Lean rear cylinder

Ernest Montague afm199 at earthlink.net
Thu Oct 1 10:04:27 PDT 2009

My rear cylinder is running super lean for some reason.  The header 
interior is clean and light grey/white with no carbon. I am running 
pump gas, which always leaves my headers a bit black.  But the rear is 
very light colored, the front the normal blackish.  It's not a head 
gasket leaking water ( at least last time water got in the combustion 
chamber via head gasket it steam cleaned everything. The chamber just 
looks lean, valves very clean and grey/tan.  I looked for missing 
vacumn plug, bad slide rubber, plugged main jet, etc.  The mains front 
and rear are 170s a range that has worked well in the past with the M4 
system and BMC big mouth race filter (factory pro design).    Even if 
it was the main I can't believe it would be so different from front to 

Also backfires and pops at cruising  2500-4000 rpm.

Bike is mostly ridden at full throttle on the track.



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