78 XS stumble and die

[xssiveone]

A friend of mine has a '78 XS, which happens to be a European import if that makes and difference.

The engine dies at 1500rpm if he gently raises the rpm to that point. If he aggressively raises the rpm to that point and past, it is fine. Any ideas? We're pretty sure it is electrical as it is just like shutting the key off when it dies. And, it will start right back up.

Things he has done:
Carb clean (several times)
New carb boots
Tank boil
Petcock rebuild (both)
Coil wire replacement
Swap in a known good TCI box
Clean and lube vacuum advance
Check pickup wires from pickup to connector
Replace stock exhaust with Mac 4-2

One thing he has noticed is that the both the mains and pilot jets are smaller then North American stock size. Can't remember if he said they were one or two sizes smaller. We think this must be because of the European origin. Any other thoughts on this? He is waiting on larger pilots and mains to see if that will make any difference.

[Mike Hart]

Will it do the same thing if he just turns the idle adjust screw up slowly?

What is aggressive? 1/4 throttle? 1/2? Full?

If idling at 1100rpm, and you quickly blip the throttle (like turn it 1/4 - 1/3 turn and let it snap back right away, does it accept that throttle input readily without bogging or hesitation, zip up to say 2500rpm, then come right back down to idle without hanging at all?

I'm thinking, gently means verrrrry slowly slightly turn the throttle - which leaves the carbs running on only the idle circuit, no load to raise the diaphragms at all, no vacuum to speak of to pull on the pickup coils.

No vacuum leaks anywhere?

Doesn't sound like electrical to me.

[skids]

It doesn't sound like an electrical problem to me either. There might be a "hole" between the pilot jet-only range and the lifting of the diaphragms because of the smaller-than-normal pilot jets. Try some stock micunis. 42.5

Originally posted by Mike Hart


Doesn't sound like electrical to me.

[xssiveone]

Just a further note on this... Mike and Sid, you were both on the money here. Thanks for pointing us in the right direction. Not an electrical problem at all. He finally got the corect jets installed, both pilots and mains and no more stumble and die at 1500rpm.

One thing doesn't make sense tho, the needle was moved up one notch, which, if I understand how this works should cause it to run richer? After he installed the stock jets, he dropped the needle down (c-clip back in the middle notch) and it started spewing black smoke. He put it back where it was and it's running good now.

[reddog]

stumble & die

Hi, first time poster but long time lurker.
I have a question & suggestion for you ,
your problem sounds fuel related.
Particularly in the pilot circuit, when you snap the throttle open your bypassing the pilot and going right to the midrange,
thats why the bike seems to run ok.
when you clean the carbs the hardest
part other than that annoying fuel level,
is being absolutely sure that ALL the jets are clear. Given the nature of your problem I would look in the float bowl,
not at the floats but the bowl itself,when you invert carbs & remove the bowls you
will find on the edge of the carb body a small brass tube extending upwards,
this is your pilot p/u tube and it goes not into the bowl but into a small chamber drilled into the side of it, and there is a VERY small jet connecting the pilot chamber to the float bowl proper. If this jet gets clogged it can cause the
symptoms you are experiencing, when I cleaned my carbs it took the smallest jet drill (we're talkin hair fine) I had.
It's located at the very bottom of the
bowl angling back, this little jet is the first step in your pilot circuit. But before you go pulling your carbs off check an obvious first and make sure your mixture screws are needle sharp & not broken off in the jet. These screws are very soft brass and even the best of us make mistakes.
Hope this helps and sorry for being so wordy, but I dipped, cleaned and set my carbs twice before I found the problem. Hey I guess this makes me a "newbie"