The Stovebolt Site Forums The Shop Area Engine Shop Backfire out Carb

I have got a 1953 235 running after a 50 year slumber. I have rebuilt the factory carb, new points, plugs, wires, cap and rotor. I also adjusted the valves per the factory manual I have for the car. I set the timing per the manual also. I have checked the vacume advance is advancing properly as well.

The problem is off idle every single time it pops back thru the carb. It will only increase in RPMs if you SLOWLY modulate the throttle by hand.

Anything more than gentle modulation and it pops back thru carb.

I would assume I need to check vacuum
reading to see if there is a leak causing a lean condition.

I also would assume I need to check the compression to make sure I am not getting a sticky valve causing fire to come back up thru carb.

The most obvious culprit could be a weak accelerator pump stream, But I have no idea how to test that. It's also brand new.


Thoughts? It's been a LONG time since I have had a 235 behave like this.

Thanks!

Try advancing the timing slightly. Turn the distributor slightly counter clockwise.

Check the vacuum advance.🤓

Check the points .I changed my points and the new points had a copper spring as well as a loose piece of spring that I thought was for a different application .the loose spring piece fit in with the copper and without it does not put enough pressure on the points and what was happening is when you accelerated it backfired because the points would not have enough pressure on the distributer shaft and would bobble around if that makes senice. Took me a long time to figure it out as I also was getting the engine running after a year of sitting.

I also think the timing is slow. To check the accel. pump, with the engine off look down the throat of the carb & work the throttle from a closed position. You should see a squirt of gas each time.

Ok.I checked all the above.

I advanced the timing as far as the distributor would go attached to metal vacuum line. It starts and runs much better but will still pop out Carb.


If I physically hold distributor from retarding back once vacuum is gone it doesnt pop.

I am assuming either the vacuum advance is falling away too quickly or I am still too retarded in general.

Could it be I need to advance my firing order one position around the cap and then set my timing back down and advance if necessary from there?

Pull #1 plug and bump starter around til it blows your finger off #1s plug hole. Look at the timing mark/BB and turn flywheel by hand to get it lined up where it should go. Now take the distributor cap off and look where the rotor is pointing. If necessary move wires accordingly. Hope this helps. It's always worked for me.🛠

I set it to TDC with BB and its pointing slightly advanced of number 1 cylinder.[i][/i]

This is beginning to drive me crazy. The car runs like a clock, minus this off idle hard throttle backfire.

One of the guys who works for me is beginning to wonder if this problem was why it was parked in '67...

If you do as Martin suggests, you will gain a lot more rotation of the distributor. Seems like the guy before you got the firing order right, but off by one wire at the cap.

Did you check for proper accelerator pump function as Wrenchbender suggested?

Intake backfire can come from timing problems, which you have investigated thoroughly. Make sure your ignition wires correspond with the proper firing order so no cylinder is firing before it is supposed to. This situation can also be a "lean pop" which happens when the slow burn of a lean mixture continues in the intake manifold rather than completing in the combustion chamber. This sounds like it might be your problem. Verify by adding a little choke to enrich the mixture before you quickly hit the throttle. if the problem diminishes or disappears, then you're on the right track. if you confirm you are too lean, then take steps to enrich the mixture. You might just need to clean the carb, adjust the mixture, or re-jet.

When I locate the BB on the flywheel the rotor is pointing at about half way between number 1 and number 4. So it is advanced of TDC, which is common I would say. The distributor is set to just past "10" on octane selector which is as advanced as it will go physically.

Secondly I put my dial back timing light on number 1 cylinder wire. I tried to dial up the timing with vacuum advance conected to see how total timing at idle there would be. Whats bizzaire is that the timing blinks very erratically and no matter how much I advance the timing light I cannot find the BB. I tried to locate the BB using both 1 and 6 cylinders and cannot find the BB with the light.

I tried advancing all the wires one position around the cap to get more
room to work but when I did that the motor would crank slow even when I retarded physically all the way back.

As for the accelerater pump, it sprays gas when the throttle is actuated. Again I have no idea to know if the shot is strong enough or long enough.



My thought again is the distributor is physically positioned too retarded. I am thinking the distributor is off by one or two teeth.

It sounds like you are very close. You could be one tooth off when the distributor was inserted. You could pull the distributor and reinsert rotating the rotor the correct direction to give you more timing adjustment at the distributor.

Do you know if the oil pump is hooked to bottom of distributor? I know alot of motors it is and you have to advance it slightly also.

I don't remember but I think it is. You can usually rotate it with a straight blade screw driver.

I checked compression this morning And I have got between 117psi and 120psi cold on all cylinders. That rules out a valve problem.

One last thing I thought of before I move distributor, if the mechanical advance weights were stuck could this cause this problem?

Long time since I had to deal with this. Though if the
weights were frozen you would not be able to move the
breaker plate, if I remember correct. Jeffrey

bigblack74:How are you adjusting the distributer? Loosen the bolt and move in the slot on the "octane" plate(+10 to -10)? Or are you loosening the clamp screw on the vacuum advance(much better (+ a lot to - a lot)? Testing the vacuum advance is simple if not hygienic. Pull the vacuum line at the carb. Apply vacuum(suck on it) cover with tongue and see if the "suction" holds. If it leaks you will know. Last but not least...#2 and #4 plug wires for some reason get crossed. Proper is 1,5,3,6,2,4. It will start and run with #2 and #4 crossed but not very well. Last suggestion: Put a dwell meter on. Should have over 30 degrees of dwell. If the dwell is bad, the point gap is wrong.

The carb insulator had a hairline crack that was discovered once is completely cleaned off. Changed that part and car runs like a top. Thanks for all the help folks.