Motorcycle Repair: carbs, vacuum hoses, air leaks

Question
QUESTION: We just clean the carbs on my 1982 yamaha 750 maxim and gave it a simple tune job. when we rev the engine to 4000 or 5000 Rpm it comes down to idle very slowly and sometimes not at all. Do I need to clean the carbs again or is it something else?

ANSWER: Steven,

-see if the throttle slides are free
in the carbs. You can probably reach
in the back of the carbs (engine off)
and lift the slides, see if they are
all moving okay up and down.

-Check for any air leaks around the carb rubbers or gaskets,
vacuum hoses or missing rubber intake manifold caps.
-Any air leaks will cause a lean fuel/air
condition and cause the engine to rev.

-Check that throttle cable has some free play.

-Check the choke operation.

-if you work on the carbs, don't mix up
any parts. Sometimes carbs have different
jets on different cylinders.

-All carbs should be synched to carb 3,
it is the third from the left when
sitting on the bike.(1234)

-the carbs  should be set
using a vacuum carb sync tool.
This makes it so all carbs are set evenly
for a smooth idle.

-Also there is a special tool used
to disable the YICS system if it has it.

-Check that the main idle screw is not turned in too far.

Check out this link:
http://members.tripod.com/dave_jack/id38.htm











---------- FOLLOW-UP ----------

QUESTION: I have my xj750 maxim running, however when I let off the gas and when the rpms are high it backfires quite a bit. Do you have any idea why this would happen and what I can do to fix it? Thanks

         Steven


Answer
Steven,

if the backfire is happening in the exhaust
system it might just be a loose pipe
or bad exhaust header gasket. If the exhaust
pipes leak, the engine sucks cool air in
and backfires. Also too little backpressure
can cause this, like if the baffles are
removed or pipes modified.

Also,the fuel mixture may be too lean.
This can be improved by increasing the main
jet size slightly or raising the carb slide
needles very slightly.
The main jet is about #120 standard.
Also check for any vacuum air leaks between carbs and
engine.

Check your spark plug color, ideally
should be light brown color if fuel
mixture is correct.
http://verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm