QuestionBike starts and idles fine. Front plug is white (not tan). Rear plug is black and sooty. When going through the gears the bike pauses after each shift but then continues to rev fine. The bike would backfire when shifting down until I unhooked the polution gear. Checked plugs for spark and plug boots. They`re fine. Changed plugs. No improvement. Changed jets. No improvement. Had dealership check vacuum and they didn`t seem to know what is wrong.
AnswerHi Cameron,
If your bike has ever run well then
it should not have been rejetted as this
only tries to compensate for the real problems.
If you have changed the air filter or
exhaust pipes with something other
then the stock units then rejetting
might be helpful.
If your spark plug is reading white it can
be dangerous for the engine. This indicates
either a lean air/fuel mixture or that
the cylinder is overheating.
This condition can burn holes through the
piston top if it goes on too long.
The cause is usually a leaking intake
boot which allows too much air into the
cylinder along with the fuel.
The black or sooty plug means that cyinder
is getting too much fuel or too little air.
This could be jets too large or excess fuel
from some other cause like a petcock valve
leak into a vacuum hose.
Also just a plugged air filter.
Low engine compression can also result
in unburned fuel.
Both cylinders should have similar compression
to run well. Within ten per cent of each other is ok.
I would start with a careful compression check.
Work on each cylinder and carb as a separate
unit to get the fuel mixture correct.
Read your plug color after a quick run
and adjust as necessary.
Make sure there are no air leaks around the
carbs between the carbs and the engine.
Check for leaks with starting fluid or carb cleaner spray.
Check for any vacuum hose air leaks.
If you cannot get any response from carb and air tuning then
check the charging system is okay as
the ignition may be lacking sufficient battery power.
Good luck!
Wayne S.
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