alcor

I have twin 454's in an early 36' Sonic (1984). They were run-out so I had them rebuilt a few weeks ago. While they run great now, I still have the same "pop" (backfire) in one of them that I had prior to rebuild. It occurs above say 3500 rpm; a single pop that may occur now and then again in 5 minutes sometime sooner sometimes later but if it's running over 3500 rpm, it's going to pop. I've swapped carbs, coils, thundberbolt IV packs. I've had 3 sets of ignition wire, plugs and distributor caps on it. Heads were replaced (old ones cracked) with new valves, seats, guides, springs. Running solid lifts & cam. Still have the pop in that engine. Appears to be through the carb to me. Will try swapping the Distributor itself this weekend. Out of ideas, any help would be greatly appreciated at this point.

Al

alcor

No, haven't swapped the fuel pump. I can try that too. At this point, your guess is welcome as I'm fresh out of ideas after chasing it for a couple of weeks now. Thanks again.

Al

blue thunder

I agree with sutphen, sounds like a lean condition. Do a plug reading promptly after you get the pop. No idling before the check. If they are white, check the fuel system. What carbs are you running?

BT

alcor

Thanks for the response BT. Running Holley 750 dual pumpers, just had them rebuilt as well. What about potential bad fuel pump? Think that could be possible? I always want to believe it's either pumping or it's not. Any thoughts? Thanks again for the help.

Al

Jersey Tom

Fuel pumps can be bad and still pump. The volume they pump determines whether or not the motor gets enough fuel. The fact that it comes and goes after a few minutes and is always above 3500 rpm makes me believe that you are running out of fuel and leaning out the motor.

shu92870

Is it a metalic sounding pop? As if a metal popcorn kernel is in the engine. I had a similar sound in my 454. Ended up being a valve that was sticking and the piston was hitting the valve. It was happening anywhere from about 3800 on up. But mostly under hard acceleration. If so I suggest checking the whole valve train. I ended up losing the engine before getting it apart to fix. DOOHHHH. Anyway just my $.02. Hope its not that. Good luck.

alcor

Thanks for the information. Iswapped the distributors tonight adn will run it Saturday in teh lake. If it pops, we swap fuel pumps. Thansk again and if you think of any other possiblities, let me know. Your "$.02" could end up saving me big bucks and end a pain in my a_s. Thanks again.

Al

alcor

No. I believe its a backfire pop through the carb; can actually feel the heat from the arrestor when it pops so pretty sure it's a backfire through the carb. The heads were changed along with all the valves, seats, guides springs cams and lifters. Since its the same pop as prior to rebuild, I keep thinking its something external. Just relaized it was your "$.02". Well, everyone's "$.02" means alot right now as this one is eating my lunch. OSO people are the best. Thanks to everyone for their thoughts & suggestions on the subject. I know "we'll" find it and I'll let you guys know what it was.

Al

homan

Alcor, if the carbs are double pumpers they sell a dual bowl with a 1/8" pipe fitting that accepts a fuel pressure gauge, I found my problem thanks to the one I had my problem ended up being the pickup tube in the fuel tank had a crack in it and when the tank went low it sucked air then gas got swished around it took afew gulps of gas and sucked some more the gauge was going up and down, luckily I was on my way in. Y could also just hook an inline fuel pressure gauge also. Mine was a perm. setup

alcor

Excllent. I'll check the fuel pressure as well. That's easy enought. Thanks Homan for insight.

Al