Hello folks,
I hope you can help me with a problem. I'm a fairly good mechanic, but am at my wits end and seek some advice. First, the truck:
1955 Chevy 1/2 ton NAPCO
Engine is a 1972 small block 400, with 80's HEI distributor and small Rochester Quadrajet, rest is stock including cam
The engine has about 22,000 miles on it, and was rebuilt in 1990. Yes, it does not get a lot of miles on it annually.
About a month ago, it developed a horrible pinging/detonation at anything above the slightest throttle. Here is what I have done so far, in this order:
- Checked timing (was set at 6deg BTDC), and timing would advance to about 34 deg BTDC at about 2200 rpm
- Checked the distributor, esp the mechanical advance. Looked OK, was not siezed, I cleaned and lubed it
- Dialed the timing back to 0 deg, no change in pinging so set it back to 6 deg
- Checked the vacuum adance, no vacuum leak
- Checked the power brake line, no vacuum leak
- Checked around intake manifold for vacuum leaks
- Replaced the distributor cap and rotor
- Took carb to local fuel systems shop, they rebuilt the carb and bench flow tested it
- Emptied the fuel tank bone dry, put fresh 91 octane in
None of the above has made any difference at all. Still pings like crazy, making the truck unuseable. I keep thinking there is something stupidly obvious that I'm missing, but I sure can't think of it. My next step is to replace the HEI and plug wires. Not sure how the HEI could suddenly "fail", but still fire the engine and cause pinging. Then again I don't have much experience with HEI other than this distributor. And maybe it's cross firing across the plug wires? They're 30 years old.
What do you think? Any ideas?
thanks,
John
