1937 UL running rough

[ldfly112]

I am still struggling with a engine miss or a hesatation at aprox, mid throttle. I have been adjusting the carb. for a couple months and switched carb. It will go away and come back when i make no changes . Is it possible that a exhaust leak at the cylinder could cause this issue. Thanks if you have any ideas

[kitabel]

What carburetor?
What adjustments did you make?

[T. Cotten]

A Linkert, Kitabel!

https://forum.antiquemotorcycle.org/...ine-hesatation

I wouldn't say an exhaust leak wouldn't do it, Idfly, but I doubt it.


....Cotten

[kitabel]

You'd be surprised at how many answers are:
1. "a Mikuni"
2. "I don't know, it was on the bike when I got it"
3. since the correct answer is M-51, and the answer given is M-75...

[ldfly112]

My carb. is a M51 it is actually the third M51 i have tried. starts starts as perfect as i can expect 2 prime kicks and starts the next kick runs a little rough until warmed up then idles fine. I rode 35 miles to a local swap meet lat Sunday ran fairly well i fired it up started for home and started missing running rough came out of it partially but still a bad hesitation rest of the way home . The carb. is adjusted per the book low speed jet turned in until runs rough then out 7 clicks and it sounds good . the high speed jet is out aprox. 1 1/4 turns when i adjust the high speed jet at 2500 to 3000 rpm doesnt seem to make much difference bike has 1200 miles on fresh rebuild. any suggestions would be very much appreciated forgot to mention i have installed the ebeyond 2000 ignition an a Dyna 3 ohm coil 12 volt cycle electric generator timing is set advanced and the mark is in the center of the hole

[dukekleman]

The book suggestions are just that, ROUGH starting points. Don't be surprised if your final tuned settings change far from those.
WIth that said, Do all the carburetors you've tried, all have the same results? If so, move on from the carburetor.

Are the fuel tanks clean of any contamination? No rust, no paint particles , no bead blasting media, no water, etc???

It is also possible you have a heat related electrical issue.

Lastly are you aware of the intake valve guides condition?

[ldfly112]

Yes all three carberators were the same. as far as the fuel i have taken off the fuel line several times and no problem with fuel flow. as far as the heat related issue very possible the coil could be getting hot it is mounted above the engine to the top motor mount with at least 2 inches clearence from the top of cylinder heads almost the same clearence as the stock location. The valve guides were new repop aprox. 1200 miles ago. the valve guides are probably V twin that came with cylinders and pistons. what would you recommend if i end up replacing them thanks for the suggestions

[kitabel]

i adjust the high speed jet at 2500 to 3000 rpm

Was this in neutral, or in gear at speed?

[ldfly112]

just got a chance to ride it again adjusting the high speed jet in gear riding third gear about 40 mph seems like 2500 to 3000 rpm I have found that it runs best with high speed jet closed still missed a bit but that is definately the best spot what would cause that

[T. Cotten]

All fuel is metered by the HS, Idfly!

If it is shut, it shouldn't run; All three carbs run with the HS shut?

Please make sure fixed jets didn't sneak in somehow. Or set screws instead of proper gallery plugs.

The contact of the HS needle to its seat can be inspected with a dry-erase marker, Prussian Blue, etc.

INKPOINT.jpg

There are other ways fuel can get to the nozzle, of course, but they are usually unique:

WORMY.jpg

It is hard to imagine all three of your M-51s could have such rare porosities.

....Cotten

[Shovelhead68]

It would not hurt to change the condenser and check your ground. I went to a shop that restores Mustangs figure if someone puts 40,000$ into a car they should have a decent condenser most are crap these days.

[T. Cotten]

Had an expensive modern kit bike roll up out back begging for charity, many years back of course, Folks,...

It was a quick diagnosis when it was arcing at the ground strap!

I don't believe I have observed such a thing anywhere else, even on my autos.

It was one of the few times it was an easy fix, just when I wanted to keep it out of the shop any way I could...

...Cotten