Wide open throttle (main jet) stutter

[DewCatTea-Bob]

By: JimF...
" 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 haven't yet been able to hear how your engine sounds when your running-issue occurs but,, from what I had read, I had let myself believe that you can run okay up to about 7/8ths throttle, but when you continue up to full-throttle, your engine then develops your running-issue, regardless of RPM. _ So that then seems to point the finger at your main-jet.
But if your issue only occurs at some particular RPM-range, then there's a possibility that your chosen plumbing (along-with the red-camshaft's valve-timing) is the actual cause.
So then, how was your issue affected when you tried it without the air-filter attached ?
__ I tend to doubt that retarded ign.timing would cause such a running-issue to occur dependent on throttle-setting.
Retarded ign.timing should merely lead to poorer power-output regardless of the throttle-setting of a well jetted carb.
However I'm thinking that maybe you possibly have more than one contributing factor involved, which combined-together, then lead to your particular running-issue.

____ Like most everyone else seeking advice Jim, you also haven't been very/too forthcoming with answers to questions asked of you, concerning your-own quest (of this thread).


Hopeful-Cheers,
-Bob

[DewCatTea-Bob]

By: Bevel bob...
" 40 degrees is a bit extreme, "

____ Ducati themselves recommend 38 to 41 degrees BTDC !
__ I-myself always went with "40" cuz I didn't want to chance the setting possibly ending-up at 42 unintentionally (during a rushed job), and also that way the tuning-job would then last longer (as the points-gap diminished & the setting fell-back towards 38-BTDC).


" Just check the points gap and note it,then increase the gap by 3 thou, "

____ It seems Jim has to advance his ign.timing by 15 to 20 degrees,
so Bob, about how many degrees do you think that added "3 thou" will provide ?
__ Even if Jim's points-gap is already set for .018" -(the max recommended), I'm sure it will still be just-find with the resulting .021", (cuz I've seen Duke-engines still run well enough with the p.gap set with far-beyond that relatively-small additional extra gap-width) !

____ I'd suggest that Jim try his Duke with the air-filter removed (and report the results), before next increasing his points-gap.


Ducaddy-DUKEs,
-Bob

[JimF]

Based on the advice I was given, I didn't try running without the air filter. I inferred that doing so would not change the air/fuel mixture significantly enough to make an assessment as to the state of the current mixture and so I moved on.

The plan now is to change the jetting to make it richer and and draw a conclusion as to whether or not I need to make the jetting richer or leaner based on the problem's response to the jetting change.

But, because the problem could also be tied to engine RPM I reconsidered my earlier decision to leave the timing alone and now I think it best to set the timing correctly and then revisit the fuel mixture issue.

Jim

[Bevel bob]

I'm not saying 3 thou will bring the timing to 40 degrees. On my 250 a thou or two will provoke a kick back where there was none. A rough check seems to be 5 degrees, so 3thou maybe 10 degrees,enough to change the response if the bad running is timing related .I also think its a combination of timing and rich mixture.

[DewCatTea-Bob]

By: JimF...
" Based on the advice I was given, I didn't try running without the air filter. I inferred that doing so would not change the air/fuel mixture significantly enough to make an assessment as to the state of the current mixture and so I moved on.

____ Well that's of-course reasonable, providing you've inspected your filter-element to make-sure that it's reasonably clean.
But I'm now curious about the cleaner-housing's plumbing-effect on intake-tract tuning (in conjunction with valve-timing & exhaust-plumbing), since your carb & manifold are not of any stock-type combination.


" The plan now is to change the jetting to make it richer and and draw a conclusion as to whether or not I need to make the jetting richer or leaner based on the problem's response to the jetting change. "

____ That's indeed a good idea to try-out, since you already have a richer m.jet to perform that experiment with.


" But, because the problem could also be tied to engine RPM I reconsidered my earlier decision to leave the timing alone and now I think it best to set the timing correctly and then revisit the fuel mixture issue. "

____ I-myself have not indicated that your retarded ign.timing could be the cause,
(since your issue's effect only occurs when going from
near full-throttle to wide-open).
But rather than taking all the time to set your ign.timing exactly by-the-book, simply opening your points-gap 2 to 5 thousandths should get the timing close enough for it to be completely eliminated from contention (for your particular running-issue) !

____ Anyhow, I'm now suspecting that neither the jetting-change nor ign.timing will make any difference because, you may possibly have a (fairly rare) valve-timing & intake/exhaust plumbing issue (which IS related to RPM).
If so, removal of the air-cleaner OR changing the muffler may quite possibly rid you of your running-issue (at least at whatever particular RPM it's been occurring at).
__ So you now have two possible reasons for trying-out running without the filter-body.
However it's a little early yet to be suspecting such exotic causes when we have yet to eliminate more common causes such as foreign carburetion.


Hopeful-Cheers,
DCT-Bob

[JimF]

I just changed the timing to about 40 degrees.

Here is a photo of the cavity behind the points plate:

POINTS PLATE REMOVED.JPG

Bob, is this what you wanted me to take a photo of before?


The spark plug looked maybe a little lean to me.

SPARK PLUG.JPG

[DewCatTea-Bob]

" I just changed the timing to about 40 degrees. "

____ Since you went to such extra trouble (instead of merely opening your points-gap a few thousandths), did you make your added work worth the extra trouble, by first (double)- checking exactly where your ign.timing WAS set at ?
If so, was it really at 21-degrees BTDC, like you had (recently) thought ?
__ Also, it would've been great if (while you had a degree-wheel all set up), you had bothered to check to see exactly how many degrees difference there is between having the points-gap set at the two extreme settings -(of .012 & .018"). _ (But I gather that you didn't bother to check that ?)
____ Now that you have your static-ign.timing set so far advanced, do you have your kick-starting procedure well established for avoiding 'kick-back' ?


" Bob, is this what you wanted me to take a photo of before? "

____ Yes but, the pic.shot's focus & lighting leave a little to be desired, (however it need not be redone).



" The spark plug looked maybe a little lean to me. "

____ Yep, either lean or the plug's heat-range is a little too hot.
But then, was that s.plug only exposed to just a full-throttle test-run, or is that the accumulation of general riding (at various throttle-settings) ?
__ What's the heat-range of your pictured spark-plug ?
The reason I had bothered to mention that it's possible that it's 'reading' could possibly be influenced by being too hot of a s.plug, is because while the insulator-nose looks to be indicative of leanness, the rest of it's surface-area inside-there looks too dark.
Ideally, the insulator-nose should be a medium-light tan, while all the rest of the exposed surface-area ought to be med.light brown to dark-brown, (but not black at all),, and (other than it's nose), your pictured s.plug looks pretty-dark to me, (being extra dark-brown or even fully black, [can't tell for sure, by merely inspecting your pic] ).
____ BTW, if it's actually true that your particular running-issue is really due to a TOO-rich fuel-mixture (at full-throttle), then the standard 'plug-reading' test-procedure (for main-jetting) would indeed reveal THAT...
Cuz an only slightly-rich mixture would not cause any notable running-issue (as you have), and only an EXTRA-rich m.jet-mixture could possibly cause any such significantly notable running-issue. _ So-therefore in such an evidently extreme case (of a really extra rich main-jet size), the std.plug-reading test (of running at full-throttle for a mile), should result with a notably black plug (and a trustworthy plug-reading). _ And so IF extra-richness is indeed the cause of your running-issue, THEN the plug will certainly have to soon turn-out black relatively quickly. _ And considering the notable-severity of your running-issue, the main-jet's mixture would HAVE to be OVERLY rich (if, that's IF 'richness' is actually the sole cause, [which I have doubts of] ).
__ Bottom-line - it would be worth running the main-jet/plug-reading road-test, to determine whether there's any possibility that extra-richness is a possible cause or not.


Dukaddy-DUKEs,
-Bob

[graeme]

Hi Jim, may I suggest you only change 1 thing at a time.
Then if you need to change something else, put the first thing you changed back to its origin.
Or you will not know what was wrong initially.
You may need to change more than one setting, but it's nice to recognize what each change does.
Graeme

[JimF]

Timing was at a measured 30 degrees. I thought I had it at 21 degrees... Could be my related to locating TDC.

Spark plug is a BR6HIX.

Picture quality is poor owing to macro focus, stability and lighting. Flash washes everything out, tripod is a PITA to get and setup.

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

Engine sputter on full throttle remains the same.

First warm start with new timing was instantaneous.

[Bevel bob]

I had a stutter at 6000, I changed 3 things, reduced the main jet,fitted a less restricted silencer and raised the fuel level. the problem was gone!!, my next problem was trying to stop the motor going over 10000 rpm when I was not watching,crazy bike,but fun.