Q: I have a Chevy 350, bored .060-inch over. It has TRW forged flat top pistons, 230-degree duration (at .050-inch lift)/.560-inch lift COMP Cams roller cam run straight up, and iron heads with 64cc chambers, 2.02/1.60-inch valves, and ported intake runners. The intake is an Edelbrock Torker II with a Performer 750 cfm vacuum secondary carburetor. Fuel delivery is via a high-volume mechanical pump with regulator. I’m also running a Mallory Unilite distributor, 1 7/8-inch primary tube headers with a full 2 1/2-inch dual exhaust, and a 10-bolt rear-end with 3.73 gears.

The problem is the motor builds power to about 4,000 rpm, then starts dropping off. I know the car runs too rich because I have to turn the idle screws all the way in to get the highest vacuum (11 in./Hg), but the plugs are always fouled. I have to advance the timing quite a bit to get the motor to run. It seems to be around 26-30 degrees, but the engine will not run with the mark anywhere near the timing tab. When I bought the heads, I was told that I would have around 10.25:1 compression with my pistons. Still, the car runs on 92 octane pump gas with no spark knock that I can hear.

I didn’t index my roller cam. But if the cam is retarded for some reason, why am I losing power at about 4,000 rpm? If it is advanced, why is the carburetor running so rich?

A: The first problem we see is those 1 7/8-inch primary tube headers. That’s a big piece of pipe for your combination; a 1 5/8-inch primary tube is much better suited to your engine. As for your carburetor running too rich, it sounds like you need to change the step-up springs and metering rods. You will have to do some experimenting to find the right combination. You should also check to see if your fuel line is kinked, or if it is large enough to adequately feed your 350. Fuel starvation could be part of the reason you’re losing power above 4,000 rpm.

As for your timing, did you replace the stock harmonic damper with an aftermarket damper? If so, check to see if the timing mark is in the correct location for your timing tab.

Judging from your combination (64cc heads and flat top pistons), you aren’t running 10.25:1 compression. The compression is probably closer to 9.0 or 9.5:1, which is why the engine runs fine on 92 octane gas.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

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