First off, the basics. 1970 Chevrolet 250 in a C-10 pick up with the following mods.; .030 bore, factory style pistons and rods, Holley 390, Offy intake, Hedmen headers, duel Flowmasters, HEI ignition, 3 angle valve job and rv style cam. She has a Langdon's water heating plate on the intake. She also has a small Mr. Gasket air filter. Everything is brand new out of the box.
We are having a backfiring problem. It is Inconsistent at idle, no pattern at all. The backfire is in cylinders 4, 5 or 6. We suspect #4 due to slight carbonn build up on the plug. When we pulled the 390 off, we noticed the top gasket was wet with fuel between the carb and spacer. The exhaust from the pipe for 4, 5 and 6 is richer than the other. It happens regulerly at 1000 to 1500 rpm. We have noticed that the backfiring seems to start from a low rumble to a mild "pop" and then all the way to a loud "pop", then she starts the same cycle over again. She runs good under load above 1500 rpm.
We have tried a 1 inch spacer and a 1/2 spacer. We have gone from 8 to 16 degrees of timing and she is now set at 14. The compression is as follows; #1 is 135, #2 is 130, #3 is130, #4 is 140, #5 is 125 and #6 is 125. We have adjusted the idle mixture screws from the factory settings all of the way in until the engine dies and back out. We have adjusted the idle speed screw in and out. We have mounted the carb with the primaries toward the valve cover and away from the valve cover. The vaves have been adjusted twice. Everything seems to be working fine with the exception of the backfiring. It seems that no matter what we try, the problem stays the same.
Thanks,
Steve Hutson (volzfan59)