i just modified my 07 screamin eagle slip ons. drilled the existing holes out a bit bigger and removed the insulation,and packing in the slip ons.....i was wondering if this happens to lean the mixture of the air/fuel how can i tell through inspection of the spark plug,if the engine is running to hot or if i'm possibly damaging the exhaust vavles? any tell tale signs????

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Less back pressure the leaner the mixture will get.....I posted a spark plug thread here that explains how to read and what to read on the plugs, try a search see what you come up with. I;ll look for it too.

 

this sure is a hard topic to find information on.........

 

This might help.
http://www.dragstuff.com/techarticle...ark-plugs.html