Make sure you have a carb that is rich enough.  I used a new '80 replacement Quad on a ZZ3 crate that ran great in my '80.  However, I started getting blowback from the crankcase at around 50,000 miles, and finally rebuilt the engine at 78,000 miles.  The 80 carb was too lean, and I found cracked ring lands and other signs of spark knock.  I called Sean Murphy Induction in Long Beach, CA (SMI Induction), sent Sean the details of the engine - heads, cam, C.R., etc.,  and he sent me a super Quad for around $350 that really gets it done.  With AFR 180cc runner heads and KB pistons to give 9.5:1 CR, I have around 400 hp and 400+ ftlb torque from 2000 rpm to redline,  no sign of sparkknock.  The SMI carb is modified to allow adjustment of the low end mixture also.