 Use care. Don't get in there with a drill bit.
05:51 Just gently poke it out and do the thing. Clean it up.
05:56 The other big obstruction, the other big
05:59 stumbling block to getting that lovely older bike to run
06:03 can be the fuel that's in it.
06:07 Because fuel is not a pure substance.
06:10 Water is a pure substance. All the molecules are the same. Two hydrogens, one oxygen. H2O.
06:17 But in fuel, there are various numbers of carbon atoms, each of which is paired with some hydrogen atoms.
06:25 And the lightweight ones evaporate first, which means you put away your bike and
06:31 the fuel starts to evaporate. And what evaporates first is the light fractions
06:37 that are essential to cold starting, because they evaporate most easily and at the lowest temperature.
06:44 So that's why my dear old mother
06:48 put her lawnmower out in the sun in the springtime, because she had found it started more easily that way.
06:56 Because she'd raised the temperature, which
06:59 evaporated more of the rather gummy old fuel. But new fuel is
07:06 often the answer. Once you've got those idle jets clear and
07:10 throttles are synced and everything is sort of the way it's supposed to be.
07:16 Fresh fuel, open idle jets. Vroom.
07:19 Ah, joy. Pure, pure joy. Yes, it is. When it fires, you're in love with life. Yeah.
07:30 Well, you're probably in love with life until your jets are stuck or all those horrible things. Now for me,
07:38 one of the things I learned over the years...
07:42 So you get a carburetor that hasn't been out in a while and you have a brass
07:46 jet in a zinc or aluminum, some, you know, moderately okay body material that's also different than the brass and it's been
07:55 soaking and evaporating in this gunk and grime. And then you have modern fuels pulling
08:01 water out of the air to combine with the alcohol and it's
08:05 corrosive and all the things and you get a stuck jet.
08:11 Don't be a savage
08:12 with an ill-fitting screwdriver.
08:15 What I found very, very successfully is I had an old screwdriver
08:20 that I'd broken the end on, you know, being a savage in a different way.
08:25 Chiseling or doing something horrible with it, right? Primitive Pete as it used to be in the movies,
08:30 in shop class. What's shop class? Who knows?
08:33 But we take that screwdriver, that broken screwdriver, and I go to the grinder and
08:38 most screwdrivers don't fit in those tiny holes. So you first straighten the edges, just a straight post.
08:43 Make it about the width of the jet. If you don't have a reference jet, make a good guess. Make it just barely fit in
08:49 the hole without scraping. And then I really square the edge off. I put a very hard edge on it and you can even put some
08:55 you know, valve lapping compound or something to give it traction. Now rinse that out later.
09:00 But you can get, when you do that, when you get a screwdriver that truly fits in the jet rather than
09:07 sliding up an already mangled piece of brass, it can make a huge difference.
09:10 Especially that hard, that sharp square edge on your grinding. So be disciplined about it.
09:16 But make a screwdriver that actually fits and you can get those suckers out of there.
09:20 Yeah, yeah, this does. It makes a real snap when it finally happens and that feels good.
09:27 Then you can reuse reverse drills and they make these beautiful tiny extractors and all that stuff,
09:33 you know, that you're trying not to mangle your poor carburetor body. The jets you may have to replace.
09:39 But I always, as a matter of course,
09:41 almost always replace the jets unless they look really, really nice. You know, I just want to, I want to know
09:48 that I'm in good shape and crisp and as clean as can be. And then you got to, you know, pay attention over the long haul, folks.
09:55 Don't leave a part too long. Yep.
09:58 I think I want to give a shout out to
10:03 good carburetor parts too, or good kits. You know, somebody,
10:07 let's start this off with the not a sponsor disclaimer, but not a sponsor, but somebody like Sudco does, you know, McCuney and Keehan.
10:17 And you know, you're getting parts that actually fit the carburetor and are made to operate in fuel.
10:23 And there are a lot of off-brand things that you can find on the great interwebs on the giant auction site or
10:29 the, you know, the, what do you call it?
10:33 Anyway, the other, the other giant place you buy stuff and it gets delivered to your door. Those kits, you know, your, your O-rings and all these things can be
10:43 really an interesting combination of sizes that are not regular sizes. You know, you can have a 1.22 millimeter O-ring,
10:51 or you can have square O-rings.
10:56 And the dimensions are not matched by the kit that you just buy wherever you, they just rough it together. Oh, it's 1.22. Well, here's a, here's a 1.5. It'll be fine. It's a little bigger. It'll scooch in there.
11:08 And God knows what it might be made of. So getting a reliable supplier. There was a guy, Randall Washington, who did Randax Cycle Shack, and he was incredibly meticulous about building kits for Goldwings and other Hondas. That was kind of his jam.
11:24 So you really want to work with people who are getting you parts that are actually appropriate for your application and good, good for use in fuel. It's a huge part of the battle.
11:34 I've found anyway, good, good float needles. You get a bad float needle and you're just like, why is that carb rich? Why, you know, just because it won't seal and the fuel level goes up and it gets closer to that hole and it just keeps dumping in there and you get, you know, suddenly, what do you, what do you do?
11:52 I went to the drags with a young man who wanted desperately to start his 500 CC drag bike on the rollers. He could have started it with his hand because it was a two stroke, but he went on the rollers and made the connecting rod look like this.
12:12 And the reason for that was that he did not pull the fuel lines off.
12:19 After turning off the petcock before putting the bike on the trailer and going home.
12:25 So one thing that you may consider about quite older bikes is that those float valves may not seal up tight.
12:34 And having in this case, it was a two stroke, so it filled up the crankcase with gasoline. Then when the crank shaft made a rotation, it shot that stuff up into the combustion chamber and stopped the piston cold.
12:49 Well, you know, liquids are about as usually harder to compress than concrete if they're in a container.
12:56 Yep.
12:57 So the hydraulic is not a good way to go. It's an end. It is an end.
13:02 What about tuning a carburetor? You know, there's a million places to find that kind of probably okay information online, but what would you say your process is? Here I have...
13:15 This is a step by step process. It begins with idle.
13:18 And if you get the thing started and it runs, you can then set the air screw so that or air screws if you have a number of cylinders.
13:32 Such that the thing idles without threatening to quit.
13:40 And then you vary the mixture to maximize the idle RPM.
13:47 Which is? Yes, sorry.
13:51 Well, it's to be determined. If it's the new Ducati single, it's an elevated number. If it's an NR500, it's 7500 RPM.
14:11 But for a normal street bike, the theory or I guess what is it, the fundamental notion of tuning that screw until the RPM is at its highest is you're making the most amount of horsepower for that throttle opening.
14:30 That's right. It is a peak power mixture.
14:33 And think of that. Like that's for me, that was just such a tremendous aha moment.
14:40 Because I like that.
14:42 It makes so much sense.
14:46 And it takes some of the mystery out of it.
14:49 Now, boy, what do you think about...
14:57 So let's say you have a twin, you have two carburetors and you screw them all the way in until they're seated lightly.
15:05 Don't screw them in tight, just lightly seat them. And let's say you go out one and a half, it fires up, but idle's pretty good.
15:15 It's doing fine, warming up, keeps going.
15:19 And then you finally turn both of the screws out to say one and three quarter.
15:25 And that's your setting where you're getting your highest RPM and then you adjust your idle speed back to whatever your factory idle might be. Let's call it 1000 RPM.
15:35 Now, what about carburetor to carburetor?
15:39 Would you just leave them the same number of turns out or would you then try to tune the mixtures on those individually for whatever that sixth of a turn or a quarter of a turn difference could be?
15:51 My experience has been that usually the geometry of the manufactured parts is good enough that if it's one and three quarters turns on this one, it'll be pretty close on that one.
16:04 But a fanatic could certainly go at it.
16:07 Yeah. And if the float levels are the same and if everything else is the same.
16:15 So starting with, you know, if you're feeling real crazy and you want the finest possible running, start with adjustment of your valves.
16:27 Check your cylinder compressions and see what they are across the board.
16:32 I think that's a really fine place to start. Make sure your spark plugs are in good condition and your ignition is in good condition.
16:39 If you have a vintage bike, I'm going to recommend stop screwing with points. But then maybe, you know, I guess we all we have like the selective Luddite thing, right?
16:49 Like what? How much BS can you cope with or do you need somehow need to cope with? Right.
16:55 So I've kind of had it with ignition points and it's not really the points. It's that the timing is so dumb.
17:05 You know, a Velocette Magneto with an automatic timing device, which is just basically weights, you know, and as the engine spins faster, it spins this thing and it, you know, the weights fly out and it rotates, you know, relative, rotates relative to the crank shaft and changes the timing.
17:24 But it's like a switch and that's how older electronic ignitions were. It was, you know, 1995 Ducati 900 SS has Cocousin ignitions that are 10 degrees, 36 degrees.
17:38 And there's there is a line that connects them on the graph in the factory manual, but I'm not sure that line actually exists.
17:45 I think it's just one or the other. And because that's why two states idle and running. Yeah.
17:53 But not optimal because you want a different you want you actually want an actual curve.
17:58 Well, that's why all the new stuff is mapped and mapping is normally done at 50 RPM intervals, but you could you could just go to town with your own dyno and all the stuff and end up with something that is that is maybe slightly better than stock.
18:19 Yeah, there are enough options now with your vintage bike that you can you can you can choose levels of cost and complexity and programmability.
18:29 I've done an electronic ignition on the Ducati and it made it carburete better.
18:35 Just just kidding. But you got the most out of it. And I think so starting with all of that really helps.
18:42 And then being crazy methodical about your float level, find out what it's really supposed to be. Be really confident.
18:50 Keep thinking about it and doing it until your float level is primo. And then I agree that generally speaking, doing your idle mixture screws the same same will yield positive results.
19:09 Tuning where it's at really, I find. Well, we were in the let's go through the process of tuning.
19:15 So we've done we've done our we've done our idle. What if we what if we have to go four turns and it still isn't great?
19:22 Well, then you need to change the idle jet. And if the if they call it an air screw, because often it controls airflow and not fuel flow.
19:33 So if you've got the air screw out to maximum and it's still not running right, you have to switch to an idle jet that is what is it?
19:49 Smaller. Yeah. Yeah. Leaner than leaner than what you have.
19:54 And then you'll you will by making these changes in the idle jet size, reposition the optimum location of the air screw so that is somewhere between one and three quarters and one turn.
20:14 Chances are that the idle jet in an engine that has recently run is not that far off. But if you're starting from cold, you oh, let's put these McCoonies on this 19 this W series Harley.
20:30 Okay, let's do it. Oh, just visit the VeloCette group on you know, whatever platform and listen to the folks trying to get the McCooney tuned in and zeroed in on their VeloCette 500.
20:43 You know, it's like, oh, it's the P1-5 needle, but it overlaps with the other needle and, you know, lots of action.
20:49 You know, there isn't a recipe and you do have to kind of start from scratch and and go through that.
20:54 Well, as you throttle up, the air velocity passing under the slide increases from what is sort of wafting along to more like a rush of air.
21:10 And that creates a low pressure above the needle jet, which is on the floor of the carburetor.
21:23 And it is barely above the fuel level in the float bowl.
21:29 And at some point, that low pressure in there is going to lift the fuel and begin stripping it off the top of the needle jet and carrying it into the engine.
21:38 And what controls that at that moment of transition from idle to main system running is the throttle slide cutaway.
21:50 And the throttle slide is not just a cylindrical obstruction, it's beveled on the upstream side so that the height of that bevel exerts an influence over the pressure underneath the slide directly above the needle jet.
22:10 And most carburetors that you'll see on street bikes have a cutaway between 2.5 and 3.5.
22:20 And on English carburetors, it's not millimeters, it's some other having to do with the distance from the king's nose.
22:29 But it is a physical measurement.
22:32 Well, it is. And on an Emil, it's a number, but it's eighths. It's 2.5/8 or something.
22:41 Yeah.
22:42 Yeah.
22:43 And by varying that slide cutaway height, a smaller height is richer, a larger height is leaner.
22:56 You can control that transition. Some people have worked over their carburetion so marvelously that no matter how fast you twist it, the engine just sparks out.
23:07 It can just be wonderful and is so satisfying.
23:11 Yeah.
23:12 And then that's mostly from very close to throttle against the stop to maybe a quarter throttle, the influence of the slide cutaway.
23:28 And from there on, you're on the needle until you're on the main jet with the throttle fully open.
23:36 And they make the needle tapers in a bewildering variety. There are multiple tapers.
23:42 There are different shank diameters.
23:45 You can get needle jets in different sizes.
23:49 But although that sounds horribly complicated and the poor underpaid persons out there who are charging $10 for a jet are discouraging us from experimenting.
24:04 You may be able to find an old timer like myself who has a bunch of jets, sell them to you.
24:11 Those little compartment boxes don't want your family survivors to put that stuff out on the curb after you're gone.
24:21 Recycling.
24:22 Yeah.
24:25 But that is the progression.
24:28 You start with the idle, then you go to the transition, which is the slide height.
24:34 And you find that out by trying to lift the slide a bit.
24:38 Does it stumble?
24:39 Does it just sit there?
24:41 You'll quickly get the idea by making the little changes that are necessary.
24:46 Would we not jump to the main in a tuning order?
24:51 Start with the make it run idle and then try and get the main set up?
24:55 Wide open throttle?
24:57 Like if you have a dino or something?
24:58 If you can't get there.
25:00 I suppose.
25:02 That's a point.
25:03 Yes, I suppose that is a point.
25:05 Because on a carburetor, pretty much there's overlap between all the stages of jetting from pilot to needle to wide open throttle.
25:15 Tremendous overlap, yes.
25:16 Everything influences everything else.
25:18 So, it's not a one and done type of process.
25:21 You go through it and then you go back through it.
25:25 Yeah.
25:26 And it's like there's an algorithm for each portion of the curve.
25:31 I guess you could call that matched asymptotic expansions.
25:35 Math people go into some really crazy stuff.
25:40 But that's the analog world.
25:44 And people like Mark was saying, people look at carbureted motorcycles and they think, "What is this?
25:53 Why would I mess with something like this?"
25:56 Because if you want to, you will.
25:59 If you don't want to, away with it.
26:04 Yeah.
26:05 But it can be fun.
26:07 Would you agree the needle and needle jet is where you really put the exclamation point on fine tuning?
26:16 Absolutely.
26:17 Needle height is the most sensitive adjustment in the engine.
26:24 One clip position.
26:26 The needle has a number of grooves at the top from three to five, typically.
26:31 And there's a little E clip that slips on there so that you can vary the height of the needle with respect to the slide.
26:42 And obviously, raising the needle makes it run richer and vice versa.
26:48 So, I never became a master of needle fitting, but I did have one really good success.
26:59 I'm sitting there with all of those McCooney catalogs and I'm looking at needle tapers and I thought, "This one looks like it satisfies the condition that I'm looking for."
27:13 And it did.
27:15 I was really delighted.
27:17 Yeah, that's really good.
27:18 You've always, over the years, told me a few stories now and again over the last 25 years.
27:24 And one of those was a related story of, you said, Irv Canemoto could come out for morning warm-up and they could start the bike.
27:34 And his guy would be revving the motor, getting ready, waiting for the rider.
27:40 And Irv would just look up in the sky and say, "We're going to need to change the needles to XYZ."
27:46 And he could just do that.
27:48 And that was one of Irv's talents.
27:50 It certainly was.
27:51 Because he was always paying attention.
27:56 He had the equivalent of spreadsheets in his mind.
28:01 They're probably still there.
28:03 It happens with riders, too.
28:05 The riders have said to me, "You know that little pothole just on the left as you enter turn eight?"
28:10 "No, I don't."
28:12 But for them, that's very real because it was a hazard to navigation.
28:20 And Irv, when he was involved with go-kart racing, built hundreds of motors.
28:28 And they used to take 12 or 15 of them to the nationals.
28:33 And so he got to associating the sound of an engine with the nature of the carburation, with the spark advance and all those things so that he had a sense of things.
28:49 I never had that.
28:52 The rider would come in and do all those riderly things, the imaginary handlebars that do this.
29:01 And Irv could interpret that into very often something that would be useful to the rider.
29:09 Wonderful stuff.
29:11 Well, you know, it's this analog machine and these analog sensors.
29:18 And there's a lot going on there.
29:20 It's an artificial intelligence.
29:23 Natural. Natural intelligence.
29:26 Natural intelligence.
29:28 And, you know, I think it's what I aspire to.
29:35 It's why I try to understand what's happening inside a carburetor.
29:41 Because as you found with your needle that solved the problem, isn't that great?
29:49 And isn't it so nice to have, you know, with my Norton Commando, an old Norton Commando, electronic ignition.
29:56 Got that all tip-top, crisp, very nice.
30:01 These carburetors called AMO Premieres, and they're just an updated version of the AMO.
30:05 It's a concentric carburetor.
30:07 And what concentric means, they finally put the bowl, the fuel bowl concentric to the jet.
30:12 So that if you put it on the side stand, it wouldn't change the carburetion the way it would.
30:17 If you put the float way over here, it's going to get rich.
30:19 If you put it over here, it's going to get lean, right?
30:21 The fuel doesn't get up into the -- so that's where the concentric name came from.
30:24 It sits concentric to that.
30:26 They updated that, I want to say 10 years ago.
30:29 They updated to this Premier.
30:31 And you had to pay a little extra money, but they updated the idle circuit.
30:34 They improved the materials, and they gave you a cleanable pilot jet.
30:39 Because the other ones -- the British industry ground them down so hard when the Premieres --
30:44 or when the concentrics were first released in the '60s.
30:47 They're like, "No, no, no, it's too expensive."
30:49 And they just kept taking the good stuff out of the carburetor.
30:51 And it was still a decent carburetor, but it had a pressed-in bushing for idle.
30:56 Unchangeable and difficult to clean.
30:58 And that's where -- I forget, there's a size that fits that.
31:01 It's like 40 thou or something.
31:04 But you just stick your guitar string in there and keep poking at it and blowing it and poking at it.
31:09 But it was nice with the jets.
31:11 And once you had that crispness of carburetion in a new carburetor, it was like, man, what a bike.
31:16 You know, easy, first kick starting.
31:19 Would idle pretty well, even off -- you know, just barely -- you know, like give it a minute.
31:25 And there would be enough heat in it that it would begin to idle properly.
31:29 And you could just ride away without trouble.
31:31 And that's -- I don't know why it's so meaningful, but it sure is.
31:36 Well, this makes carburetion sound a bit out of reach.
31:46 But the fact is that that ability of the mind to associate this set of changes with this set of results is not that different from how we look at the face of the person with whom we're negotiating the purchase of a house for cues as to how we should behave and how we shouldn't.
32:08 Everyone has this.
32:10 So if you choose to apply it to these antique devices, you may be surprised at how good you become at it.
32:20 It's always nice to have a skill.
32:22 Yeah.
32:26 You know, so if you're working on a Japanese motorcycle and you're going to take apart your Makuni carburetor or your Kian carburetor, please get yourself a JIS screwdriver, Japanese industrial standard.
32:39 Yes.
32:41 The feeling of putting a JIS screwdriver into a JIS screw head, usually they have a little dot on them.
32:47 It looks like a Phillips, but it's not a Phillips.
32:49 And if it has a dot, for sure it's not a Phillips.
32:53 Just be kind to your car.
32:55 That dot appeared in 1968, as I recall.
32:59 Excellent.
33:00 But it is. It's wonderful.
33:01 The thing just clicks in there like putting the side plate into a Smith & Wesson so that the gap disappears.
33:09 It's perfectly smooth.
33:11 Yeah.
33:12 So if you get a Japanese motorcycle with mangled Phillips screw heads, it's because they're not Phillips JIS screwdrivers.
33:19 So just invest a couple bucks.
33:21 You can even get one that's an impact.
33:23 There you go.
33:24 Here we are.
33:25 And there's a reason that every Ducati 900SS out there, practically, that's still running McHughney carburetors has Allen head screws in the flow bowls.
33:35 It's because people took them apart with the wrong screwdriver.
33:39 Yes.
33:40 I did it for years.
33:42 Yeah.
33:44 So customizing your own needle or doing that thorough carburetor tuning is a real art.
33:52 And there are people out there. There's a guy named Mark Salvesburg, who's factory pro jet kits.
33:59 And he's one of the old school tuners.
34:02 He was making his own dynos.
34:07 I talked to him recently about Ducati 900SS McHughney carburetors.
34:13 And I called him and I said, "Hey, I'd like to get this, your jet kit and everything.
34:19 And then what do you think about the pilot?"
34:21 And he's like, "Well, man, every bike is different. You got to find out what the bike's asking for."
34:27 That's so important.
34:29 And it was really cool to talk to a tuner guy who's selling something who doesn't say that, "No, this is the one path to God."
34:36 Like this right here. Just put these in your carburetor.
34:39 Because every bike is different.
34:42 And it's been proven out on his website where he's got lists and lists of people who are saying, "Yeah, I live here.
34:48 I ride at this altitude and this is the setup that works on my bike."
34:51 Because people cut the tops off the airboxes, etc.
34:55 And he makes an important point that I'm going to share.
35:00 We have a dyno at Cycleworld and it has a sniffer.
35:04 It's a Dynojet 250i and it has a pump and temperature resistant hoses and a big long copper tube.
35:12 And on a non-catalyst bike, you can stick that thing up and try to get a real clean signal.
35:18 And then you can get an O2 sensor off a bike that doesn't have an O2 sensor reading.
35:24 And on that Ducati, I was like, "Yeah, well, I'm ordering the jet kit."
35:28 And I'm like, "Yeah, I'm just going to tune this thing."
35:30 And I got a dyno. I'll just take some readings.
35:33 And he's like, "Look, man, don't try to tune to a number."
35:37 He's like that, you know, like Stokey, you know, going Stoke, you know, 14.7 to 1.
35:42 He's like, "You know, you got to find out what the engine is actually asking for."
35:45 Because it isn't. It's not a number. It is not a number.
35:48 And so it's a good point.
35:51 Your 5-gas analyzers and your O2 sensors, those are really excellent tools.
35:56 And you can, like with a 5-gas, you can get into the happy zone where the thing's actually running really nice.
36:03 And you could go on the lean side of that to get the best gas mileage.
36:08 Or you could add a little extra fuel to give it that bit of margin when you really whip the throttle open.
36:16 Two miles per hour on top.
36:19 Well, those guys at Polaris, I went up there to talk to the team that developed the Victory motor originally.
36:27 And they made this point.
36:31 They said, "We were trying to get the digital fuel control, the fuel map, the way it would work best.
36:42 And we couldn't do it with a motorcycle and a rider out there and the laptop and an engineer back here.
36:50 So we attached the sidecar and we put the engineer and the laptop in the sidecar."
36:57 He operated the laptop while wearing a helmet.
37:02 And that way they could look at one another and the rider would shake his head or nod,
37:08 according to how much, whether that change was improvement or catastrophe.
37:15 And the reason I tell that story is because a lot of this kind of thing is very much ad hoc.
37:24 You think about it, we're not getting anywhere. What's in the way?
37:28 Well, they said, the engineer and the rider are too far apart. They can't communicate. Sidecar.
37:36 Or you can call them Arbazaz and fly them out.
37:43 Well, that's the thing about development. And you are a developer.
37:48 Even if you're pulling your XS650 out of the shed, which I'm going to do soon.
37:53 Even if you're pulling your XS650 out of the shed that hasn't run in forever and it's got stock carburetors.
37:59 You are the developer. And even though the jetting is probably right, it's a dirty business.
38:07 Motorcycle development, even on the most professional level, at the greatest gigantic manufacturers,
38:13 it's still really sausage making. It actually is a dirty business where you have to practically solve problems.
38:21 It's not some clean, perfect lab where we just model everything in a computer, print out the parts, 3D print the parts,
38:29 and then there's your bike and it runs perfectly. There's just too much going on.
38:33 There is the infinite every time you do it. And that's what's enticing about it.
38:40 Yeah, it feels good.
38:42 And then when you get it, it's so good.
38:48 All right, so we're getting up to the main jet. Then what?
38:52 Well, obviously on the needle, if you have the reason to believe that that's the correct needle for that engine,
39:00 for example, if it's what SUDCO recommends for the application, that might be a good thing.
39:08 But then again, your exhaust system or your camshaft or what have you may be different.
39:16 So. You can vary the height of the needle until you get best running.
39:23 And on the main jet, if the bike speeds up slightly, if you roll off just a little bit, it's rich.
39:36 And the reason being that the air responds quicker than the fuel.
39:44 So you end up with too much air and it will or rather more air.
39:53 And that if it improves at that point, you know that it's too rich.
40:00 And I watched a guy wreck a cylinder on a TZ750 at Daytona because his buddy told him,
40:08 there's no reason you shouldn't ride on the needle because he had the wrong gearing on,
40:14 but he didn't want to waste the rest of the practice changing this, changing this bracket.
40:19 So he rode on the needle. Oh, dear. That cylinder looked terrible.
40:24 But on a four stroke, it's lubricated from central casting and it can't do all those horrible things that two strokes used to do.
40:37 And this diagnostic works and go up or down with main jets until you get what you like.
40:48 And there is also the possibility of varying the needle jet size.
40:54 This is for the extreme perfectionists.
40:59 I was given a routine for this by Harry Hunt and my rider and I went to Loudon, New Hampshire,
41:06 and we tried different needle jet sizes until we got a size that gave us between a quarter and a half a second a lap.
41:16 Amazing.
41:17 And so, there is no surefire method for getting through all of this, but there are generalizations about it.
41:30 And you then fill in the blanks and find a way to the truth.
41:38 How do you know if it's the truth?
41:41 Do you smile?
41:44 Well, of course, super fine drive feeling.
41:47 That's what they used to call it in the early days, Honda manuals.
41:51 Yeah.
41:52 But of course, in a modern engine, a digital fuel control engine, the oxygen sensor tells you, tells the computer,
42:02 oh, you need to feed in a little more fuel or a little less.
42:06 But in the carburetor days, we had to determine the fuel mixture by looking at the spark plug,
42:15 which in effect is a quickly removable part of the combustion chamber.
42:21 That's true.
42:22 It enables you.
42:23 Yes.
42:24 It used to be, by the way, that when our forefathers raced Max Norton's and G50 Matchless's,
42:35 they frequently removed the head because they wanted to look at the piston crown.
42:42 Lots of pictures of those people in the basement of some famous hotel on the Isle of Man.
42:48 Somebody's got the cylinder head in his hand and is looking in at the piston.
42:53 But looking at the spark plug is a shortcut, is an approximation.
42:59 Irv Canemoto used to have the heads off after every practice and wanted to look under the pistons.
43:05 Two strokes, piston temperature is determined by, I mean, it's a much more critical value.
43:14 But the business of spark plug reading is a complicated subject all on its own,
43:23 which considering the state of the clock, we'll talk about another time.
43:29 But there's evidence in the spark plug.
43:32 And people like the famous champion man could look at your spark plug and tell you all kinds of things like...
43:46 With a loop, magnifying glass, something to really get up in there and a bright light to look down the insulator in the center.
43:53 And a bright light, yes.
43:55 One finger on the spark plug to maintain the focal distance.
44:02 At one point, Bobby Strauman looked at the plugs out of my TZ750 and he said,
44:10 "I think you can increase the ignition advance maybe quarter, half a degree."
44:17 And I said, "How did... What are you seeing that makes you think that?"
44:23 And he said, "Oh, the center wire."
44:28 The center wire is sheared during manufacturing, so it has sharp edges when it's new.
44:33 He said, "If those edges are rounding, like when you hold a glass rod in a Bunsen flame,
44:41 if they're rounding and the spark plug heat range is correct,
44:47 your ignition timing is adequate, combustion chamber temperature is enough to slowly soften the tip of the center wire."
44:57 This is on a full throttle plug job, mind you.
45:02 But I love that because the great thing about these pursuits is that the more you learn,
45:11 the more questions you uncover, so you don't need to be bored.
45:17 It's just the way it goes.
45:23 And I'm sure that there are two kinds of scientists.
45:26 There's the kind who want to be right about something.
45:29 And of course, that's very satisfying.
45:32 It answers your questions.
45:34 It becomes like a religion.
45:36 Okay, that's handled. Good.
45:39 The other kind want to be surprised.
45:43 And I think people who are involved in motorcycle tuning typically belong to the group who want to be surprised.
45:52 They're hoping for surprises.
45:54 "I don't understand this, but it's great.
45:57 What is causing this?"
45:59 And when you've come up with an answer that works, you are so pleased with yourself.
46:07 Absolutely.
46:09 Two things I want to say is your needle and your needle jet are wear items.
46:14 It depends on the carburetor and how you're riding.
46:16 If you're rattling around town a lot and you're kind of doing such and so, like that kind of mid-speed, there's a certain mileage.
46:25 And the intake process is making the needle rattle back and forth in there, too.
46:29 Right.
46:30 It's horrible.
46:31 Easy on the graphic gestures, right?
46:34 Sorry for those listening on Spotify.
46:37 Well, anyways.
46:39 Yeah, so they're wear items.
46:41 And it's a good idea to really, really examine the needle.
46:44 And if you see a shiny spot or if you can actually see something that is different, that you could see a deformation in it, then it's really bad.
46:52 And don't forget the needle jet, because that gets hogged out, too.
46:56 It's the softer metal. It's brass.
46:59 Yeah, and so if you're messing around with that stuff.
47:01 Also, corrosive fuels can etch a needle.
47:05 And if you can definitely, if you can feel something with your fingernail on the needle, get a new needle.
47:11 Read the number on the needle, go to a reputable supplier and get good quality parts and start fresh there.
47:20 The English call them stockists.
47:22 Stockists, yes.
47:24 Go to see your parts stockist.
47:26 And when I get stomped, if I'm having some kind of problem that I can't solve, when I'm working with the fuel system, I start, I go back to zero and I start at the gas cap.
47:40 And I say, does the gas cap vent properly?
47:44 Absolutely.
47:46 Because if you're taking fuel out and the air can't get in, eventually the engine will stop and the tank will still have fuel in it.
47:54 And you'll say, this can't be.
47:57 Now I can see fuel, then I move to the petcock.
48:00 I can see fuel in the fuel line, but how fast does it come out?
48:05 And so that's where you take, I take a measuring cup that I've commandeered from the kitchen and it shall never return.
48:12 And I run fuel into that and I say, I go, I might do some calculations, pounds per horsepower hour or whatever the heck it is.
48:22 Yes, one half a pound per horsepower per hour for a typical number.
48:27 Yeah. And so you can know, and you can kind of know that if you turn the petcock on and it's like drip, drip, drip, that you might want to like pull out the petcock and get the muck off the screen.
48:41 Hopefully there's a screen even there.
48:43 Yes.
48:44 So these are the things you want to verify, trust but verify.
48:47 But that's where, you know, for me, when things are going wrong and I feel confident in the other parts of the engine, that compression is good and that your valves are adjusted right.
48:55 And you, you know, all that other, the ignition is tip top, you know, start at the gas cap, is fuel coming out of the petcock?
49:03 Does the fuel shut off when you shut off the petcock?
49:06 Yeah, great.
49:07 And the other thing you can do when you get into the float bowl is you can drop the float, take the float off, put your cup under the carburetor and reopen it because the floats going to be hanging down.
49:16 You should get mass flow in there.
49:18 And if you very gently lift it back up, does it stop?
49:22 Anyway, lots of tips and tricks.
49:24 We got something.
49:26 If it has a bowl sump plug, you can just unscrew that and put the cup underneath there.
49:33 You don't have to take the whole bowl off.
49:36 If you're real crazy, you can make a fitting that goes in there that has a hose barb and you can put a clear tube off the bottom of the carburetor and make a U-turn and run it up the side of the carburetor and you can visualize the float height relative to the rest of the carburetor.
49:54 Yes, seeing is believing.
49:56 And there are some references out there.
49:58 You know, I happen to know that there's a point on the side of a McCooney carburetor, CV carburetor, which my friend Ed calls "Slovenly" because he's a big flat side fan.
50:10 There's a key in flat side conversion.
50:13 You can rack them or you can put them on the cylinders.
50:15 And apparently there's all glory in doing that.
50:19 But I, you know, I like it stock.
50:21 You can run that tube out and you can find out where the fuel is supposed to be and you can know for sure.
50:28 And then you can know that your carburetors are very, very close.
50:31 Yep.
50:32 So many things to learn and read about.
50:36 Kevin Cameron has written a fantastic story about carburetor tuning that goes into great detail and you can read and reread.
50:43 And I did, I have a collection of carburetors at home and we were in the midst of COVID.
50:50 So I got out the camera and I shot this variety of carburetors and I tried to get the camera way up in there so you could see the orifices and you could see the needles.
50:58 And I lined them all up.
51:00 I'll put the link to that story in the description of this video.
51:14 And that came from the late Hurley Wilbert, who was Kawasaki's great rider, engineer, tuner.
51:32 And he had it from an old Australian speed man.
51:33 He said in that accent that no one who is not Australian can properly imitate.
51:37 He said, this works.
51:39 I don't know if it's right, but it works.
51:43 Fair enough.
51:45 It works.
51:47 Well, have fun tuning your carburetors.
51:49 There will be a video about this XS650, I swear.
51:53 We're going to pull it out of my barn back home and try to figure out why it won't run because I know it won't run.
51:59 And we're going to dig that out and go through the process, the dirty process of making the XS650 run and ride again.
52:09 And so we'll talk about more specific parts and then changing carburetors and other options.
52:15 That's one of the fun things is that it's not overly complicated to slap a carburetor on an engine.
52:24 It is a little complicated to tune it properly for the engine, but it can be really fun to mess around.
52:30 And you can really get some different running qualities without spending a lot of money.
52:34 One of the things about motocross carburetors, typically that's where flat slides and flattish oval slides and other sorts of things come from,
52:46 is that a motocross bike's rear wheel is not on the ground all the time.
52:53 So during the brief moments when it is on the ground, you need something with really sharp throttle response
52:59 so that you can feed some power during the opportunity for acceleration.
53:04 And those flat slides have very little volume under them so that it doesn't take much of a pulse to suck down the pressure above the needle jet and get the fuel flowing.
53:20 And in fact, when they tried putting flat slides on NSR500 Honda GP bikes, two strokes, they found that the throttle response was too immediate.
53:32 And so, horses for courses.
53:38 That's a great point on the flat side. The volume of air under it is smaller and it takes less.
53:43 And you get used to the CR, the round slide, the classic McHughney round slide, CR carburetor.
53:50 You get kind of accustomed to the way it responds.
53:54 But man, when you get a good set of flat sides on the same engine, crisply tuned, there is a vast difference.
54:02 And that's why Ed calls CV carburetors "Slovenly".
54:06 Yes.
54:09 Well, because you turn the throttle and the carburetor looks around at you and says, "Did you want something?"
54:15 Something, yes.
54:20 Well, thanks for listening everybody. Have fun tuning your carburetors.
54:26 Visit the website for that story.
54:28 Don't be scared of taking it apart and trying to make it better.
54:33 You'll get used to doing it three times at least. And then you won't be disappointed when you have to do it two times.
54:38 Well, the thing is that that's how anyone who has any expertise whatsoever about these things got that way.
54:47 Screw up, screw up again, think about it, get it right. Now you're your own expert.
54:55 You are your own expert. Do it everybody.
54:58 Thanks so much for listening.
55:00 Like, comment and subscribe. As they say, we do appreciate so much everybody listening through the podcast.
55:06 It's been doing great. And we do have a great collection of the questions.
55:11 We'll address those next week and we'll look at topics or something.
55:15 This was actually a topic that was suggested by a listener.
55:19 And we have more of those. So keep the discussion going. We really appreciate it.
55:24 And again, thanks for listening.