I recently bought a basket case 1990 Force 90, and rebuilt the power head myself (my first time, but I think I got it right). Cylinders now have good compression. It starts and idles, but it does not do well when I start getting up in RPM. Smokes profusely and sputters. I have played with the timing and linkage but nothing seems to work. It is winter now, so I can't put it under load. I have adjusted full open timing with a light, as described in the manual, but still no good. Could I have a crankcase seal leak (I did not change any seals when I did the pistons)? 