Popping thru Carb at low RPM

My newest Project pops back thru the carb. can't hardly keep it running till I get above 15-1800 rpm when I get to cruising speed it starts coughing again, add a little throttle and it runs beautiful.
I've never ran into this before. it has a stock fuel pump backed up with an electric pump. makes no difference with or without the electric pump
it's a 375 horse 455 Olds with a stock quadrajet carb

 

valves just a little tight, a smidge too advanced at idle?

 

It's a distributor/timing/points/electrical problem yet you don't tell us what those parts are?

Point's? adjust the dwell first...
Pertronics? put the points back in...

 

Popping through the carb would make me think that an intake valve is out of adjustment, bent or burnt allowing compression back into the intake. Usually an ignition problem or miss will be heard in the tail pipe.

 

cam worn out lobes

 

Also a stoped up exhaust port will make it pop through the carb

 

I've found that backfires through the carb usually indicates a lean condition. If it's a valve or ignition problem it would be there no matter what RPM. Check for vacuum leaks.

 

What cam?? transition circuit oft needs richening with hot cam. The hole is deep in a brass pressed in thing that is REALLY FUN to extract...they carefully made it so the metering hole is 1/8" farther down than any little bitty drill you can find.

 

How new is the motor, what parts were just changed before it started doin this.

 

Hey DrJ, where you been?

 

i had that happen on a bbc and found out it was a worn cam lobe.....juat my opinion....but i did stay at a motel 6

 

My guess is a mis-adjusted valve.

An idle mixture is lean and requires quite a bit of advance to run properly at idle and low speeds.

Are you running vacuum advance?

 

Are you using Ethanol? Try using 100% gas?

 

Same problem a few cars back, flattened cam lobe. Fix was new cam and lifters and thorough cleanout of sump.

 

The fact that at speed it runs fine leads me to belive it could be a idle air bleed stopped up with debris as this makes for a very lean condition, just my two cents.

 

Could be a vac leak making a cyl or two run lean. I would still check timing, ignition, points, etc.

 

I have just the opposite problem. I bought a new crate motor and broke the cam in on an engine test stand before installing it in my coupe. It ran fine with no apparent problems. Now after it warms up I get a strong popping out the exhaust from my left bank. I changed the fuel to make sure it was fresh and no ethanol. The distributor is new as all the parts used. I haven't checked the plugs yet.

 

Sounds like you have an overly tight valve adjustment that comes in with parts expanding due to a temperature rise.


A compression test could tell you a lot.

 

Flat cam lobe. I've seen this before. Start it up, remove the air cleaner, and run it up to 2000-2500 rpm & you may see small flames above the carburetor.

Lucky667

 

check the easy stuff first follow the k.i.s.s. rule.
I always have to remind myself of this when I am diagnosing problems.
good luck and let us know how its going.

 

After the EPA eliminated lead from gasoline and most of the ZDDP (zinc-dithoiophosphate) from motor oils, automobile manufacturers had to go to roller cams to prevent warranty problems due to cam shafts going flat.

If you have a fresh engine with a hydraulic or solid flat tappet cam, you should use GM EOS (Engine Oil Supplement) only for break in and Valvoline VR-1 Racing Oil or Shell Rotella -T truck motor oil. However, you may find it difficult to find Rotella-T in a viscosity you need on the street. If you do not exceed 270/100 lb. open/closed valve spring pressure with a flat tappet cam, you can use no lead pump gas and regular all season motor oil without having problems with eroded exhaust valve seats and flat cam lobes.

I use Valvoline VR-1 Racing Oil exclusively and never used GM EOS. I have a Comp Cams camshaft and valve springs that had 320 lb. open / 125 closed pressure at set up. The engine has about 25,000 miles on it with no problems. I cannot attribute that to the oil because I have never used anything else in other engines for comparison. It may have been fine on Valvoline All Season or any other brand of oil. Who knows?

 

Popping threw carb could be caused by. Worn out cam , retarded timming, lean fuel mixture bad wire or plug. Ok evryone else said the same thing

 

Maybe just a worn out Q-jet. They're great carbs when they work properly but when they don't are usually a real pain in the ass