Pop in rebuilt 454
 

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

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

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 :cool:

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

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.

stay on top of
 

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.

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

Re: stay on top of
 

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

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

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

Al

If you do nothing else, do a spark plug reading after hearing the pop. I had a fuel pump volume problem earlier this year and it showed as surging at WOT. Would drop about 200 rpm and come back up, then drop again.

BT :cool:

Check out the compression...
 

Any time you have a problem you need to check the condition of the engine with a simple compression test. They all need to be the same and a warm engine should have at least 155 lbs in each cylinder. The valve springs should also be checked for tension or maybe one is broken.
Narrow it down to a cylinder by inspecting the plugs, it could be a vacuum leak between the intake manifold and heads or underneath the carburetor.
The solid lifter camshaft may have a valve (intake) adjusted a little too tight (when it is hot the valve is hanging open). There is no good reason to run solid lifters on a low rpm marine engine.
Fuel pressure needs to be at least 3 PSI at full throttle
(really, if there is any pressure at full throttle then you have more than the carb can use, right?).
Sincerely
Dennis Moore

Thanks Dennis & BT, good stuff. I'l be checking those plugs and the compression this weekend. Not sure why the engines had solid lifters but decided to replace them when the engines were rebuilt as they had two different cams (both solid lift ) in these engines. They (the engines) were a real mess once we got into them, crack heads, cracked crane, cracked rod, cracked intake manifold, a piston pin that had drifted into the cylinder wall etc., not sure how they were running as good as they did!

Anyway, I just can't get away from the fact that this exact pop existed prior to replacing everything inside this engine during the rebuild. Makes me keep thinking it's got to be something external. Well regardless, I'm trying everyone's suggestions at this point as none of my thoughts have worked yet. Thanks for the help and I'll let you guys know what I find. Other that this occasional pop, they run great!

Al

Ok, update on that poping 454. Swapped Distributors, that wasn't it, still pops ocasionally above 3500 rpm. Pulled the plugs (after pop and no idle) and this is what they look like. All of them pretty white except #8 which appears black with a little brown. Any thoughts based on this? Getting gauge to check fuel pressure to see whats happening there, will let you guys know.

Al

The plugs are lean. Fuel pressure check alone will not eliminate the fuel pump as the problem. You need a large volume of fuel at 3500 rpm and above. A pump can have pressure but lack volume. Buy a new one or swap with the other one to be sure.

Thanks for the resposne JT. Local dealer was closed before I could get to them so, swapped the fuel pumps and will try them in the morning. What do you think about that #8 being so dark? Valve adjustment?

Al

Thanks again Bill. Swapped fuel pumps last night, will try them this morning. Also cleaned plug and put it in another hole, will see what that shows. Changed plug wires last night as well, will check old ones as suggested to see what I find. Thanks again, this ones wearing me out. Being able to swap stuff is at least taking it easy on my wallet.

Al

FOUND IT! Well them! First, cracked #8 spark plug, second bad wire and third fuel pump, was pumping but apparently not enough volume. Anyway as each one of these were found and fixed, it got better till it was gone.

Thanks a bounce BT, Dennis Bill, Jersey Tom and everyone else that took the time to help out on this one. If you guys ever get to New Orleans look me up. Office 504-887-2800

Al

That's great Al. Now get out there and have some fun!

BT :cool: