'72 402 backfire through carb - at wits end..

[LujMK]

Hi...many years ago, as a gift, I bought my father a 72 GMC with a rebuilt 402 and carter (now edelbrock) 1407 carb. He's been tooling around in it over the years and recently it made a popping noise (per him) and he parked it. I returned from overseas and together we noticed low compression and oil - bad head gasket. We changed both head gaskets (heads were nice!), and after noticing fuel leaking from the carb linkage (manual choke area), father rebuilt the carb. We put everything back on, used the faqs on this site to get TDC, installed the HEI carb accordingly, put the fluids in and attempted to start the truck. What we got was a backfire/flame through the carb a couple times - it would try to start as we'd advance/retard the dizzy but still just skipped and burped out the carb. We're both at a loss on this one. New plugs, new wires, valves (comp cams rollers) were spaced (well - as rollers are) according to the faqs here and we both swear we had the right TDC .. but .. any thoughts? I've got to deploy again in 10 days and hate to have the old truck sitting for another 12 months.

Cheers.

[68 Stepside]

Pull the distributor up out of the cam gear, rotate the rotor 180 degrees from where it's currently at, stick it back in and try that, just be sure you're fully seated into the oil pump drive gear when you drop it back down. I've installed the distributor 180 degrees out numerous times.

[68 Stepside]

Also, if that doesn't work for you, find TDC again and give it another shot.

[leddzepp]

Are the plug wires in the correct firing order?

[GASWAGON]

Check the stuff the others have said first. If everything is correct, pull a valve cover and check your valve adjustment. If the valves are to tight you can experience what your describing.

[LujMK]

Thanks...we've just ran to TDC again, dropped in dizzy and will give it another shot. Jeepers I never thought this would be such a bugger

[cdowns]

when you did tdc did you make sure you r on the compression stroke??finger over the sparkplug hole you feel the air blowing out

[au03z71]

Sounds like you're 180 degrees out. It happens to everybody at least once. Do as 68stepside says and it should take care of it.

[stevencvn72]

Stuck intake valve (open)...maybe, posibly? Let us know what fixes it.

[Bpokey]

I have heard that if you over adjust the rocker arms it will let the valves stay open just a little bit and cause a backfire through the carb.

[Bpokey]

I just noticed that GASWAGON beat me to the valve adjustment solution.

[LujMK]

Re-did TDC and got her to start up and idle nicely. Went to adjust the valves and there is significant noise from the header gasket...looks as though the header is not being completely sealed by the gasket causing blowby of exhaust - and a nice loping sound (not metal to metal). The sound is too loud to adjust the valves so going to find a good gasket to put there. The valves are the roller/hydraulic setup and we've no clue the cam in the engine (bought rebuilt with no specs), so best we can do is go off of sound -- or any other thoughts? Thanks so much!

[JRBECK64]

LujMK,
I bought a 50% Crush Gasket from a local High Performance Auto Parts house and I cant remember the Name brand but since I put them on 6 Months ago I have had no problems at all with leaks. Im sure if you ask your local parts guy about them they will know what they are. Good Luck and Congrats on getting it started.
JRBECK64

[beebster]

I had a similar problem and it was 2 plug wires crossed.
but in this case "rockers overtightened" sounds about right.