Wide open throttle (main jet) stutter

[JimF]

Hi Bevel Bob and others

I have an aftermarket but visually authentic looking megaphone that by some cosmic good fortune has a removable baffle in the center. I am a little worried to pull out the baffle as I don't want to irritate and alienate the neighbors or the police.

I plan to order a half dozen main jets tomorrow, perhaps three to either side of what I have now.

I don't want to come off as a dolt, but what do you mean when you say you raised the fuel level? Are you saying you changed the quiescent level of the fuel within the float bowl to bring the level up a bit more?

So as I see it now perhaps I have five variables:

1) less restrictive exhaust (possibly leaving me in the undesirable position of having neighbors carrying torches and pitchforks chasing me through the village.)
2) less restrictive air intake (for testing only, this would be nothing more than a temporary removal of the air filter.)
3) re-size the main jet
4) change the spark plug to a different heat range
5) change the fuel level (once again, I guess this means change the float so as to have more fuel in the bowl to draw from.)

I am more than happy to entertain other possible variables and advice on what variable to try first.

Jim

[graeme]

I may have missed this bit,,,,,
Did it run well without the miss at full throttle previously?
Or is this something that was always there?
graeme

[DewCatTea-Bob]

By: Jim...
" Timing was at a measured 30 degrees. I thought I had it at 21 degrees... "

____ So then as it turns-out, you've been running your engine with the ign.timing set half-way between the static & full-advance settings (for later 18-degree AAU equipped models), which seems to be a coincidental compromise.
__ Anyhow, bothering to have taken-note of the (previous) ign.timing can most-always provide some useful clue.
Since you've been getting your engine kick-started with such an advanced fixed-timing setting and without complaints of kick-back, leads me to assume that you've previously learned the best method for kick-starting your type of engine (rather than just having had good-luck most every kick-start attempt).


" Spark plug is a BR6HIX. "

____ I believe a '6' is too hot of a s.plug,, a 7 should be hot enough, (normally).
And for the main-jet plug-read testing, a number-8 should be hot enough (if the main-jet isn't too awfully rich).


" First post-retiming cold start took a few more kicks than usual. "

____ Unless the very-first attempts were spoiled with kick-backs, there's no good-explanation for that odd occurrence.


" Engine sputter on full throttle remains the same. "

____ As ought to have been expected, especially since the ign.timing was only about 9-degrees retarded.


Duke-Cheers,
-Bob

[DewCatTea-Bob]

By: JimF...
" but what do you mean when you say you raised the fuel level? "

____ Those of us who recall a lot of what we've read on this forum,, will recall that B.bob had rotated the bowl on his SSI, so as to slightly raise the fuel-level just below the throat of the carb, (thus making it somewhat easier for the fuel to be sucked-up into the air-flow).


" I am more than happy to entertain other possible variables and advice on what variable to try first. "

____ STILL waiting to learn what effect removal of the current air-intake/filter setup has on your running-issue !


Hopeful-Cheers,
-Bob

[JimF]

I have not tried it with the air filter removed, but I will.. I lost a good part of the weekend dealing with a relative's health issues.

I did however, just minutes ago, slide out the baffle. A very quick run up and down the road on the unrestricted megaphone exhibited the stutter, and if anything the stutter may have occurred slightly sooner (perhaps 500 rpms sooner) than with the baffle in place. That's unscientific though, and I would need more data to be sure.

Excuse me, but a mob of angry villagers are at my front door. I wonder what they want?

[DewCatTea-Bob]

" A very quick run up and down the road on the unrestricted megaphone exhibited the stutter, and if anything the stutter may have occurred slightly sooner (perhaps 500 rpms sooner) than with the baffle in place. "

____ With the resulted less restricted back-pressure, the preexisting fuel jetting-mixture should've then been reduced towards increased LEANness, (as increased AIR-flow became allowed without the baffle) !
So if your full-throttle running-issue was indeed made worse that way, then it would of-course seem that more fuel is required and so richer main-jetting ought to be what's actually called-for (rather than leaner jetting).
__ However, now that it's seemingly been confirmed that your fully wide-open throttle running-issue doesn't begin until such high RPMs are reached, that detail seems to indicate a fuel-supply issue (more so than a main-jetting restriction). _ So it should be checked that none of the fuel-passageways (all the way from the inside of the tank, down into the carb.float's bowl) are possibly restricted in any way.


Hopeful-Cheer's,
DCT-Bob

[JimF]

A quick run up and down the road with no air filter seemed to have no effect on the main jet stutter, which appeard at around 7K RPM which is very much the same as with the filter in place.

Jim

[DewCatTea-Bob]

" with no air filter seemed to have no effect on the main jet stutter, which appeard at around 7K RPM which is very much the same as with the filter in place. "

____ Okay then, (I assume that was with the baffle reinserted back into your muffler), it thus-then seems that not-only was your filter not restricting air-flow any excessive amount, but also it now seems that my notion of incompatible-plumbing is no-longer a probable possibility as well.
__ So this latest outcome now leaves the main-jet notion as the most suspected open-end left to eliminate. _ And because of your baffle-removal experiment, a richer m.jet seems to be what should be tried first.
And-so since you already have a #190 m.jet, why not give that jet a try (before choosing more m.jets) ? _ Cuz if that larger jet doesn't cause any notable change at all, then adjusting your Mikuni-carb's float-level may then of-course likely be more important than the main-jet's particular size.
However if the task of swapping m.jets is less fun for you than installing a colder s.plug and performing a standard full-throttle plug-reading test, then that trial ought probably be done first, instead.
____ Also, (now that it's better understood that your running-issue has been more strictly tied-down to being merely a HIGHer-RPM issue), have you checked your fuel-passageways to make-sure that your carb isn't possibly becoming starved for fuel-flow, (during high-RPM) ?


Hopeful-Cheers,
-Bob

[JimF]

I called Sudco and they reminded me that I had bought a 180 and a 185 main. I could not find the 180 but I found the 185 and installed it. It was like the old days, where the bike would not rev past 6000 rpm.

I had been thinking I was too lean on the main at 140.

At 185 I am too rich.

So either 140 is indeed too lean and I lept over the sweet spot above 140 but below 185, or maybe 140 is too rich and I need to go smaller.

So perhaps I will order a 120, 130, 150, 160, and a 170.

Jim

[DewCatTea-Bob]

" It was like the old days, where the bike would not rev past 6000 rpm. "

____ Alright but, so exactly how does that influence the engine's performance in relation to your running-issue ? _ Is it the very-same only much worse, or is your latest symptom something-else different ?


" So perhaps I will order a 120, 130, 150, 160, and a 170. "

____ If those m.jets aren't cheap, (I've heard some cost upwards of 10-bucks each), then try to run the std.plug-reading test, (to first confirm that you're indeed running rich).
__ If you're really too-rich at full-throttle, yet not so rich when at 7/8ths-throttle,, then you ought to try lowering the needle-jet's needle-valve, which should then slightly lean-out the mixture (at higher throttle-settings), so that you can then get closer to FULL-throttle before your running-issue becomes noticeable.
However if lowering that needle then causes your established running-issue to become even more notable, then that would help confirm that you've already been running too lean.
__ Also, you may wish to consider also ordering another needle-valve of the next increment-size, while you're bothering to place an order.


Hopeful-Cheers,
-Bob

PS. - It may be too-much to expect but, I hope that it's been noticed that my previous-posts have been edited (with additional-wording) once I've signed-off with ''DCT'-Bob' added onto them.