The Stovebolt Site Forums The Shop Area Engine Shop Cough out carburetor

I have a 1956 Chevy half-ton, 235 engine. It has an occasional cough through the carburetor. It doesn't do it every day and when it does cough, it will do it only once and then not again for the rest of the day. Sometimes it's enough to stall the engine. Always happens on acceleration, either leaving from a stop or accelerating in second gear.
Everything was rebuilt or replaced about seven years ago. I adjusted the valves and set the timing a little over a year ago. The truck gets driven every day, though it did sit for all of last summer while I was out of town. The cough started some while after I got back.
Anyway, I researched the forum on this topic. I understand that an intake valve must be open at the wrong time in order for this to happen. But I've also read that a lean condition can cause a cough out the carburetor, too. My question is, based on the info I've provided, does anyone have a suggestion on where to look first? Haven't started working on it yet but just wanted some advice on where the most likely culprit might be. Don't know if this is related but the exhaust is loose and rattling somewhere, sounds like it's down by the muffler. This started about the same time as the cough. Based on a quick check, it sounds like something is loose inside the muffler. Just thought I'd mention it.
Thanks.

Look at your accelerator pump circuit.

Ed

Start by doing an "upper cylinder lube" procedure. Get a can of Marvel Mystery Oil or some other lightweight tuneup oil, run the engine at a fast idle, and slowly dribble the oil down the carburetor throat until you get a heavy cloud of smoke from the tailpipe. Then dump enough oil in quickly to stall the engine, or shut the ignition off after giving it the heavy dose of oil. Let the engine soak overnight, then crank it with the throttle wide open until it starts. Goose the gas pedal several times and run the engine at fast idle until the smoke clears. If there are carbon deposits on the intake valves that cause them to stick intermittently, this procedure usually cleans them up. Warn your neighbors what's going on in advance, or you just might get a visit from the local fire department- - - - -the smoke screen will be epic!

If that doesn't help, then dig into the carburetor and inspect the accelerator pump circuit.
Jerry

Nice, Jerry

Thanks

Easier than Jerry's suggestion: first check the dwell. A burned projection on one of the points will change the dwell, which will in turn change the timing.

If nothing here, then try Jerry's suggestion.

Jon.

cough out of carb is generally a lean condition on the low end.

Thank you for the replies. I will be following these suggestions and will report back.

Hotrod,

Would the "upper cylinder lube" procedure be a good thing to do anyways for periodic maintenance?

Thanks

If a vehicle is run regularly, at least a few times a month, there's really no need to do that. I give my vehicles a good dose of Sea Foam fuel additive on occasion. It can't hurt anything, and it will probably keep deposits from forming on valve stems, etc. Since the OP's truck started giving problems after a fairly long sleep, it's probably got some gum, or maybe a little rust on a valve stem or two. The heavy dose of oil will free that kind of problem up, at least pretty often, without having to do a valve job.
Jerry

I had a 1967 327 V8 that would cough up the carburetor. It was caused by a bad exhaust lobe on the camshaft. The exhaust valve did not open. When the intake valve opened the compressed air would enter the intake manifold and out the carburetor.

Rick

If it were mine, I would pull the valve cover and make a visual on every valve opening and closing, a sticky or tight valve can give you this symptom.

If I do this, will I have oil running everywhere? I've never run it with the valve cover off before. Going to do the Marvel Mystery Oil thing tonight. Under cover of darkness.

Ive run mine with cover off in the garage, no problems. Thats how some old-timers would adjust the valves. (Not me!)


Good time to check for a bent push rod.

No it is not like a small or big block chev, no squirts, nothing will escape the head

The top end of these engines are dribblers, not sprayers. There is no way for oil to escape with the valve cover off.

I did the upper end lube with Marvel Mystery Oil as suggested. The cough is still there. I notice it's very responsive to the accelerator, like the second you step on the gas, you get the little cough. As I said, it's pretty rare, but it does happen. Days will go by and there's no sign of it and then out of the blue it will happen and then disappear for the next few days. Anyhow, I'm going to take a look at the points and then turn to the carburetor. The points are about a year old, but I suppose they could be messed up somehow. Or maybe the gap isn't right anymore due to wear on the plastic lobe. Anyway, a quick look at that and then on to the carburetor.

Just throwing it out there maybe the accelerated pump has a defect or dirt , or maybe the little ball bearing that is down there and can’t remember what it does.

I'll be checking the carburetor here pretty soon. I understand that I should look down the throat of the carburetor and operate the throttle and look for a good squirt of gasoline. If that is present, I assume I will need to disassemble the thing and look further, is that correct? Since it's suggested that the accelerator pump could be the problem, I imagine I will have to have a good look at the piston, cup, spring, etc. Any suggestions on what I should be watching out for aside from the obvious broken or worn parts?

Hello, still waiting to hear what you found in the carburetor. If that solved the problem please let me know. Ed

Getting back to this a year later (or so). I checked the points and found them slightly off. I cleaned them with a points file, reset them, and adjusted the timing. That took care of the problem. For a while. It's been about a year and the cough is back. It did sit all summer again (second summer in a row) without being run. I would say i got a few months' worth of trouble free driving before the problem returned. Too busy to deal with it now but I will be getting to it soon.

Did you "treat" the gasoline in the fuel tank over that one-year period of "rest"?

What kind of gasoline was in the fuel tank (what % Ethanol)?

I've never seen an "intermittent" flat cam lobe.
Jerry

Yep, no one has come up with self-healing steel yet.

I didn't do anything to the gasoline that was in the tank. No idea what percent ethanol it was, either, though I've cycled through a few tanks of gas since. Back when all this was happening I adjusted the valves. They were all almost perfect and very few needed any adjustment.
I'm wondering if the leather piston in the carburetor might have dried up while the car was sitting and maybe now has some kind of problem where it doesn't always function correctly. The problem is really intermittent. It will do it about once every day and that's it. Temperature doesn't make a difference. It does it hot and cold. It does it in all gears. And it happens only when I step on the gas. Just a short cough out the carb. The carb is a Rochester model B that I rebuilt about ten years ago and have never had any trouble with. But I do recall that the throttle shaft seemed a little loose in the body of the carb and I've read that this can cause the problem I'm having. I stated in an earlier post that I did the Marvel Mystery Oil upper end cleaning last year.

If you can buy non-ethanol gas in your area it will definitely help with your carb not gumming up during the periods that your truck just sits. Ethanol will start to coagulate in six weeks. I don’t know it that is your problem but it will be if you use gas with ethanol in it. I use non-ethanol gas and I also treat it with Stabil gas treatment. My truck sits thru the winter here in NY and I haven’t (knock on wood) has issues with the carb yet.

Thank you.

A sticky valve with cause a backfire through the carburetor. A valve can stick from varnish coming from an old gas tank. Ethanol fuel seems to be a wonderful solvent for 70 year old varnish on the sides of the tank. This dissolved varnish makes its way to the valve stem and gums up the guide, causing a miss. It can be intermittent, such as when the engine hasn't warmed up yet.

I had the gas tank chemical dipped when I rebuilt the truck, so I don't think varnish is an issue.

joeflanagen You might look for resricted exhaust system by taking your air cleaner off ,then speed it up and watch for gasoline to fly-up out of the carburetor when it comes down. Also will be doggy.

Could a problem in the muffler have any bearing on this? Because it sounds to me like something has come apart inside the muffler. An exhaust restriction maybe?

War Eagle1 A 3.88$ quart of marvel mystery oil would be a cheap test to answer a question. In the old days they would grind the valve seats with just a 45 degree stone and leave them wide. Didn't know any better,that will contribute to a cough. When they lapped em they had no way to narrow the seat either.Would give it a pint to a tankful and see,also check other stuff mentioned.

I just discovered a pretty good sized crack in my exhaust manifold. Could this have anything to do with the problem?

Usually a cracked or loose exhaust manifold will produce a backfire upon letting off on the foot feed, especially when going downhill.

Ed