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I have the same stain in mine. The intake and exhaust valves being open at the same time causes this. This exhaust gas stain is in a place that robs horsepower. I use a anti-reversion plate beneath my carb. and seems to help. The collector length has to be just right to time the returning pulse wave and sonic wave to keep from pushing the residual exhaust gas to the intake. I also need some tips on how to stop this.

Thats what my friend the cam man thought, He thought it was the overlap vs my short header causing the problem, we are cut on a 106 lsa & installed on 106 maybe more retard due to chain stretch The large load from the big tires dropped us down in rpms where we usually don't spend any time causing the stain