My '05 bike has stock exhaust and airbox/filter but had always previously had trouble lurching or cutting out at WOT. I first assumed fuel starvation and wasted a lot of time checking pipes, tank breather, and fuel taps. Then I tried the 'recommended' main jet enlargement from 130 to 135 and the problem got much worse....at that point I had the eureka moment realising that the root of my trouble was *rich* WOT *not* fuel starvation. Currently with 125 main jets the problem is nearly solved and I have a feeling that 122.5s will be perfect, but I am wondering why a bike with a reputation for running lean is finding the stock jetting too rich at WOT. Then it struck me that I remember from checking the valve clearances that both exhaust valves had well over spec clearance - about 0.12mm. As they were not tight or clattering I just left them, but could this exhaust valve restriction be the cause of the rich WOT with standard jetting ? Over winter I might re-shim to bring them back into the official spec range, but doing it now will involve a week off the road as I will have to wait for the new shims to arrive after measuring the old ones. Apart from the slight performance loss, is leaving the exhaust clearance so large likely to cause any other problems ?

Nope. a couple of thousandths extra exhaust valve clearance will not cause this (or any other) problem.

I would be curious how you determined it rich at WOT. Have you shimmed the needles? Or replaced the with aftermarket? If so, maybe go back to the stock needles/position.

Of course if it was in fact rich with the 130 jets and it runs right now then that means the air velocity is higher than it would be for a normal GS. Or maybe you are at very high elevation?

Anyway, I think your problem lies elsewhere but I'm not very knowledgeable about the 3-ckt carbs. But it's not caused by exhaust valve clearance.

Hiya, thanks. As far as I am aware the exhaust, airbox, filter and carb setup I inherited are all stock. Elevation, climate and fuel quality are all normal. The WOT stumble/cutout/lurch problem has been driving me nuts since I got the bike, and after checking all the obvious stuff like vacuum hoses, petcocks and tank breather my first instinct was to follow the 'conventional' GS wisdom and increase main jet size from 130 to 135 - which made the problem vastly worse. I then went back to 130, followed by 127.5 and 125 with the problem decreasing all the time. 125 has almost eliminated the issue and I am pretty sure 122.5 will be pretty much perfect. I am just a bit curious as to why my basically stock bike should want to deviate so far from the standard jetting, and the only thing I could think of was the out-of-spec exhaust valve clearance causing a fractional restriction of airflow at WOT. When I made the initial trial of 135 mains I also went to 20 on the pilot and 1 groove lifting of the needle which have made big improvements in idle and mid-range as expected.

05 has 130 mains stock. You may have had a float drift high if you have it rich @125.
Or a non stock needle. Needle has grooves in the top ? That could be a DJ needle.
Cool.
Buddha.

Yeah, 130 was stock. Float heights appear pretty close to spec and show no signs of unevenness, I measured them with the verniers. Given what I know about the history of the bike I would be really surprised if the jet needles had been randomly changed...I think I am the only owner who has had a taste for tinkering. The K5 parts diagrams I have seen show the clip rather than the 'solid' type, but I am aware that both variants are out there. The air filter, although of the paper type and stock design does not appear to be a Suzuki original, so maybe it is a little more restrictive than OEM, although TBH to my eye it looks a bit thin......oh well......at the end of the day whatever works works I guess......I am not really bothered about optimising it for ultimate max power as that ain't really my thing anyway. I just like the relative rugged simplicity of the GS design.

Quote from: olosh on July 07, 2018, 09:41:35 AM
When I made the initial trial of 135 mains I also went to 20 on the pilot and 1 groove lifting of the needle which have made big improvements in idle and mid-range as expected.

Moving the clip on the needle probably caused the rich at WOT problem. If I had to guess, you were lean at WOT and bumped a two pilot jet sizes and also moved the needle clip and wound up way rich at WOT. Go back to the stock needle position and go to 132.5 mains and I bet everything is all better.

The WOT problem was there before raising the needle and changing the pilots. With the needle raised one notch and the size 20 pilots the idle and midrange are much smoother, but the pre-existing WOT problem was not exacerbated.

OK, whatever. You're not following my logic.

I suggest the original problem was you were lean at WOT, and you applied the incorrect solution, and wound up with similar riding symptoms as before and concluded you went the wrong way. BTW I did the same thing recently, made a guess as to what was wrong and applied the wrong solution and made the problem worse, and BTW it was about jet size and needle position as well, so this is a recent memory for me.

To try to solve it, you did two things at once: change the needle position and change the jet by two sizes. You needed to only change the jet by one size. Now it's too rich, which mimics the symptom you had before. Move the jet back down to 132.5 and put the needle back to stock and I bet your issue goes away.

The thing that fixed your idle and "midrange smoothness" whatever that means is the bigger pilot jet. The needle had absolutely nothing to do with it. The needle position only affects WOT.

In fact the only thing that really matters at WOT is the main jet size and needle position. "midrange", which I guess means throttle between 1/4 and 3/4 is affected by main jet and needle position as well. You are compensating for too small of a main jet by moving the needle, which is why it still works in the midrange.

The needle position relative to the jet meters fuel from the jet. Raising the needle increase fuel at all throttle positions above about 1/4 throttle until you run out of capacity due to rpm based on main jet size. Bigger jet gives more capacity at high rpms AND increases fuel above 1/4 throttle. So if you are lean at WOT/high-revs and you raise the needle without changing the jet it won't fix WOT/high-revs, it'll just make richer at 1/4-WOT, lower revs. If you increase the jet without changing the needle you will get richer across all revs at 1/4-WOT, which is probably what you want. If you wanted it actually richer only at WOT/high revs then you need to move the needle the other way and increase the jet.

It's either a) your bike is somehow very special and requires a much smaller than normal main jet and needle position doesn't matter except to make your "midrange" "smooth", or b) the needle/jet combo you started with originally caused the problem. My money's on b.

This probably is why you are making logically incorrect conclusions: The size jet is not set according to the AMOUNT of airflow or somehow the restriction of the air filter etc. It's a result of the VELOCITY of airflow. More velocity -> more fuel from a given jet size. The reason you need a bigger jet with a "lower restriction" air filter is not because you are getting more air so you need more fuel, it's because you have slowed down the air so it's not picking up as much fuel. You need a bigger jet to deliver the SAME AMOUNT of fuel as before, because you are getting the same amount of air only slower. One way or the other, if you don't increase displacement, change the head/valves/cams, or increase engine rpm, you won't actually flow more air, no matter what.

I'm getting off of this thread. I've done all I can to help. Good luck.

OK cheers, interesting about the velocity. Yes, Indeed I changed 2 factors at the same time which may have been unwise as makes diagnostics more difficult. You sound like you are much more expert than me, and I do not want to contradict you. Yet what you are suggesting seems to contradict the 'conventional wisdom' of carb tuning 101, which states that in essence the idle circuit controls mixture up to 1/4 throttle, needle jet controls mixture between 1/4 and 3/4 throttle and main jet controls mixture between 3/4 throttle and WOT. If this lore is fundamentally incorrect then of course I need to ditch it ASAP, but in the past it has always served me well as a guide. According to this model, at WOT the needle will be so high that it will not restrict fuel flow at all, regardless of its clip position. Is that not true ? I have raised the needles on many bikes and never before seen it have ANY noticeable effect on WOT mixture, let alone enriching WOT mixture sufficiently that the mains have to be decreased by about 4 sizes to compensate. But if other lines of enquiry do not yield any good results I might buy some 132.5s and try your suggestion. If it works I will be quite willing to eat as much humble pie as seems appropriate :-/

I'm curious on if / how valve clearances could affect air fuel ratios.

I could see how if the INTAKE clearance was off, it could affect how much air/gas gets into the combustion chamber, but not the composition of that mixture.

If the EXHAUST clearance was off, that would only affect how much combusted gas gets vented, no?  Or is the thinking that if the combusted gas isn't vented completely, it mixes with the intake material and thus alters AFR?

Isn't the majority of air fuel ratios determined by the carbs, with the cylinders just burning whatever mixture is sent their way?

Hiya. It is well known from practical experience that less restricted airflow either end requires up-jetting...eg sports exhaust or replacing stock air filter with K&N. My theory was that with the exhaust valves not quite opening fully (due to over-large clearance) this might give a slight restriction in airflow, which may require slight down-jetting similar to going back from a sports exhaust to stock. However, this theory, whilst seeming plausible to me, does not seem to have found much traction here. Yes, now you come to mention it, it does seem a bit counter intuitive that the exhaust config would have such a big influence on what goes on on the inlet side, as the exhaust valves will always be shut when the inlet cycle is happening.....but there we are, engine tuning has always been a bit of a Black Art.

Quote from: Endopotential on July 08, 2018, 11:03:36 AM
I'm curious on if / how valve clearances could affect air fuel ratios.

It probably wouldn't.

If you had zero valve clearance, such that the valves didn't close all the way, then you'd have very poor vacuum and that would affect the mixture along with a lot of other worse things.

Quote
Isn't the majority of air fuel ratios determined by the carbs, with the cylinders just burning whatever mixture is sent their way?

Well sort of, but the carb slide lifts and opens the main jet via the needle in response to vacuum, and vacuum can be affected by valve clearances, but not by too much valve clearance. As long as the valves are sealing then you will have vacuum sufficient to operate the carburetor.

Quote from: olosh on July 09, 2018, 10:59:36 AM
Hiya. It is well known from practical experience that less restricted airflow either end requires up-jetting...eg sports exhaust or replacing stock air filter with K&N.

While this may be well known from "practical experience" it is also commonly misunderstood with respect to CV carburetors, and there's a lot of mythology about this that holds over from the days of 2-cycle engines and that kind of thing.

But the mixture is all about INTAKE air velocity and vacuum. Air velocity draws fuel from the jet, so lower velocity means less fuel from the jet. Vacuum raises the needle from the jet, so more vacuum means more fuel. If you do something to reduce vacuum then it will lean the mixture. Likewise if you reduce velocity you will lean the mixture.

Exhaust system is typically tuned acoustically to produce a low pressure "rarefaction" between acoustical pulses (at certain rpms) to maximize scavenging, increase velocity and increase vacuum as a side effect. If you reduce the "restriction" of the exhaust, you likely actually slow down the exhaust which has multiple effects, but the main effect is going to be the exhaust will cool in the pipe which raises the resonant frequency so it changes scavenging. If you change exhaust and "need" a bigger jet then it means you reduced intake velocity or vacuum (or both) at the RPMs where you need it, and this must be caused by higher pressure at the exhaust port when the valve is open. What you did was move the scavenging higher in the rev range by increasing the resonant frequency of the exhaust column. There are a hundred other factors, things like the location (or existence) of the crossover pipe, collector size and position, interaction with the other cylinder, etc.

Anyway...

Quote
My theory was that with the exhaust valves not quite opening fully (due to over-large clearance) this might give a slight restriction in airflow, which may require slight down-jetting similar to going back from a sports exhaust to stock. However, this theory, whilst seeming plausible to me, does not seem to have found much traction here.

Yeah, that's because you are thinking purely in terms of restriction but really it has nothing to do with that. And actually, if you did something that necessitated a smaller jet, by not changing the valve timing/lift or displacement, then it means you increased either vacuum or intake velocity or both. You are not flowing less air, or having more restriction. You are just flowing the same amount of air faster, so it picks up more fuel and doesn't need as big of a jet. One way or the other, within reason, the engine is going to draw in 487 ccs of air+fuel on each cycle. You want to "flow" more than that, you need to increase displacement.

Quoteengine tuning has always been a bit of a Black Art.

Not really. But it just requires a level of understanding that's pretty far above the level of the shadetree mechanic. So we shadetree mechs tend to use trial and error and magic formulae to produce results, and then attribute it to "art" because we don't really understand what causes it. And it's one thing to understand it, and quite another to be able to design or predict it. There is a lot of stuff interacting so trial and error is often more reliable than design anyway, at least for hackers like me. Then we readily fill in the blanks of what we don't understand with unprovable (to us) theory that may make sense but can still be totally wrong. Happens with just about any complex system.

I have designed tube guitar amplifiers, and it's largely the same thing. The system is so complex and with so many variables that even though I know what I'm doing, it's just so much easier to try a couple of different capacitors and see how they affect the feel than it is to do all the math and modeling and deterministically identify the right one. That's the same thing as this jetting. Who cares about the theory, it's pretty easy to just try jets until you find the right one. Only reason this is relevant is that you wound up with such a different set of jetting than everyone else, it must indicate another variant configuration.

Dang it, and I was going to get off of this thread.

Update: 122.5 mains perform perfectly. No issue at WOT, no issue with 'roll-off' stumbling, plug colour on the rich side of perfect. Not going to mess with it any more. Maybe in the fullness of time I will definitively find out why my bike likes a smaller main jet than standard, but for this season I am now just  going to get on with riding it.