Carb tuning question.

[user11235813]

I installed an almost new (3000kms) set of carbs a few months ago which have been running fine with stock jets 17.5, 60, 130 with stock muffler and stock airbox. Although it's likely been running lean. Petrol milage was good.

Started stumbling a bit during starting, no big deal I just noted it. I was running with the vacuum valves electric connection unplugged and noticed no difference so during a recent jetting upgrade to 20, 60, 132.5 I removed the whole vacuum plumbing and solenoid set up. While installing the jets I notice a tiny bit of gunk in the bowl, really very slight, which I cleaned out. I also moved the clip position on the jet needle to one position down.

I set the air/fuel mix to 2 1/2 turns out and the starting stumble has gone away and the bike does feel to pull better than it did with the smaller jets and it runs nicely and idles perfectly at 1150 when warm. OK all good so far.

I've recently checked the float heights using the clear tube method and they appeared to be set right. When I inspected the plugs after some time with the new jets, they look fine, if anything they may even look a bit lean no carbon build up and white on the electrode which is sort of how it always has looked. This surprised me a bit, fuel consumption has not been noticeably different.

I recognise that I would have been better off making the changes one at a time but I didn't due to it being a fairly standard adjustment and it's too much work.

However, I have noticed a little problem that occurred a few times that I now would like to address I only noticed this recently during some high speed, (that's GS high speed which is not really that high!) highway overtaking, where a car was doing just under the limit, say 95km/h and as I go to overtake they do the usual thing and speed up so not I have to drop down a couple of cogs and give it a handful and this is where the problem has shown up, the engine stumbled a little bit, not catastrophically so but enough to give me pause.

It has done this a few times, however when I tried to emulate this on the open highway with no traffic around I was unable to replicate the stumble, but during an actual overtake it would happen again.

I never noticed this before the jetting upgrade but at the same time it would never pull as well either, so I think the jetting up grade is good but there's something not right yet that I'd like to address and so here I am looking for suggestions.

Does this look like maybe I should go up to a 135 main jet?

I have not set the air/fuel mix screws while the engine was running but I'm going to do that today after I get a tool made to do it and I need some advice on that, basically I am wondering how this works with two carbs, I mean do I just do them one at a time to get the highest idled speed or do I adjust them both together?

However I'm not certain that the air/fuel mix would be affecting this at WOT anyway.

TL:DR,

...will 135 main jet correct a bit of high speed WOT stumble.
...air fuel adjust, one carb at a time or both together.

edit: added photo of plugs.

[The Buddha]

This is a detailed and articulate post user112 - so who're paying to write em for you

The plugs are a teeny bit lean, which is exactly what we like to keep them @, but remember - this is an idle plug pull not the rpm you have the issue in. It should be exactly this way @ idle, just a wee wee bit lean, helps hot restarts, cos it its not restarting, you can use 1/2 or a tiny bit of choke anyway. If it was rich, it would be a bigger issue for hot restarts cos it has no "leaner outer" lever.

To do aload and rpm plug pull - a lot harder. You'd need to hold that rpm under load for 5 sec, hit kill switch and pull the clutch in @ the same time, then pull plug to check it - and yes it has to be done on the road, cos you'd redline @ 1/8th throttle in your driveway, so its not where your problem is.

OK a stumble/flutter like that can be 1 or 2 things.

Stumble under acceleration is rich. Not lean. Lean problems show up @ steady throttle. So my thought is - is your air filter clean or has it got a bit of gunk. If its clean - skip this and try the next one.

You may be getting a slide that's rising too fast. They used to supply these retarded plugs for the 1 hole in your slide. It does work, but man is it very very unnerving to put a drill/tap to a perfectly good and $$$ part.

If that's the case - train your wrist to only open it as fast as it doesn't stumble. It actually gets a lot worse if you rejet it with pipe or K&N and pipe etc etc.

Cool.
Buddha.

[user11235813]

...snippety snip

OK that's great, yes it does restart well, I only use a little bit of choke in the morning for a cold start and not for very long.

I guess I've never done the L and PP, due to having to wait for the engine to cool down but maybe I'll use my special chopping gloves that I bought that I never use and do this. So what is the best method exactly to do this test, do I just roll on in a high gear from a lowish speed and hold WOT for 5 seconds, then kill? Then what am I looking for regarding plugs. I'll take pictures anyway.

You've given me a good clue there, perhaps it's my throttle technique that is wanting, maybe hitting WOT too fast? That would explain me not being able to replicate it because during an overtaking a little panic sets in when there are cars coming towards me but far enough away for it to be a safe overtake, but then you know like as I'm overtaking the car driver will think "ooh someone is overtaking me, that must mean I'm going too slow",

I adjusted the air/fuel to be 3 turns will do the highway pee pee test tomorrow and report back.

[The Buddha]

The throttle open - you can try it on an open road - without the panic.
It wont turn up in a plug pull test. So do that before you burn your fingers or get run over on the side of the road.
Cool.
Buddha.

[user11235813]

OK I took it out for a run, WOT in top lasted for about 6 seconds and as I was going 130kph I didn't want to risk any more, kill switch clutch in checked plugs. They didn't look a lot different maybe a poofteenth less lean but hard to say.

Made some accurate marks on the throttle for more testing. Going up a very slight incline bike runs at a constant 95kph at 1/8th throttle. Opening it up to 1/4 position and it steadily accelerates to 115kph opening it to 1/2 throttle and it keeps pulling well but I couldn't go further due to it not being a good place to speed.

Later I had a chance to test it out further and up to 3/4 throttle it pulls well. All feels good. I need to go out again and test the last bit of throttle but I'm not sure of the best way to do this. If I open it right up and back off a bit there is a momentary dead spot it doesn't really increase in speed but I have not had a proper go yet.

I'll take it out tomorrow can you suggest any specific testing for the final 3/4 to WOT position. I'm at sea level and the needle was on the middle of the five slots, and there was a factory thick washer under the fat plastic donut like washer on the needle, I left all that on but moved the clip down one position.

I reckon the pilot jet is about perfect.

Anyhoo any advice appreciated.

[The Buddha]

You managed a WOT run for 6 sec ? Yikes, I cant even imagine that.
Anyway that 3/4 steady state will tell you all the way to WOT, If you're anything darker than milk chocolate there - you're fine through WOT, cos the stock needle and mains are metered better than we can ever pfook around and improve there. So they do 3/4-WOT with no other transitions.
Also you got winter coming over there - good time to test it, find a humid and cold day and let-er-rip.
All the rest is near spot on.
So that leaves the training to your wrist now - jetting is near perfect - BTW you would have trouble with the stumble if you were burning black as tar or white as snow (AKA 1 jet size rich or 5 lean), not anywhere in the ash-chocolate zone. Lean bikes make good power, and work very well under changing throttle conditions. They act up at steady throttle. So your stumble isn't from it being lean anyway.

Cool.
Buddha.

[user11235813]

Okie dokie. Winter up here in the deep north, it's like 24C during the day but a brisk 16C early morning.

So if I'm going fast at 3/4 throttle, and I roll on a bit more more to WOT, I would not expect much more to happen?

And if I'm say doing 100km/h in top and I drop down a couple of cogs and g e n t l e y roll on to WOT, I should expect a decent acceleration if all goes well and I'm easy on the wrist, yes?

I will try a plug pull tomorrow and remember to bring the camera with.

[herennow]

A nice steady uphill section (if you can find it) is great for WOT chops. Stops speed climbing too fast, and you can often do it in a lower gear to keep from getting too crazy fast.

But whatever you do - be careful. High speed, killing the bike, braking and pulling over is all a little risky. I do it but I don't like to ...

[The Buddha]

You need steady RPM, it cant be varying. So basically you have to get it to speed and keep it there for 5 sec, that's why WOT plug pulls are hard and dangerous.

Now if you're at 3/4 throttle and you open to WOT - you will let in more air, then as the rpm climbs the fuel mix will compensate by adding fuel.
What I meant was that it will be metered the same as it is @ 3/4 throttle because there are no transitions in the 3/4 to WOT region. So for example: @ 3/4 you're letting in a crapload of air, and 14:1 fuel. @ WOT you will let in 1.33X crap load of air, and 14:1 Fuel. Or whatever the ratio is as determined by your plug check @ 3/4.
Bottom line if metering is right @ 3/4 it will be right @ WOT cos there are no transitions in that region, and we didn't fook around there. Like for example if you pulled out the emulsion tube and polished something and modded the cross holes ... even so it would mess it up @ 3/4 ... Oh yea and you modded the needle to compensate @ 3/4 ... yea far fetched but just making a theory to why jetting can be good @ 3/4 and off @ WOT or vice versa. No, you're fine, stock parts = WOT and 3/4 are the same AF ratio.
Cool.
Buddha.

[gregjet]

OK My take is a touch different.
In my opinion tuning CV carbs is safer and much more accurate done on a dyno. Because the throttle opening is not linked directly to the throttle, because of the vacuum actuated slides , you can never be sure of the actual opening/needle position. Experienced people like Buddah can probably do it with some accuracy, but most can't. I have been tuning  bikes for years ( my own and friends)  and rarely would attempt do a proper carb  tune on a CV carb bike. Finding a place where you can hold a bike at or near wide open with a consistent load , is a lot harder than you think. That is pretty fast. Don't forget most of your load comes from wind resistance. Load in 3rd gear wide open is a lot less than load in top wide open.
Add to that that plug resding modern fuels is not as easy as it used to be as they don't colour up as consistantly unless really rich. 2 strokes still work because of the added oil, but 4 strokes are very light shaded indeed.
Do let me stop you, but I suspect it will be more frustration that easy.

[The Buddha]

Its actually easiest with an O2 sensor in the exhaust and the highway preferably with open stretches. Dyno is a safe environment to do a WOT run, but tell you nothing about the state of tune. O2 in the exhaust is still the way you read mixture. I did exactly that for over a year to come up with a formula.
Cool.
Buddha.

[user11235813]

OK some good advice here I will see if I can get this done right and report back. But as Buddha said, I'm prolly as good as it's gonna get and my dead spots were just too much air.

However I don't understand if I'm getting 14:1 at 3/4 and not getting any more fuel at WOT then is WOT just meant to lean the bike out? Why not just call WOT 3/4 and stop there. Sort of like the reverse of turning it up to 11.

[herennow]

As I understand it Buddha is saying that WOT you get more air AND more gas, but the ratio stays the same.

[mr72]

However I don't understand if I'm getting 14:1 at 3/4 and not getting any more fuel at WOT then is WOT just meant to lean the bike out

As long as the slide is not all the way up at 3/4 throttle then it will get more fuel at wot or higher rpm.

Fuel requirement is a function of throttle, load and rpm.

Sent from my Pixel 2 using Tapatalk

[The Buddha]

As I understand it Buddha is saying that WOT you get more air AND more gas, but the ratio stays the same.

I would say - if Suzuki did it right which I believe they did - then yes. Since we didn't pfoock round with it in that aspect.
We put larger mains, we lifted the needle a bit, we put bigger pilot, bigger air screw both of which count for a tiny amount @ 3/4 or WOT - so correct @ 3/4 = correct @ WOT. Besides I have run it with O2 sensor in frigid cold canadia on an 89-00 and its pretty much the same 3/4 vs WOT. Unless you drop in a weird profile needle its gonna be the same @ 3/4 vs WOT. I can see how you can make WOT weirdly richer than 3/4 but not other way round even with a funky needle.
Cool.
Buddha.