Inliners International Forums General Discussion Engines PLEASE HELP!!!

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)

Just to verify: it is backfiring through the carb and not out the exhaust?

Sorry, The backfiring is through the exhaust.

check the dist. cap and the rotor.you could be having a issue there?? As for the wet gasket take a mirrow and look into the carb for a out of ajustment float level. hope this helps}[oooooo]

Thanks for the replies Twisted6 and Bald Eagle. The dist., cap, rotor plus wires and plugs are new. I will pull the cap and check though. We thought that it may be dumping to much fuel into the carb. We'll check the float levels.

Steve

Twisted6, Can you give me the basics on checking the float levels with a mirror?

Thank you,
Steve

You could be running a little rich but you may also have an exhaust leak on the rear cylinders ingesting air to cause the popping.

back firing through the exhaust could be a cracked head with water getting in the in take port and going through the motor. had that happen in my vette. and a customers truck.

All your doing with the mirror is looking into the carb to see if fuel is leaking into it. That right off would tell you the floats are to high.And that they need ajusted. Or you can just take off the side screw On the carb.If the float is ajusted right The fuel should just barely trickel over the treads. Hope this helps }[oooooo]

Up-date. We checked the floats. While they did need some adjusting, the backfiring problem is still there. I'm begaining to think that bob308 is on to something. My question would be, with the compression well within specs., could I still have a cracked head?

Check for a vacuum leak, especially on rear port. Brian