Motorcycle Repair: 1978 GS400X Starting, clymer manual, float bowls

Question
I purchased a 1978 GS400X.  The bike has no electric start.  When I first purchased it fuel would pour out of the carb overflows.  I pulled the tank, rebuilt the petcock, pulled and fully cleaned the carbs and reassembled.  I no longer have the fuel leak.  The bike will fire but then bog down.  I set the air pilot screws to factory (1.25) turns from closed.  I cannot seem to find what the factory setting would have been for the throttle adjust which I think may be the problem.  I did pull and re-gap the new plugs when the Clymer manual came which seemed to help it fire up.  I think I am close to getting it to fire and idle.  Any ideas you could provide about the throttle adjust?

Answer
Hi John,

Always check the compression before tuning any bike.
It could need valve clearance checked or
need engine service.

The float level has to be correct and
check if your carbs are filling with fuel.

If the fuel level is high enough in the float bowls
and the carbs are very clean then it should fire
if the spark is strong on each cylinder.

You have to determine if you are getting
fuel into the cylinders. Are the spark plugs
wet or dry?

Are the carb slides lifting without any
restrictions?

Is the center screw between the carbs set
so both throttles open and close at
the same time?

Are the throttled closing most of the way?
Try turning your idle speed down until
you get it running and then set it so
it idles. If the throttle is opened
when starting it defeats the function
of the choke or enrichening system.

Is the choke lever allowing the choke
plunger to lift and close properly.

Adjust the carb idle speed and mixture
screws after you have it running and warmed up.
They only affect the idle so it should still
run if you get it going.

If it won't run then you have to check for
fuel in the carbs, good spark,
good compression and you may have to
physically clean the center carb emulsion
tube holes and main jets.

The carb slides also have rubber diaphragms
that must be good.

The carb float needle seats on these
bikes sometimes have some small screens
under them that can plug easily.
I would check those if you didn't when
the carbs were off.

If it fires but stops then try some carb cleaner
spray or a bit of starting fluid
in the carb mouths to see if it runs on that.

If it will run with that then your fuel
system needs checking.

If it is flooding then check the float levels
and float needles/seats.

Sometimes all it takes is a drop of moisture
in the fuel to stop up the main jets.
Try draining the carbs and try again with
fresh fuel.

With some tweaking it should run as long as the
engine is in reasonable condition.

Good luck!
Wayne S.
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