Mikuni 'sputters' at throttle wide open

[JimF]

Using Sudco's recommendations my Mikuni-carbed 250 is running great. It revs up high, pulls strong and starts easy.

Now that I have it running well, I noticed that when I really twist the throttle WFO (Wide F-ing Open) the engine sputters. But if I climb the engine revs using moderate throttle twist there is no hesitation.

My question is should I suspect that it's too lean or too rich on the main jet? Or does it not matter as the troubleshooting is the same regardless? I am thinking I need to move the needle up one and down one from its current position to see what makes it better. Is this the correct thing to do?

If I find a needle position that works do I call it "done" or do I buy a different main jet and try to bring the needle clip back to center?

Thanks,

Jim

[Rick]

The answer may be in the old Henny Youngman joke:
Patient says, "Doc- it hurts when I do this."
Doctor says, "Then don't do that."
Slide carburetors just aren't very good at responding to the sudden change in air pressure and velocity that results from whacking the throttle open. If the jetting and setup is good except for the fast response, then it's probably as good as it'll get. A smaller bore carburetor would help. The industry solution was those miserable diaphragm carbs, and finally computer controlled fuel injection.
There may be a solution, but every slide carburetor equipped motorcycle I've ever owned would stumble if I whacked the throttle open.
Rick

[double diamond]

This is why Dellorto developed the PHF/PHM carbs with an accelerator pump. It retains the advantages of a throttle controlled slide (rather than vacuum controlled as on CV carbs) but addresses the sudden lean condition of rapid throttle opening by adding fuel to the intake charge with a diaphragm pump when the venturi effect at the needle jet cannot keep up. Some other slide carbs (slide type Bings on BMW's come to mind, some of the Japanes carbs used on off-road bikes) have an accelerator pump in the circuit for the same reason. You might realize some modest improvement by fine tuning jetting (needle with more taper if you think richening the mixture at larger throtle openings will improve the situation) but you're never going to completely eliminate the sudden lean condition of whacking the throttle open without another fuel circuit (i.e. accelerator pump). There's no advantage to keeping the needle clip in the center position. If it runs best with the clip higher or lower, that's where it needs to be. You could move it back to the center position by changing the needle jet, but you'd resort to this only when when a top or bottom clip position doesn't resolve a performance condition and you need to take the mixture up (or down) a notch. The main jet will affect the needle/jet performance to an extent, progressively more as the needle is raised, so raising the needle might require a smaller main jet. Matt

[JimF]

The quality of answers one gets here is incredible. Thanks. You guys are great.

[DewCatTea-Bob]

" The quality of answers one gets here is incredible. "

____ I've held-off posting here myself, waiting to see what others say and how Jim responds to what he gets,, (since I have no past good-experience with adapting Mikuni-carbs myself).
__ I hadn't expected that Jim would be so happily pleased with the response-info (posted so far), cuz I had expected that Jim would've already been well familiarized with that fairly-common effect which one encounters whenever a handful of throttle is suddenly grabbed-opened with such type of carb, (as with the SSI-carb that Jim likely has on his long-owned Mach-I).
So having thus-then figured that Jim certainly must be experiencing something quite notably even worse (than that with the SSI),, I therefore naturally expected that someone would more likely address a notably far less obvious momentary lean-condition, possibly due to his particularly jetted Mikuni-carb (rather than the TYPE of carb), and-so more-fully fulfill Jim's particular issue-concern.
So unlike Jim, I-myself am somewhat disappointed that nobody has yet posted more specific info concerning the likely probability that Jim's particular issue is not simply due to his carb's TYPE.
__ That Jim seems to accept that the answer is simply due to his type of carb, NOW leads me to think that THAT may indeed be all there is to it, after all. _ However, after all of the quite notable troubles I've heard-of in the past from others who've tried to adapt a Mikuni to their DUKEs, I have come to expect such more unique running-issues (such as Jim had become concerned with).
So I'M not satisfied that Jim's concerned issue is not possibly an actual one. _ Especially since he had mentioned his carburetion-issue only concerning his Mikuni and not also with the SSI as well.
So I'm thinking that his Mikuni is already operating at a rather lean mixture to begin with, and is thus more susceptible to the kind of issue he's experiencing.
____ I don't feel really qualified to give advice to anyone trying to get a Mikuni to provide absolutely CORRECT carburetion, but I'll suggest a direction to try improving such an issue.....


By: JimF...
" I noticed that when I really twist the throttle WFO
the engine sputters. But if I climb the engine revs using moderate throttle twist there is no hesitation.
My question is should I suspect that it's too lean or too rich on the main jet?
I am thinking I need to move the needle up one and down one from its current position to see what makes it better. Is this the correct thing to do? "

____ Basically, yes. _ It's likely that you're then getting a sudden EXTRA-lean condition, and you could very likely improve that (suspected) abnormal condition by raising your needle-valve's position (by lowering it's clip all the way), so as to find-out if the issue is then lessened.
And if that helps, going a couple sizes bigger on the main-jet could be okay as well.


" If I find a needle position that works do I call it "done" "

____ Well it may then be "done" well-enough for you but, actually probably not really DONE,, cuz Mikuni has such a very vastly wide selection of graduated jetting-parts, that certainly there's an even better jetting-selection which would work more optimally for you. _ However, a lot of extensive testing would need to be done in order to fine it, (and that's fairly-well beyond what any single person could ever get accomplished).
__ Also, it's quite doubtful that simply raising the needle will completely solve the issue.


" or do I buy a different main jet and try to bring the needle clip back to center? "

____ Assuming you prefer the best PERFORMANCE rather than being environmentally-friendly, lowering the needle back-down won't help that running-issue, even IF a much bigger main-jet somehow helped lessen the issue.
__ If your carburetion-issue is mainly occurring when at low-speed (under 30-MPH) with low-throttle, then you're likely running on mixture controlled by the throttle-slide and even the idle-jet...
So-then changing to richer increments of those jetting-parts (more so than the main-jet), would help reduce the lean-out switch-over, as well.
____ Reminding you, that this is mainly-general stuff, and is likely only a useful direction in the possible event that your running-issue is indeed fairly significantly worse than the same type of effect that's experienced with a similarly sized SSI-carb.
In such case,, I suggest trying richer settings, one mixture-metering throttle-section* at a time,, until satisfied. _ (* The various throttle-sections [in order of most likely probable significance, for this issue], are: throttle-slide cut-away; needle-jet/needle-valve; main-jet; idle-jet.
__ If your spark-plug doesn't already show-up consistently as a dark-brown, then going with richer jetting is very LIKELY the way to go, (since there would then be room to do so).


Hopeful-Cheers,
-Bob

[Harvey]

Not only but also. The jetting that is talked about, Idle, needle, needle jet, main jet, are all to set the mixture for steady state running. Snap acceleration is controlled by the slide cutaway, and the well of fuel, that is held around the outside of needle jet. As the throttle is opened the vacuum moves back, from the slides front edge, at the idle/slow jet, back to the slide cutaway. As the vacuum moves back across the needle jet, it pulls the extra fuel from around the needle jet.

This is the fuel that is needed to compensate for the sudden air supply when the throttle is snapped open. Two stroke carbs use a shoulder around the needle well to assist, due to the low vacuum of the two stroke.

Harvey.

[vintagetour]

Jim,
What jets did you end up with? I am in the process of jetting my 250 as it runs well but is much richer than your previous posted sizes.
Thanks
Tom

[DewCatTea-Bob]

" I am in the process of jetting my 250 as it runs well but is much richer "

____ So-then exactly which model of carb are you using Tom ?
How about listing all the jetting-sizes which YOU are currently running with ? _ And also let us know if your s.plug-reading/shades are dark, medium, or light brown.


By: Harvey...
" the jetting that is talked about, Idle, needle, needle jet, main jet, are all to set the mixture for steady state running. "

____ That's certainly quite true, as those differing metering-sections are each intended only to control the air/fuel mixture-ratio for their respective throttle-settings,, and of course I had not meant to imply that altering any of them to richer settings could be done to possibly cure the concerned sudden opened-throttle issue (without any downside).
However, IF the current jetting-sections are set on the lean side already, THEN the quickly-opened throttle issue could be HELPED (to be less of an issue), by adjusting them towards richer settings (as can be done without getting TOO awfully rich).
__ Whether this can be done (as I believe it probably can, in Jim's case), depends on how rich his mixture actually already is. _ If his s.plug-reading is black, (which wouldn't make sense, since he had his issue with his Mikuni and not also his SSI),then certainly he would not wish go even richer ! _ But, if his plug-reading is light-gray or even light-tan,, THEN he has sufficient room to jet richer, so as to lesson his issue (with snapped-open throttle).


" Snap acceleration is controlled by the slide cutaway, "

____ I wouldn't say that this particular mixture-section 'controls' the issue in question (while the others don't have any control at all),, but I do agree that the slide's 'cut-away' size is probably more effective than the needle-valve setting, at having the strongest effect on the issue in question.
It's just that it's usually easier & cheaper to mess with the needle than it is to get another throttle-slide(s) to try-out,, and the needle can be adjusted to help-out (with the issue), even if not the best target to take aim.
__ I hope Jim will let us know if raising his needle-valve (as far as it goes) helps-out the severity of his issue,, cuz if it indeed does, then he can more-so suspect that it would be worth his while to try-out another throttle-slide with a lower cut-away size.


Hopeful-Cheers,
DCT-Bob

[vintagetour]

Mine is a VM 26. Started with a 190 main, 22.5 pilot (1.5 cutaway on the slide) If I wrote it down correctly I think Jim posted he had a 140 main, 30 pilot (with a 2.5 cutaway) to start with. My 190 main was too rich with a black but not sooty plug. I dropped to a 180 and it is starting to get a little brown on the plug but still too fat on the main. I am leaning towards Jim's settings but wondered if he has made jet changes from his posted. I just picked up a 25 and a 27.5 pilot to try and I think I'm going to go ahead and put the 170 main in tonight.

Anyhow I'm just trying to fine tune a little before I try to sell it. Even though it starts first kick hot or cold my wife cannot start it as she has a weight problem (not enough lead in her butt) I guess we will have to get her one of those new fangled bikes with an electic start button.