Backfiring through the carb - Solved!

[sargentrs]

Slightly off track but you guys are my "Go To Guys" for anything on all my vehicles.

1964 Fairlane 500. 289/Cruise-O-Matic. Autolite 2100 carb. Carb is under 2 years old. Bought from Guaranteed Carburetors. I've bought 2 other carbs in the past from them and they do excellent work with great customer service. Anyway, she's been running fantastic for the past couple of years. Drove her to work Monday morning and had no issues. Got in her to go home that afternoon, pumped the pedal and pressed it to the floor to set the choke like always and she fired right up. Started easing down the road and I noticed she just didn't feel right. You know how it is. You just KNOW something's different. As she shifted into 2nd there was just a little "Pffft" through the carb, just once, and I noticed a little stuttering. Got up to 30 mph and she smoothed right out. The whole hour drive home at 60+ mph she ran great. Every time I'd stop at a light or stop sign and start moving again, she'd stumble just a little. Sometimes the little "Pffft", just once, sometimes not. Got her up to 75 mph a couple times and no problems although that pretty much maxed her out and she didn't want to go any faster. Leaving a stop, if I stay in the gas accelerating constantly, there's no backfire. But if I ease into it, she stutters and then "Pffft".

I've replaced the PCV and air filter and have a new oil breather cap on the way. Been a year or so, thought she might be having a little trouble breathing. Didn't help. I've had a couple of injuries so a little hard to get around lately. Before I start pulling plugs thought I'd get you guys' opinions. Carb or ignition? Think the ethanol gas might have gotten to the power valve or accelerator pump? Any and all input is greatly appreciated. Advance timing a degree or two, might have slipped? New chain and gears and Duraspark ignition.

[colnago]

My very old, very tired 352 sometimes backfires through the carb on start-up. I did a compression test, and #1 and #3 were way down (I don't remember exact numbers, but all others averaged out to ~125; #1 was below 100, and #3 was below 50). When I checked vacuum, the needle bounced all over while idling; if I revved the engine, it smoothed out some. The conclusion was that the intake valves on those two cylinders weren't closing all the way. I tried the SeaFoam treatment, which made a lot of smoke, but didn't clear anything out (I was hoping it would dislodge any carbon build-up on the intake valves; it didn't).

I guess I'd start with compression/vacuum tests, since those are easy, and they don't require new parts.

Joseph

[1972hiboy]

Three things come to mind., vac leak, valves, or carb. Whether the carb is new or not, n-e-w don't spell good. Id give a look at the accell circuit. Even with fresh rebuilds ive done sometimes the cover on the accell diagram leaks after a small amount of time causing a vac leak there. Then move on to the others. should be fairly easy to nail down though with your skill set and the vehicle in question.

[sargentrs]

Thanks for the input, guys. Vacuum test will be first on the list. Should tell me something about the condition of the valve train. She's got almost 165,000 miles on the 289 so I know her time with us is getting about like my time left on this earth. I don't run like I used to either.

[cep62]

Drove her to work Monday morning and had no issues. Got in her to go home that afternoon, Started easing down the road and I noticed she just didn't feel right

A couple thoughts come to mind , but just speculation.

A cracked or carbon tracked distributor cap ?

A leaking power valve ? , I know old Holleys could act up and I think autolite carbs have a similar valve.

Do you still have points ? maybe a condenser , or electronic module?

Corroded wire?

[sargentrs]

Drove her to work Monday morning and had no issues. Got in her to go home that afternoon, Started easing down the road and I noticed she just didn't feel right

A couple thoughts come to mind , but just speculation.

A cracked or carbon tracked distributor cap ? Possibility. That would be one of the things I'd replace when I do plugs.

A leaking power valve ? , I know old Holleys could act up and I think autolite carbs have a similar valve.

Do you still have points ? maybe a condenser , or electronic module?

Corroded wire?

All possibilities. I'm leaning toward fuel related rather than spark/electrical related unless maybe a fouled plug. Right after I got the car I replaced the carb. About a year and a half ago, I swapped out the points for a Duraspark set up. All new components including wiring. Shortly after that I replaced the headlight switch mounted fuse panel with a 12 circuit ATO fuse panel and all new wiring. All connections were crimped, soldered and shrink wrapped. But like HiBoy said "new" doesn't mean "good". Hope to get to further diagnosis this weekend. In the meantime, it's just a little annoyance but if left untreated I'm sure it'll get worse. She's my only car and my daily driver so she has to run reliably or I have bigger personal issues. Like....work!

[colnago]

She's my only car and my daily driver so she has to run reliably or I have bigger personal issues. Like....work!

Meh. I find that whole "work" thing to be highly over-rated. I can certainly do without it ... but the wife and kids get a bit cranky if there's no food in the house.

Back on topic, I keep thinking it's a timing/ignition thing, rather than fuel. The only source for spark is (or should be) the plugs. If you're getting backfiring through the carb, then there has to be a path open between cylinder and carb. Intake valve not closing, or cam off a tooth, or timing is too advanced, or ... ? I can't see it being related to fuel, unless there's something really, really hot in the intake that's causing it to ignite before getting into the cylinders. That's my thought process, anyway.

Joseph

[sargentrs]

Thanks for the insight, Joseph! I've just been lazy and haven't done any real troubleshooting yet. I'll start with the vacuum test and then move to the plugs. I do have one little known issue in that my #4 plug threads in the head are barely hanging in there. The plug is snug but it's not torqued. I give it a wiggle test every few months. Been fine for 2 years but might've worked it's way lose.

[farmallmta]

In general if the engine backfires through the carb one or more intake valves is open during ignition on one of the cylinders. This can happen under load when:
1) timing chain is worn
2) rocker arm/s is maladjusted or worn, ditto the rest of the valve train
3) slightly burned intake valve face or seat, or worn valve guide
4) vacuum leak in fittings or hoses
5) vacuum advance diaphram in the distributor is leaking, failing to advance the distributor when you require.
6) crack in the distributor cap or carbon track
7) misfiring spark plug due to crack or fouling or spark-jumping of the spark plug wires, as if crossed and feeding one another intermittently.

Start with the simplest and most accessible possibilities first (#4-#7), then advance to the more complicated and costly possibilities (#1-3).

Hope this helps.

[sargentrs]

Thanks, farmall! Well, I put a new timing chain and gear set in a little over a year ago so at least #1 should be ok. Distributor and vacuum can are also new, not even reman, so #5 is not likely. Of course, as mentioned before, new doesn't mean good so still a possibility although small. That only leaves 5 others to look for! I feel better already.

[Manny]

Looks like time to pull the plugs and do a compression test. Be the best way to sort out the other 5 fast..

[sargentrs]

Thanks for the input, Manny. Haven't gotten to that yet. Progress report.... Since the carb has a lifetime warranty, and is the easiest/fastest thing to do, I pulled it and sent it back for rebuild. Installed it yesterday and dialed it in. I'm getting 20.5 Hg pressure on manifold manifold and holding steady at idle. Carb works great. She purrs and has good throttle response all the way to WOT with no issues, in park. However, when I took her for a test run, the backfiring is worse now under rapid acceleration or going uphill under load. Easing into it on level ground and accelerating slowly, she run good. Next up, I'll check the timing and replace the cap/rotor to see if that helps it.

[sargentrs]

Bingo! Distributor cap/rotor. Put on a new one and no more backfiring. Is that what y'all meant by carbon-tracked?

[cep62]

If you look closely you might see a faint line , like a pencil mark by one of the terminals ,
that will cause the spark to jump to the wrong cylinder at the wrong time.
If it jumps early it'll backfire through the carb.

Damp weather can cause this if moisture gets under the cap.
One morning I made a short trip and stopped ,when I went to restart it was popping back through the carb ,
replaced the cap and good to go.
I think the engine got a little warm , not hot enough , which caused condensation in the distributor.

[sargentrs]

Interesting. Thanks to all who helped and offered valuable advice!