Hey, I've recently purchased a 1955 F100 that had been sitting for three years. Before it sat the previous owner had just installed a brand new Edelbrock intake and carb. The engine is a 390 that had a fairly stock rebuid several years earlier it also has an FMX auto transmission. The problem is that the engine will backfire out of the carb once right away if I accelerate hard. If I don't back off the pedal it will continue to pop. It does this whether the engine is cold or warmed up. If I back off the throttle and accelerate gently it goes fine- it idles well and runs great otherwise. Since I bought it I've installed a new gas tank, replaced the fuel lines, fuel filter twice, fuel pump, plugs, points, cap, rotor, wires, distributor vacuum advance unit, re-set timing to spec., adjusted idle mixture, etc. It has 17-18 " of vacuum at idle. I'm suspicious of the carb. as I feel like I've ruled out a lot of the other possibilities. I haven't really taken apart an Edelbrock carb before but was reading about the "step-up springs" and wondered if they could be a possibility- otherwise do these carbs have an internal fuel filter? Any one have any other ideas. Jay
Could be a vacuum leak around the intake gaskets or carb base and since the previous owner just installed them I'd be suspicious. On a 390 most of the intake gaskets are under the valve covers. To check for leaks under the carb spray some carb cleaner around the base gasket with it running. If you spray a spot with a vacuum leak the idle speed will jump until you quit spraying the leak. This works for the intake gaskets even better but on that FE you'd have to run it without the valve covers on. Make sure the accelerator pump nozzles are both squirting the same. Double check the manifold for missing plugs or vacuum tree ports that are missing or leaking. I have seen some very warped "out of the box" new aluminum manifolds. Always go over new aftermarket parts with a fine tooth comb before you install them even if it is from a reputable manufacturer. If you can't find a vacuum leak try borrowing a known good carb and stick it on and try it. Hope this helps. Brad
FE motors are bad about bending push rods, sometimes stuck valves on an engine that has sat for a while.
45 Shooter is right. If it keeps popping unless you let up it is either an intake valve is stuck open or a burnt intake valve. You can find which cylinder it is by doing a compression check or pull the plug wires one at a time and gas it. When you pull the wire on the bad cyl it won't pop back through the carb. Also with a cold motor, you can start it up and let it idle, feel of the exh manifolds right where each exh port exits the heads. The bad cyl will be cold, the rest will burn your fingers.
Agree with 45shooter. I had this EXACT problem with the 350 in my dads stude. A compression test revealed a cylinder down. Ended up being burnt valve...which could act like bent pushrod, stuck valve. In my short years I have found my compression tester to save me days worth of guessing. When I run into something I cant figure out pretty quick the compression gauge comes out next.
Late ignition timing is also a possibilty, or dist. vacunm advance stuck, since its been setting awhile. I say this because you have to take the dizzy out to swap intakes on the FE's. Maybe it never got set right?
I don't know about great minds, but turning wrenches for 35 years you're bound to learn a thing or 2. That KY Jelly is some great stuff though, huh?
Damn I was meaning KentuckY...Never thought of it this way when I signed up. RYAN...Can I change my name??? just kidding. We always wondered why they named that stuff like that. I think at the time Kentucky had the highest teen pregnancy rate of all 50 states. Maybe that had something to do with it??? Sorry for the hijack but I just couldnt let that one go... Oh yeah...check your compression...did I already say that
Since this thread started about 20 months ago I suppose it is water under the bridge by now but there is always another 390 needing work out there some where. I was just going to say if the engine just had a new intake put on it that means the rocker arm assys, have been off so it never hurts to ck to see if the rocker shafts were installed correctly. ((with the cotter key in the end of the shaft "heads up" if it looks upside down then the lube holes are on the top of the shaft instead of at the bottom and you know what happens without proper lubrication. ( galling on the shaft, metal in the oil, bent push rods etc,) Also if you happen to break a valve spring you will most likely bend a push rod right away and probably get some popping if it is on the intake. Also there are some points to ponder before setting the in=manifold on .... some thing that I have learned along the way with my alum intake. Let me know if anyone has any ?s
Ive have had a couple of engines that acted that way. Ran a compression check everything fine. Pulled the plug wires one at a time it stopped popping on no 3. took off the valve cover and peved it up and you could see the intake valve float. Changed out the valve spring and cured it. OldWolf
Wow! It's been a while since I first posted this. Just to put a "pin" in this thread...I never really did find out the problem. I fiddled around with the timing some more and got it to stop doing it almost all of the time. It does still pop every once in a blue moon if you punch it from a dead stop but it's very uncommon. It happens so rarely that I've just learned to live with it. It doesn't concern or bother me enough to keep chasing it...
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