I put together a 496 rat and installed it in a '75 Blazer. It only has two
problems and I've done everything I can think of to fix them. From my
observations off the balancer, it will run perfectly smooth and then it
advances a quarter to 3/8 inch, and retards the same distance, and then it
returns to a smooth idle. Also, my light gun is measuring it at around 70
degrees advance. I know it shouldn't be that high. In face, that's about
twice as much as it should be. I'm wondering, since the balancer is the only
thing that isn't new on the engine, besides the block itself, could the
balancer be worn out to the point that it's slipping on the rubber buffer
and that's what's responsible for the bad readings and the resulting
resonance is causing the rough idle?
The engine is a 10:1 with a 750 Edelbrock Performer carb, and performer
intake. There are no vaccuum leaks that I have detected, and the fuel
mixture is set correctly. I'm running ACDelco R45TS and have switched out to
R43TS but the problem remains. The fuel is 91 octane with a bottle of 104
octane booster mixed in the suggested ratio.
The only other thing I can think it might be is maybe I put too much oil in
the cylinders during construction and just need to get it out on the road to
burn it out.