Wide open throttle (main jet) stutter

[JimF]

Thanks you guys, this forum is a life saver for me.

I can roll on my throttle and climb the RPMs up to 8K and beyond in any gear with the engine under load so long as I never get on the last quarter turn of the throttle. I equate the last 1/4 turn to mean "main jet."

If I open the throttle full the engine sputters.

You can hear what I am talking about 15 seconds into the attached audio file. That's wide open throttle and the engine sputters. A few seconds before that and one gear down I took it up to 8 grand, then a few seconds later in another higher gear I again take it to 8 grand but both those times I am staying off W.O.T.

(I have attached an audio file captured by my Blackberry. My PC plays it with QuickTime." It is an AMR file and the board won't allow me to upload an AMR so I put the AMR in a zip file. You open the zip file and pull the "sputter_duc" AMR out. Then right-mouse click on the "sputter_duc" file and select "open with" and then Quick Time Player.)

Duc Audio.zip

I am going to guess that there are two possible issues:

1) The main jet is undersized and at full throttle the engine is starved for fuel.

2) The main jet is ovesized and at full throttle the over-rich mixture is fouling the plug.

If I had to figure this out on my own I suspect I would get out on the open road, open the throttle full for 10 or 15 seconds and then kill the ignition. I am guessing that when I roll to a stop the spark plug will either be white from a too lean condition or black from a too rich condtion.

My fear is that if I am too lean and I hold the full throttle too long I might damage the piston.

That's why I am asking the forum's sage advice...

Thanks,

Jim

[DewCatTea-Bob]

____ Well, it's looking like you're going to have to do some searching Jim...
In one of the few other threads you started concerning your Duke's Mikuni-carb, someone-else listed all the jetting of his-own Mikuni, which was claimed to be well adjusted & set for his Duke, apparently without any carburetion issues. _ So whatever size main-jet HE employs, may be a good point to try-out next.
I did a little search myself, and this: viewtopic.php?f=3&t=1078&p=7296#p7327 , is unfortunately not of the thread* I had meant to find, (* which was a couple months older yet).
__ Was your Mikuni-carb's original main-jet, larger or smaller than the main-jet which you currently have installed ?
Can you list the size-numbers of both of them ?


Hopeful-Cheers,
-Bob

[JimF]

I think the difference between this post and my previous one is a small but distinct difference.

Months ago I was cracking the throttle wide open quickly and feeling the stutter, to which I was well advised that without an accelerator pump this kind of issue would be common. I accepted that and moved on. If I didn't whack the throttle hard, I didn't have the problem. Everything made perfect sense because when I stopped whacking the throttle the problem seemed to go away.

Until recently...


What I have found is that without the quick throttle twist, but rather with a slower steady throttle twist, I still get the stutter in the last 1/4 turn. I think I had subconsciously learned to avoid the stutter by staying out of that throttle region, getting up to 7 grand without the wide open throttle, shifting up and climbing again.

What has made me reconsider my issue is that now I am beginning to push the speeds on open roads, getting into 5th gear and slowly getting to full throttle but finding that my stutter issue remains. So this is not caused by me cracking the throttle from closed to open in 1.5 seconds, this is really a fuel supply issue when I am running on the main jet.

So what I thought was a problem caused by cracking the throttle wide open wasn't really that at all.

Here is where I am at now:

140 main
30 pilot
182-02 needle jet
5DP7 needle
2.5 slide
0.5 air jet

I will try to find the post you mentioned that spedicfied a well set up Mikuni's jetting.

Jim

[Bevel bob]

My suspicion is that the main jet is too large .

[JimF]

Is this the kind of situation where a "throttle chop" and a plug read would be useful?

If so, how long should I stay on full throttle so as to insure a useful plug read before I kill the ignition?

[DewCatTea-Bob]

By: JimF...
" 140 main
30 pilot
182-02 needle jet
5DP7 needle
2.5 slide
0.5 air jet
I will try to find the post you mentioned that spedicfied a well set up Mikuni's jetting. "

____ I've already spent three search-sessions looking for it. _ I recall it being posted back around the time not long before you decided to order a Mikuni-distributor's recommended jet-set, and the post was posted by a fellow-member who has been with us for at least a couple years but I can't recall his name now (without seeing it), and he had listed his set of jets in the same manor as you have above (and not within the text of his post).
I THINK his main-jet size was 160, but not sure,, and also back at that time, the carb.throat-size was not well-established, for either Mikuni-carb , (or, yours was unknown then, and his was a 28mm,, just not sure anymore, which is why I was so interested in searching for that particular post).

____ Also, while I was searching-through the forum, I came-across various posts by you which together have led me to believe that you-yourself must own (in recent times) three different DUCATI-Singles - (one 4-speed & two 5-speed models)... Is that correct ?


Duke-Cheers,
-Bob

[DewCatTea-Bob]

Is this the kind of situation where a "throttle chop" and a plug read would be useful?
If so, how long should I stay on full throttle so as to insure a useful plug read before I kill the ignition?

____ Yes but, it's been my-own experience that such 'plug-reading' is much more undecisive than is usually claimed. ...
__ THE procedure is to install a new/fresh spark-plug and as soon as the engine is started, take-off & run at ONLY full-throttle for (AT-LEAST) a mile or two, then kill the ignition (to stop the engine) and coast to a stop, then remove the plug & see if it's insulator-nose's running-color is either: 'light-gray' -(thus 'lean'); 'med.brown' -(thus close to 'normal'); or 'black' -(thus 'rich').
However my-own experience with performing that test-procedure leads me to conclude that you'd have to run the engine (solely at full-throttle) for more like 10-miles (at least !), in order to obtain an actually-USABLE 'plug-reading' from an engine which is carb.jetted anywhere near reasonably close (to what's considered to be optimally normal). _ In other words, that fairly-well-known plug-reading procedure won't really tell ya jack-shit, unless your main-jet is way-the-hell TOO F.n RICH !! _ As that's the only way your new plug is going to get black-colored with merely-just 1-mile of full-throttle riding !
So yes I'm telling ya that if your carb's main-jetting is only 'slightly-rich' OR 'perfectly-normal' OR (any amount of)- 'lean',, THEN (in such non-excessively-rich cases), any test-distance under 2 to 10 miles of purely FULL-throttle running, is not really going to provide you with a 'plug-reading' which you can fully trust to be completely-accurate ! _ (Cuz the plug's white insulator will not have had sufficient time to have fully become it's new shade of running-color ! [UNLESS the main-jet is extremely/excessively RICH].)
Therefore it's my good opinion that such short-term road-testing can only be useful for JUST revealing a main-jet mixture that's EXTRA-rich. _ Cuz any OTHER mixture will take a lot longer (than just a mile or two), to place a TRUSTWORTHY running-color onto a new/fresh spark-plug !
__ I suppose however, that if one already had good suspicion of whether the main-jet mixture is too-rich or too-lean, they could then, (in the case of too-lean), run the test with a new spark-plug that's a couple heat-range increments colder (so as to obtain the expected lean plug-reading sooner, without the plug itself tending to taint the results). - (Cuz a spark-plug of which it's heat-range is too 'hot', will tend to make a normal-mixture 'read' pretty-much the same as if it's too-lean.)
____ Does anyone (who has actually tried/done such plug-reading testing [extensively]), disagree with my stated-opinion on this topic-issue ? _ Cuz if so, then I'd like to hear/read of your thoughts on this, (cuz I-myself never got any real satisfaction from such 'plug-reading' tests ! _ [Cuz I could never run solely full-throttle for a long enough distance.] ).


Dukaddy-DUKEs,
DCT-Bob

[Bevel bob]

You could put it on a dyno and monitor the ex emmisions at the various throttle positions.

[graeme]

Yes Bob, I have done that too, it makes setting carbs easy.
You just need to find a friendly dyno owner.

[JimF]

(Here's a quick answer to the question of my "Ducati" holdings, all of which are or have been 5-speed...

I have the '64-ish 250 cafe'd and devalued Diana which is the subject of these posts of late.

I have a '65-ish 250 Mach 1, restored but still very original.

I HAD a '69 450 Mark3D Desmo widecase.

I also have a round case twin.)


The entire task of performing the plug chop which includes installing a new plug, the 'hard to orchestrate in an urban setting' long uninterrupted ride at full throttle culminating in an ignition cut and coast to a stop, etc. which may or may not result in a worthwhile reading 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 know no one with either a dynamometer or an exhaust emissions analyzer.


Bevel-Bob, you think my main is too large (too much fuel.) 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. 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?





Jim