Wide open throttle (main jet) stutter

[JimF]

The jets were about USD $3 each. I had to make about a $25 minimum order so that was alright, I ordered several sizes larger than my 140 and several sizes smaller in increments of 10.

The jets arrived and I had a 140 main so I went up to 150 and the stutter occurred sooner.

So I went down to 130 and the stutter was mildy noticable.

I went down to 120 on the main and the stutter is gone. I can twist the throttle wide open in any gear and the engine wants to pull strongly to redline and beyond.

I will try a plug chop when time permits.

I could use a stronger spring in the Mikuni though as the one that's in there doesn't want to push the slide down against the drag of the cable (negligible), the Super Practic and the grip, but that's a minor issue.

[Bevel bob]

Great, so its not far off the 110 that I sugested, I would not be surprised if you finish up with a 110 for clean combustion. I always understood that the Mic carb is a more efficient carb and would have guessed it would have a stronger pull on the jet , meaning a smaller jet would be needed than an SSI.My Mic has the jets missing, so no help there.

[ecurbruce]

Jim,
Which head is on your 250 with mikuni? Is it big valves or small valves?
Also wondering about compression? You may have said somewhere, I'm not finding it ...
You said it has a megaphone exhaust, right?

Thanks, Bruce

[JimF]

Hi Bruce,

As yet I don't know if I have the big or small valves. The bike came to me with just 2000 miles from a friend that I feel is truthful and a story that the bike was laid up after killing its first owner hence the low miles (and a non-stock front fork.) Given the poor performance that the aftermarket Mikuni provided without the proper jeting I can see why little mileage was attached to the bike post-rebuild.

No compression data either...

Supposedly it was a Diana, but everybody and their brother says that so I give little value to the statement. Dew-cat-tea Bob knows alot about the bike from past troubleshooting topics and he says many elements support that this particular one was indeed a Diana. Having heard this from Bob with no bias either way I can put some faith in it having a higher performance lineage. Too bad it was cafe'd, it destroyed much of the value it that's the case.

I do have a megaphone exhaust, but an aftermarket that I have not seen anwhere else; I bought it used and it looked to be much older than the new breed of megaphones that are available now. What makes this one a bit different is that it has a baffle that slides up the back end and is captured in place by a hook that is held by a spring's tension to a tab on the bottom. I have an original megaphone and there is no baffle or provisions for one.

Jim

[DewCatTea-Bob]

" I will try a plug chop when time permits. "

____ Not merely-just "when time permits" but also 'distance' as well, I'd say...
As I've previously pointed-out, the standard main-jet/plug-reading test will indeed reasonably*indicate whether a full-throttle fuel-mixture is too RICH, (* by relatively soon turning the plug-tip black),, but when the air/fuel-mix is fairly-CLOSE to a normal/optimum-mixture, ya then have to run the engine full-throttle a lot longer than just a mile or two, in order to tell if the mixture is still rich (or lean, [and even much longer still, if a lean-condition is to be TRUELY determined !] ).
__ Also, don't forget to use a s.plug that's not too hot, or it will tend to mask an otherwise good plug-reading.
A BP8HS (or B9HS) was the plug of choice for the test (back in the '70-s), as the engine should then be running at it's hottest-temp (thus helping to keep the plug clean).



" As yet I don't know if I have the big or small valves. "

____ The (standard) 'Mark-III' did not employ valves any larger than those in other 250-models, (which were 36 & 33mm).
The large-valves -(40 & 36mm) were first employed for the Mach-I (which was introduced for 1965), and it's top-end was then shared with the 1965 'Mark-3'.
And Jim's motor-number is a little too old to be a 1965-model.


" No compression data either... "

____ The '250 Mark III' has a model-unique piston specified to be "10:1" ! _ While the '250 Mark 3' is lower (at either 9.8 [n-c] or 9.5 [w-c] ).


" Supposedly it was a Diana, but everybody and their brother says that so I give little value to the statement. "

____ It's indeed too-bad that the name "Diana" has become so abused. _ (An example of such next follows.)


" Dew-cat-tea Bob knows alot about the bike from past troubleshooting topics and he says many elements support that this particular one was indeed a Diana. "

____ Actually, I did-not indicate that your Duke is a "Diana" ! ...
While I'm certainly fairly convinced that your Duke-model is a real '250 Diana Mark III' , a 'Diana 250' is a completely DIFFERENT model (which is about as different as two street/road-models can get).


" Having heard this from Bob with no bias either way I can put some faith in it having a higher performance lineage. "

____ Next time you have your Mic.carb removed from your Mark-III, you should then note that it's intake-port measures as 28.5mm, (not 26.5 -[Monza] or 29.5 -[Mach-I] ).


Duke-Cheers,
-Bob

[JimF]

I am feeling jinxed. Anyone following this thread knows that I had nice running setup of a Mikuni on a 250 narrow case. For a long time all was good except that at wide open throttle the engine would stutter. Initially I thought the stutter was from too a twist to wide open, but later realized that at the last quarter turn when running solely on the main I had a problem.


I bought different sized main jets above and below what I was running and determined that increasing the jet size made the stutter occur sooner and decreasing the main jet size made the stutter occur later until finally I could roll the throttle side open and suffer no ill effects. The bike pulled strongly through any degree of throttle opening up to and including wide open.

A week passed...

Then something strange started to happen - I was mid-throttle accelerating and I felt a stumble like I was running out of fuel. I instinctively reached for the petcocks to twist them to reserve when it occurred to me I had just filled the tank the day before. By this time I had de-accelerated and as I twisted the grip again the problem was gone. "Weird" is what I thought.

So I tested the throttle throughout the range and everything worked as it should. But in my mind I was worried I had not heard the last of the problem, and I was right.

Days went by and then it happened again. It felt exactly like I was running out of gas. Just as before I de-accelerated and just as before the problem was gone when I opened the throttle again.

"Could be fuel-related or it could be electrical..." is what I was thinking.


Yesterday however the problem came to stay. Opening the throttle 1/4 turn or more and engine just has no power. Up to 1/4 turn and everything is fine, the engine can get to about 7OOO rpm on that little amount of range albeit slowly.


So this seemingly says that something is wrong with the the main jet and or slide cut. However the combination of these items was fine at the onset and the ill-running condition came upon me slowly which is what is confusing me.


If this is an electrical issue I am thinking it would occur independent of throttle opening, which is to say independent of which carb jet my fuel is being delivered through.


The spark plugs looks OK to me, maybe a bit lean but I am thinking to change to a new plug just to see if that has an effect.

plug.jpg

I am thinking to pull the main jet out and clean it with compressed air.

I have also noticed very light-colored 'smoke' upon occasion emanating from the exhaust pipe while waiting at intersections. I don't know if that's related to this problem at all.



Jim

[Lonestar]

Jim,

Sounds like you're on the right track suspecting an obstructed main jet. A good cleaning and making sure a fuel filter is in place may fix it. Could also be that the needle escaped from its clip and dropped down, although that's a long shot.

Dave

[DewCatTea-Bob]

By: JimF...
" I am thinking to pull the main jet out and clean it with compressed air. "

____ As I read-through your post, I also came to suspect that your newest issue is as if your main-jet gets blocked with some small (m.jet-hole sized) particles.
It's certainly possible that you had purchased some contaminated gasoline (that previous day).
So I wonder what the ratio of new fuel is to the fuel you had been using before ? _ In other words,, when you recently last filled-up your tank, how much fuel was already in it, (half a tank, or on reserve,, or what, roughly) ?
Also,, did you already have installed, any fuel-line fuel-filters that provide finer-filtering (than the screens over the stock petcock-inlets) ?


" The spark plugs looks OK to me, maybe a bit lean but I am thinking to change to a new plug just to see if that has an effect. "

____ I've now added a brightened & zoomed-in view of Jim's presented spark-plug pic...
Now that we can better see it's tip-end, we can more confidently 'read' it.
__ I agree it's reading (quite unquestionably) indicates a "lean" fuel-mixture/burn-environment (overall/averagely), however I also suspect that that s.plug's heat-range is somewhat hotter than need be.
What is the number-designation of that particular spark-plug ?
__ If my brightened-up version of Jim's pic still really shows the actual color of his presented s.plug, then I'd suggest lifting-up the needled-jet's needle-valve to it's most raised (richest) clip-setting, (that's the lowest slot on the needle), so as to turn the plug-tip's reading towards a tanner color (and obtain a more normal fuel-mix ratio). _ (UPDATE-note... That's assuming of-course that the reading-condition of Jim's spark-plug was not produced solely during just the time when his m.jet had become plugged-up with the debris-particle he later found.}
__ Also, a cooler s.plug may provide a less tainted reading.
__ (However all my conjecture on the above is based on the assumption that the picture I've posted really does look just like Jim's s.plug appears in person.)


Hopeful-Cheers,
DCT-Bob

[graeme]

I would also suspect the bad lot of fuel.
Clean the carb jets and drain the tank and check for water etc

[JimF]

carb_stuff.JPG

It sure feels good when you diagnosis something from the symptoms and you're spot on.

This only happens to me once in ten years or so, so the bad news is for the next ten years all my diagnostic cconclusions will almost certainly be wrong.