Idle and smoking.... 1982 Yamaha Seca 750

 Yep, me again! Trying to get this motorcycle to work... After
adjusting the float bowls, now we are having an issue that my man is
mostly concerned with the white smoke it is blowing. (For those who
haven't seen my posts before, this bike hasn't been working in 10
years... and we are determined to make it work!) Thus far, rebuilt the
carbs, adjust float bowls, changed oil, changed spark plugs. It is
running, but won't have a sitting idle. Needs to feather gas to keep
it running... but is constantly blowing smoke...
Please reply asap, as I'm sitting in the garage trying to help him
figure this out!! Thank you ahead of time!

 


Good post.
I probably would have waited until tomorrow to answer but since you said
"Please answer asap.........." , here goes:  ;-)

You likely have about 0 compression because the rings are stuck and oil is
pumping past the rings and being burnt.  It's likely bluish smoke but the
light has to be just right to tell the difference.

A complete top-end rebuild would, no doubt fix the problem.....rings and
valves.

 OR .....take a long shot and get some MMO (Marvel Mystery Oil).  A quart
will probably be more than enough.  Run it long enough to get the heads hot
(or very warm).  Shut off.  Pull the plugs.  Put about 2 ounces of MMO in
each cylinder.  Put plugs back in about 1/2 loose.  Turn it over ONE
revolution (approx.).  Then don't touch it for 24 hours.

Turn over one rev. again.  Remove and clean plugs.  Put a rag over (NOT IN)
the plug holes and turn it over vigorously to blow out excess MMO (but not
likely there will be much left).  Install plugs and see how it runs.

If no improvement, it's probably tear down time.......after a compression
test to confirm a ring problem.

If it's better but not good yet, repeat the MMO treatment (repeating it past
twice is probably a waste of time though).

If you get lucky and it's REALLY running better, warm it up good and change
the oil. MMO is NOT good for wet clutches and a good amount will, no doubt,
make it's way into the oil.

If the MMO treatment works, you might want to add a little to the gas for a
tankful or two to finish the process.  DO NOT PUT MMO IN THE CRANKCASE OIL.

Good luck!

 
Besides stuck piston rings, it's possible that the valve guide oil
seals are leaking a bit of oil down the valver stems. Smoking caused
by valve guide oil seals leaking will stop after the engine warms up.

 
It seems that it doesn't blow smoke when it's just sitting running,
only when he rev's it up...

 

Look for a lot of oil inside the airbox or an oil-fouled air filter.
If the piston rings are stuck, the crankcase breather will blow oil to
the airbox.