Motorcycle Repair: 91 gsxr 1100 running terrible, suzuki gsxr 1100, vacuum hose

Question
hey i have a 91 suzuki gsxr 1100 kitted to 1250, it starts and drives but it backfires like crazy and when you roll back at all on the throttle it revs but you dont accellerate and it sounds weird and sputter/boggs but will rev high without taking off, carbs are good, but both spark plugs on the left were dark and a little wet and the right side were new looking white if anything, i think its a fuel problem like the petcock or something, its an aftermarket dual valve one but there are no vaccum lines but there are nipples under the tank one right behind the petcock and one at the back end towards the seat, and the petcock has a little one on the opposite side that you turn it to pri or res, i really need some help wayne, thanks

Answer
Hi Tucker,


Usually backfires are caused by lean fuel mixtures.
This can mean a lack of fuel or an excess of intake air.
Other causes can be loose exhaust header pipe or bad
pipe gaskets. Also having no backpressure from the
mufflers or having open mufflers.

The fuel petcock usually needs a vacuum hose
attached to open the fuel valve.
There is a diaphragm inside which blocks
the fuel until the vacuum opens it.
If the diaphragm is leaking or you have
no vacuum hose on it then the fuel valve will
not open and allow fuel to the carbs.

Check that all the cylinders are running and getting
hot by checking the exhaust pipe heat.
If one cylinder or more is not running well
it will be cooler and that could cause backfiring.

Check that the compression is similar for
all cylinders.

Check that all the carbs have about the same amount
of fuel in them. (fuel level)

If you get plenty of fuel to the carbs
then do a fast run with clean spark plugs.
Check the color and condition of the plugs
for clues on fuel mixture and running.

Check the carb intake connections for air leaks. (Use carb spray or wd40)
The engine will speed up if there is a leak.
Also check for any open vacuum hose fittings since
it sounds like the fuel valve vacuum hose may be off.

If all cylinders are running equally it may need
the carbs synchronized and maybe rejetting unless
it was running okay at some point before.

Richer mixtures can help to curb backfiring
but first make sure your spark is good,
plugs are good and your carbs are getting enough fuel.

Check the petcock by applying vacuum to the smallest
hose opening. You should be able to see the
vacuum diaphragm chamber on the valve and
see where the smaller vacuum hose should go.
The other end of the vacuum hose goes to
the intake pipe in front of one of the carbs.
When the engine starts the vacuum will open
the fuel valve.
Also check that all the carbs have fuel hoses
or fuel getting to them from the tank.

So to sum up, check the cylinder compression,
fuel supply, cylinder pipe heat, plugs and
fuel mixture by observing the plugs.
If you get all cylinders running, getting
them synchronized with vacuum sticks will smooth
things out.

If one cylinder is not heating up equally
check the spark, carb and compression there first.

Good luck!

Wayne S

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