Q: I built a 6-71 supercharged 355 c.i.d. Chevy. It ran fine for two summers until I took the car to the track to dial it in. On the fourth pass, the engine misfired through the blower. The blow-off valves on the supercharger intake manifold released like they were supposed to. When I got back to the pits, I found that the distributor shaft bushings had failed, which caused one of the points sets in the dual point distributor to close up, causing the backfire.

After replacing the bushings, points, and condenser, I reset the points and the blowoff valves. I also changed both power valves on the dual Holley carburetors. After doing all that, the engine would not run properly. The total timing before the misfire was 34 degrees; now the engine wanted more timing to run decently. I stripped the engine down, inspected all the internal parts, then reassembled it with a new block and crank, balanced the reciprocating assembly, and lapped the valves. The engine ran just the same—it wanted to run 42 degrees total timing before it would run smoothly.

I checked for vacuum leaks, but didn’t find any. The engine starts good and runs smoothly, but obviously detonation is a severe problem with that much timing. Should I do a leakdown test? Should I check out the supercharger for any problems?

 

A: We think your problem is electrical. When the distributor shaft bushings failed, they may have damaged the distributor body more than you thought. What we think you need to do is the dump the dual point and get an MSD electronic distributor and an MSD 6BTM ignition box. The distributor is extremely accurate, and the 6BTM has a Boost Timing Master control on it that will automatically adjust spark timing based on how much boost the supercharger is making. The 6BTM can adjust spark retard from one degree per pound of boost to three degrees per pound of boost, with a maximum retard of 15 degrees.

There is a possibility you have a problem with the harmonic damper. The outside ring may have slipped during the backfire, causing the timing marks to be off. You might want to check that out using a degree wheel with the engine at TDC on number one cylinder.

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