QuestionQUESTION: I recently prepared my 82 yamaha for running again, after sitting idle for the last 3 years. So of course I knew the carb would be gummy and I disassembled it and cleaned it (like I done maybe 5 times in the past). I think I need to trying disassembly and cleaning of the carb again because the engine hesitates at high rpms (around 6k). At low rpms the engine runs smooth and powerful. Is there anything I should look out for when cleaning it this time... or should I be looking elsewhere?
ANSWER: Alan,
First,
-try a new spark plug, check the wires
and plug caps, the caps should be under 5k ohms.
Some Yamahas have had problems here.
-have you checked the fuel flow from the tank,
the petcock or fuel tap may be partially plugged.
Make sure the tank vent or gas cap vent is
allowing pressure/vacuum out.
-If you do remove the carbs, check the
center main jets thoroughly and the emulsion
tube jets above them. These jets have many
small holes in them. Use a fishing line or
fine guitar string wire to make sure they
are clean.
-check your plug color for clues on the fuel
mixture. Black=rich, whitish=lean, tan=good.
Do this after a brief run at the speed it hesitates,
pull in your clutch and kill the engine quickly.
A new plug may be best for this test.
---------- FOLLOW-UP ----------
QUESTION: You gave me a lot of good tips...
Re-cleaned and checked the carbs... I'm confident they are clean and all jets / holes are clean.
New spark plugs and a brief run resulted in the left plug looking clean and tan... the right plug electrode looked tan but the base ring looked black and sooted, suggesting some right cylinder misfire. So I checked the plug wires and plug caps. The left plug cap measured 11.9k and the right plug cap was 8.5k. The left plug wire showed mild corrosion where it connected to the coil and the right was shiny clean.
I would have expected the left plug to show black soot because of the higher resistance of the cap and the mild corrosion of the plug wire, but it was the right instead? Does this make sense?
How bad is 11.9k and 8.5k for the caps?
Thanks!
AnswerAlan,
even though there was some corrosion
on the coils, it might not be the
reason the right cylinder looks sooty.
A loose plug can cause this.
An interesting thing to try might
be swapping the coils or the plug caps,
see if anything changes. See if the
plugs read the same or switch conditions.
-are the compression readings the same for both
cylinders or is one higher/lower?
-plug cap resistance varies a bit, it would
be nicer if they were closer to 5k ohms for
a stronger spark. How it runs is the real factor
regardless of the ohms.
-i always thought these bikes ran a little lean
even when they were new. It may pay off
to raise the carb needles a notch or maybe
even increase by one main jet size.
