Motorcycle Forum / General / Yamaha / January 2007

I've been told you need to. By more than one place. If you change the
back pressure/exhaust flow... or the intake flow, you affect the
dynamics of the carburation. Hence rejetting is necessary.

I have my pipes sitting in box here, and getting a hypercharger next
payday, which comes with a jet kit. Getting it all done at once.

Dave

Loud bangs (backfire) on trailing throttle probably means that your
pilot circuit is running too rich.  OTOH if you are getting a series of
small putt-putt type backfires on trailing throttle accompanied by some
minor surging (stumble) at slow cruising speeds you're running too lean.

Typically, going to a K&N filter and less restrictive exhaust will
encourage a lean condition, but one that is more pronounced at increased
throttle.  The fact that you are running fine except at closed throttle
suggests that those are not the problem.

You (or someone else) may have mis-adjusted the pilot needle screws or
the pilot circuit may be partially blocked.  Usually a blockage in the
pilot circuit results in a "too lean" condition, not too rich.  BTW, due
to the small size of the passages, and the sensitivity to blockage, the
pilot circuit is one of the first areas in the carb to exhibit problems
when gummed up due to old gas.

The best way to adjust the pilot screws (after a thorough carb cleaning)
is using an exhaust gas analyzer (at idle).  If you can't find someone
who has one, or want to fix this at home on your own, you can use a
Gunson Colortune.  Google it.  I've used one of those for adjusting 4
cylinder carburetted engine quite successfully.

Ummmm...  you just said you are getting backfiring.  Don't you consider
that a problem?  Or are you just as happy to emulate the big American V
twins, many of which seem to suffer this same problem.