info from the website on the weight difference with the muffler. Then again, you have one of these and I don;t. Just passing along what I found by checking ti out. Could be bad info, but this is what I got from the site.

Irvine 18.2 oz. W/M, 1.7 BHP, 8.59 cc
46 FX 16.45 oz. W/M, 1.62 BHP, 7.45 cc