Wide open throttle (main jet) stutter

[Lonestar]

Fit a main jet two sizes larger (safer to be overly rich than overly lean), and run the bike at full throttle. The stutter will either diminish or get worse, so you'll know in which direction to go to find the perfect size.

[double diamond]

Jim, my experience with plug chops has been on racetracks where you can run a few laps to get the motor up to operating temperature and “cleaned out” which is to say get any fuel deposits from the starting/warm-up procedure burned off the insulator, then run the engine wide open on the main jet in top gear on the longest straightaway on the course. Right when it’s time to shut off for the upcoming corner at the end of the straight, you’d hit the kill button, pull in the clutch, snap the throttle shut and coast/brake to a stop. Hopefully you’d be able to coast into the pits. As you can see, this procedure is difficult to employ on the street. But on a race day there’s limited time to get jetting sorted out and your jetting is already pretty close if you’ve run the track before (or your fellow racers are forthcoming with ballpark jetting suggestions) so this method is employed. In your situation, you have the benefit of unlimited time to sort out your jetting so just start plugging away at it. It sounds like you have a main jet issue, which is actually pretty easy to address. I don’t recall exactly what your float bowl looks like, but some Mikunis have a cap to access the main jet. You simply unscrew the cap (with a clean catch pan for the gas that will drain out) change the main jet, replace the cap and pour the gas back in the tank. Takes no more than a minute. If you don’t have the cap on your float bowl you’ll have to remove the four screws to drop the float bowl, but there’s probably enough room to do this with a short screwdriver. So put in a 145 main jet, if it’s worse, go the other direction. If you notice no effect, go up or down to a 150 or a 130. Eventually you’ll see what direction you need to go or if changing the main jet is even having any effect. I’d imagine going to a 130 or 150 is going to have an obvious effect and indicate what direction you need to go in. If there is little or no effect, you know your issue lies elsewhere (perhaps that retarded ignition timing you’re employing). I have a bag of small hex Mikuni main jets, more than I’ll ever need. Is this the type of main jet you’re carb uses? I’ll have a look this weekend and see if I have the range you need, be glad to send you several. BTW, if your air filter is one of the scrambler wire gauze type, I doubt removing I will have much effect on your jetting. If it’s a paper type element, it could actually be your problem if it’s dirty or deteriorated. Matt

[DewCatTea-Bob]

By: JimF...
" The entire task of performing the plug chop
seems way too complex when compared with the simplicity of just changing the jet and seeing if the bike runs better. Or am I missing something? "

____ I, for one, don't think you're missing anything,, as the 'plug-reading' test would likely only tell you if your main-jet mixture is overly RICH. _ So then if you did that test and then found that your s.plug became black, you'd then be able to REALIZE that your m.jet is too rich, (but otherwise, you'd have to ASSUME that it's too lean).
So it indeed seems that it would be more direct to just go-ahead & change the main-jet size.
__ I had asked for confirmation of the size of the main-jet that came with your Mikuni, but I gather (from an older post) that it WAS-then a #190.
So I was going to suggest that you try reinstalling that m.jet to find-out it's effect (whether positive or not) on your full-throttle issue.



" If I take off the air cleaner for a quick ride, to my thinking I would lean out the fuel mixture with the added air. "

____ That's reasonably true of-course but, a normal -(relatively non-restrictive) air-filter (that's not gotten plugged-up), really shouldn't make a big enough difference to be so outwardly noticed.


" If the bike runs better then I could surmise that you are correct and I could order a few smaller jets to try. Does this sound feasible or am I overestimating the amount of extra air the fuel would be infused with if I remove the filter? "

____ I think that would indeed be overestimating the amount of the (then available) increased air-flow,, but if your engine then actually does become more normally responsive, I'd then certainly suspect that there's EXCESSIVE flow-restriction by that filter.


Hopeful-Cheers,
-Bob

[JimF]

I like the idea of going bigger on the main for reasons of engine safety - good advice. The bike will get better or it will foul, but either way I'll have my direction nailed down.

That's what I'll do then.

Bob, as you mentioned I know I have a 190 in the parts drawer but I suspect that's way too big over my current 140. I'll probably call Sudco on Monday and order two jets larger and two smaller.

[DewCatTea-Bob]

By: double diamond...
" So put in a 145 main jet, if it’s worse, go the other direction. If you notice no effect, go up or down to a 150 or a 130.
I’d imagine going to a 130 or 150 is going to have an obvious effect and indicate what direction you need to go in. "

____ Due to my lack of Mikuni-experience, I really don't know about that,, but it's been my understanding that the increments between Mikuni-jets is so very small -(about 4 or 5 times more greatly incremented than Dellorto or Amal ), that ya have to jump SEVERAL sizes in order to notice even a slight change. _ So if that's actually true, then it's fairly doubtful that those suggested size changes would be worth trying-out.
So since Jim ought to still have the original #190, then that's what (I think) ought to be tried-out first. _ And if that size doesn't happen to work-out, then try a #110, and next a #160 or 170.


Hopeful-Cheers,
-Bob

[DewCatTea-Bob]

By: JimF...
" Bob, as you mentioned I know I have a 190 in the parts drawer but I suspect that's way too big over my current 140. "

____ Unfortunately Jim, you beat-me-to-the-draw with your last-post, before I got my related-thought posted.
__ When you do shop for main-jets, then ask about the difference in size-increments (issue I've brought-up).
Cuz I've always found that the incremental-steps which Dellorto & Amal supplied, were more than adequate,, and if Mikuni really does increase the number of incremental-steps (by even just double [for the same overall-range]), then the difference between THEIR number-sizes could be quite misleading !
However if Mikuni-increments are actually stepped by 1s (rather than the 10s which have been noted thus far), then my increment-issue is already being jumped-past. _ In other words, if Mikuni also offers main-jet sizes such as #139 & #141, (instead of JUST step-jumps such as 130 & 150), THEN my stated concern has already been compensated for. _ (But if not, then ya shouldn't expect any substantial-difference between a #140 and a 150 or 130, [and THEN a #190 would seem to be a more reasonable jump in increments].)


Hopeful-Cheers,
-Bob

[double diamond]

Dellorto main jets are marked according to the diameter of the hole so if a Ducati scrambler with an SSI27 runs well with a 112 main jet, then a 26mm Mikuni would require something close to a 160 Mikuni small hex which measures about 1.12mm. A 190 Mikuni is about 1.25mm (or equal to a Dellorto 125), 180 Mikuni is about 1.20mm. Matt

[Bevel bob]

My 250 runs with a 110 on an SSI, and i have measured the jet using a sewing needle and micrometer at 0.110 mm. It was a 118 jet that i soldered up and re-calibrated with a sewing needle. A 265 needle jet measures 2.65mm , but solder is too soft for these.I have a Mikuni that was fitted ,I will look see what jet it has. Ugly thing though .

[JimF]

Just asking, this may be a dumb question but I have posted more stupid things, regarding the high RPM stutter I am experiencing: Could it in away way be related to my ignition timing being at 21 degrees rather than 40 degrees as it should be for a non-AAU equiped engine?

I am thinking to reset the timing and then move on the carb issue...

[Bevel bob]

Yup. It could be.40 degrees is a bit extreme, try 37.Just check the points gap and note it,then increase the gap by 3 thou, if there is a significant improvement you will know the way to go.Easy to go back if it flops.