Engine backfiring under load when hot

mstover

mstover

Member
Ok. Fixed all the other problems.... here's a new one.

Car had been running pretty good and I decided to take her out for a short drive this afternoon. It's a hot day around 86 degrees and the car started out just fine. A couple of miles down the road, however, the car sputtered, popped and backfired when I stepped on the gas as I pulled away from a stop sign. When I pulled over, the car idled just fine. I started again and a few blocks away the same thing happened. With the backfiring and popping becoming embarassing, I pulled over and stopped the car. After 10 or 15 minutes, I started the car and it ran fine... for a couple of minutes. Then the backfiring started again. The GAV was only 1/8 to 1/4 turn open and the spark was about 1/2 to 2/3 advanced. Any suggestions?

Thanks,
Mike
 
Check the spark to be sure you have at least a 3/8" hot blue spark. My car had an intermittent miss and backfiring and it was the poor contacts in the repro ignition switch.

My car also has poor running and acceleration from stop lights on hot days if I get stuck having to use gas with ethanol in it.
 
It's likely and ignition problem as suggested but I still have to ask a question. With all the issues cleaning the tank, etc., did you install a new gas line to the carb? If a new line is installed incorrectly (too far into the ferrules) it can restrict gas flow. With the car not running or engine speeds low the float bowl will be replenished but normal driving will run the level down and do just what you describe.
 
Sounds like an intermittant spark. A friend had a similar situation. He had the wireless style bottom plate in his distributor. The contact plate normally has some "spring" in it so that it keeps contact with the shoe coming off the points. One side of the contact plate had cracked, and it was no longer sprung up tightly against the shoe. At certain spark advance settings and bumpy road conditions, he'd loose power and then it would backfire and run fine for awhile. We found the problem on the side of the road and jury rigged it by sliding some folded up paper under the contact plate to prop it up.
 
I did replace the flexible fuel line to the carb, but not the short, steel line into the carb itself. I have had some problems with the float valve inside the Zenith carburetor. It has a rubber piece inside that warps after sitting in ethanol ladden fuel for a couple of days. This causes the float value to stick... sometimes open and sometimes closed. If stuck open, would this flood the car enough to cause these symptons?
Thanks!
 
Backfire under load..........seems like a similar situation I had with a flathead V8 8ba engine and the timing was too retard. Advanced the timing a little and fixed it. How's your timing, set properly?

In our case the car (47 tudor) idled really nice, on the road & shift to second gear, backfired like hell. Advanced the timing and fixed it.
 
I experienced same prob awhile back - runnning great then without warning, and in traffic, began backfiring through the carb - got off the road and it would idle great, start right up, but the minute I tried to get under way the backfiring would begin. Got home on the back of a rollback. I too have a Zenith which had gas pouring out of it - I believe I had a hung float (unconfirmed). It has not happened again which worries me. Nothing like a nasty intermittent problem which you know can/will reappear at any moment....:(
 
Mike,
As several have suggested, backfiring under load is a classic ignition problem; it takes a better spark to ignite the mixture when it is under the increased pressure caused by opening the throttle. The whole ignition system is under suspicion, but why not check the things that are expected to fail every once in a while, like the condenser? Also, cheap repro points are expected to wear down and close up the gap, giving a spark that is both feeble and out of time. Both condenser and coil are known for sometimes failing in a temperature dependent way, but thermal flexing can cause intermittant shorts or opens in any dicey contact. Somebody once reported that the junction box terminal bolts shorted to the firewall when the engine compartment warmed up. A friend was ready to drive his car into the river until we found that ALL of his plastic distributor housings had conductive scratches and tracks, and that one of his plug straps was shorting to the housing. Stories abound. After checking out the usual suspects, get systematic and Persist!

Steve
 
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I'd bet on the condenser. If that is the case, it will get worse. Another option is poor fuel flow causing a lean fuel situation.
 
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