I drilled two 1/2" holes across from each other and the sound is much improved over stock.  Do larger holes produce a lower tone?

The backfiring on deceleration seems to be worse now, but it may be that it's just louder.

I ordered a Sigma6 jet kit to see if it helps.

Got two appx. 1/2 inch holes in mine courtesy of a previous owner. It's still not very loud. In fact I'd had it for a while before I even noticed that it had been altered. I've never heard an undrilled one run though. I don't really get the "louder is better" deal myself, but to each his own.

PhantomII wrote on 07/10/13 at 06:30:54:


You may need to rejet.....but backfring is not necessarily the measure of proper jetting.  The big single doesn't get enough air fuel when the throttle is completely closed even with proper jetting, and the lean mixture does not ignite in the cyclinder and builds up the muiffler and then ignites.  Try keeping the throttle opened just a tiny bit as you shift gears or decelerate....it will help to keep the bike from backfiring.  You don't need to have the throttle opened far....just enough to allow some extra fuel and air into the cylinder.

Well I was very mindful of trying to keep the throttle slightly open while decelerating, but the back fires continue.  I'm pretty sure the jetting on my bike is lean because i already had to turn the low speed screw out over 8 turns just to get it running acceptable before drilling holes on te muffler.  I ordered a jet kit today hopefully it will arrive in a couple of days.