QuestionI drained the old gas from the system, replaced the gas filter, new plugs and cleaned the carbs. The problem still exist, rear cylinder runs like a clock but front has trouble firing. Front cylinder is only luke warm. On D Cell front cylinder backfires bad and the idle drops down slower than usual. I checked for air leaks with gumout spray while running, found nothing in idle change. Checked front plug not fouled but brown in color not tan. I was told it could be a air and fuel mixture problem but don't know how to adjust that. I have contacted you in August about this problem and cleaned everything but up against a wall on this front cylinder Thank You
AnswerHi Scott,
(Just wanted to mention that you can sometimes
swap the ignition coils to see if the problem moves
to the other cylinder. This will only work
if the problem is spark related and the coils have
a problem)
There are a few things that can cause a deceleration backfire.
The most common is a lean idle or pilot circuit in the carb.
The front cylinder gets more air and more cooling so sometimes
it needs more fuel than the rear cylinder.
The second is leaking intake valves or leaking exhaust header pipe.
In rare cases the camshafts can have flattened lobes.
Usually a compression test can help to pinpoint a leaking valve.
In other cases a leak down test has to be performed to see if the
engine is losing compression somewhere like a valve or head gasket.
If your cylinder pressures are near equal then you can start
tuning the idle circuit on the carb.
If the bike is running on all cylinders at mid throttle but not at idle
then it indicates either mechanical problems or the idle circuits
in the carb are too lean, that is too little fuel or too much air.
The idle mixture screw is up under the front side of the carbs.
Originally these adjustment screws are covered with a plug
to prevent tampering. The plug can be drilled and pryed out
carefully so as not to damage the adjuster screw head underneath.
The screw can be turned outwards or anti-clockwise to richen the
idle fuel mixture.
Turn it to where the engine runs the smoothest.
If you get no response to adjusting this screw at all then
it means the idle circuits in the carb are probably still
plugged up. In this case the carb will need complete disassembly
and soaking in some Yamaha carb cleaner overnight.
So, verify your engines mechanical condition with compression tests
or better yet a leak down test.
Check for intake and exhaust pipe air leaks.
Check valve clearance.
Adjust the idle or pilot screw to verify a response.
No response, clean or replace the carb.
Check for a damaged o-ring on the main jet holder.
If these are hard or damaged you may get fuel
leaking past the main jet holder at idle which
will foul the spark plug.
When adjusting the idle mixture screws keep the
idle speed low otherwise the needle jet circuits will
take over. If the engine starts to speed up then
lower the idle speed as required.
Make sure your plug, plug wires and caps are good.
Hope something turns up for you, the carbs idle
circuits are very fussy about being super clean.
Fuel tends to gum up quickly these days in less
then a month sometimes.
Fuel stabilizers can help or additives like Seafoam etc.
If your compression is not ideal it may be hard to tune
but if the idle mixture screws are reponsive you may
be able to adjust it to idle steady with some fine tuning.
If you seem to be getting nowhere with the carburetion
then the pulse or pickup ignition coil may be
failing. These sometimes cause more problems when hot.
If your spark looks strong at all times then I would continue to
examine and tune the carbs.
The battery needs to be strong and your charging system should
put out about 14 or more volts at a fast idle to support
the ignition unit. Low battery voltage can cause missing too.
Wayne S
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