gas coming out of back of carbs as I try to start my Yamaha FZ600

 Hello, looking for some advice. after cleaning my carbs and adjusting
floats on my 87 Yamaha FZ600, I have some gas leaking out of the back
of the carbs when I am attempting to start it. I wanted to know if a
small amount of gas is normal and pulled back into the engine when it
starts. I had been trying to start it without the air filters on it
because I was going to sync the carbs. I have had the carbs apart 5
times re-cleaning and checking my adjustments. Any help please?

 On Sun, 15 Jun 2008 11:44:46 -0700 (PDT), fzbuilder


I'm not familiar with your bike and am not a carb guru, but it might
be time for teardown number 6.  Unless you have a fuel line leak or
perhaps a loose fitting, fuel dripping out of the carb usually means
the float needle valve is leaking.

The fuel goes into the carb, passing through the needle valve as it
fills the float bowl.  The float in the float bowl rises as the fuel
level in bowl increases.  A metal tab on the float presses on the
needle valve needle as the float rises, shutting off the fuel flow
when the fuel level in the bowl reaches the specified level.  You
probably measured the float setting when you were in the carbs to
ensure it was as specified.

To my knowledge, a properly functioning carb won't have fuel running
out of it, even when the fuel enricher is engaged.

Hope this helps,
Robert

 One of three things:
Bad needle valve seal
leaky float
float level still not set properly

To test #2, drop the floats in a cup of gasoline and see what happens.

To test #3, you can deliberately set the floats lower than you think
they should be.

The most likely cause, however, is #1.  You can clean all day long, but
if the 21 year old orings are shot (and they probably are), readjusting
all day long won't help a bit.

 
Only for old piston port 2-strokes with wild intake timing ;-)

In the carburetor department, I would have to say it's a problem with
the float valve sticking, trash on the float valve seat, sticky
floats, leaky floats that sink,
or a misadjustment of the float level.

The float valve height specification is measured from the gasket
surface on the carburetor body to the bottom of the floats (which is
on top with the carb upside down on the bench.

Some amateur tuners misread the "gasket surface" as being the surface
of the gasket and set their floats measuring from the gasket. The 1 mm
thick gasket would tend to make the floats shut off the gas flow too
early though.

If your carbs aren't screwing up, the only other thing I can think of
is that your intake cams are timed right and the intake valves are
open too early at the end of the exhaust stroke.

If the timing chain is off by one tooth on the cam sprocket, that's
about about 12 degree too far advanced.

 Only for old piston port 2-strokes with wild intake timing ;-)

Ahhh ... them were the days ....