My engine has suddenly developed an occasional "caugh" out the carburetor. When first starting the engine it is quite pronounced and lessens as the engine warms but does not go away. Adjusting both the timing or mixture seem to change it but not eliminate it. Compression is between 37-42 psi on all cylinders and plugs are cleaned and gapped at .025". Anybody have any ideas?
Weak coil, dirty or worn timer would be my guess.
A sluggish intake valve occasionally not closing on time, therefore the "caugh" is going out the intake hole instead of the exhaust hole.
The sluggish movement is probably caused by ethanol gas deposits on the valve stem.
When this happens to my car a half ounce of Lucas Fuel Treatment to 10 gallons of fresh gas stop the "caugh".
Its a cheap and easy first attempt. If that doesn't stop it, then start troubleshooting and swapping parts.
From the symptoms that you describe, I'd say you are running a bit lean. A lean fuel-air ratio burns slower than an optimum mixture, thus there is still a bit of fire in the combustion chamber when the intake valve opens for the next cycle. This lights up the incoming charge and you get a pop or cough out the carburetor.
If I'm correct that your cough is due to lean mixture, then it will be most noticeable when the engine is cold because of the poor vaporization of a cold engine. And it's particularly prone to coughing when you add throttle.
There could be other reasons for your cough, as Richard describes. But the easy first step it to try making your mixture a little more rich.
What kind of Carb? What do your plugs look like?
I cant say ive had a model t cough from a lean mix, they just run rough or die.
Intake manifold leaking? If there is a manifold leak this would cause a lean fuel air mixture and would be hard to impossible to adjust the carburetor properly.
Ken ..........Carb is origional Holly which I recently put a kit in (quite a while before this problem). Plugs have always run black with minimal carbon. After xmas I'm going to clean the timer and if that doesn't cure it I'll pull the coils and send them out.
what on the plugs is black? the insulator or the body?
the insulator should never be black. you either have a extremely lean mixture, or a intake valve not seating.
if your insulators are black the you are running quite rich, which will not cause a backfire.
I highly doubt your problem is ignition related.
I had a Stromberg M-1 set too lean on the Fronty, and it coughed as the engine died just as I was driving it half into the garage. Because it was a temporary carb, I had no air cleaner or screen on it, and that cough started a fire. It was pretty ugly.
My guess would be a leaky intake valve.
Valve, Spring or Seat. Cross fire from a plug wire or coil. Start with a compression check (wet and dry) and sort through the wires and coil box.
In my opinion 37 - 42 is low when 50+ is expected as normal. Try adding 10 ml's of oil to each cylinder and re-test.
Have your plugs always been gapped to 0.025"? If you changed them recently, sometimes that number is a little too close especially on 'X', doesn't matter what the box says. I always set them at 0.030". With good coils it will jump this extra easy.
Here's what I'd do.
First I'd bubble check the plugs. Others say it doesn't matter but I always bubble check them and seal them if necessary...eliminates a variable.
Pop the timer and take a good look. Clean it, check for ridges, look at the roller/flapper. Grease it a bit while it is open.
Start it, let it get warm, throttle down and try some carb cleaner at the intake glands. Speeds up, you have a leak. If you find a leak, you can try to retighten glands, but that usually doesn't work, so change them.
Then I'd listen good and figure out what cylinder it is by shorting out a cylinder at a time. Once found, Swap a coil, see if it moves.
Finally, if all else fails I'd slap on a trusty old NH from the parts box and then look for and listen for differences...at least that way I eliminate the carb.
Others may disagree, but running too rich is better than running set too lean. You can always tune back rich once all else sorts out. It just gives you sooty plugs provided it is not too rich to bog you down.
Your profile indicates that you live in Chico. That is not far above sea level, so your compression is low. Your engine must be getting tired. Nevertheless, it is fairly even. If you ever tried to start a 1950's Chevrolet 6 you will notice they tend to cough when cold. This seems to be caused by lean fuel mixture. You might try to enrich the fuel adjustment. If that doesn't fix it, try cleaning the carburetor. I have never run a vaporizer type carburetor, but it is my understanding that they are harder to start when cold. If you are running a Kingston L4, the car should be easy to start either cold or hot. And if a Holly NH, you will need to choke a bit more than with a Kingston.
Good luck. Let us know when you find the cause so we can all learn.
Another thing I thought of after posting the above, you might have a partial blockage of the fuel line. This would cause the float level to drop when going fast or pulling hard. A low float level will make the car run lean at speed but richer at idle. If you are running any type of filter other than the original sediment bulb with screen, it could be a filter slowing the flow of fuel. If you have the sediment bulb, it might be dirty. Worth a try to check out.
I went out to clean the timer and just for the heck of it I fired the engine and guess what! It ran normally with no coughing. I hate things that fix themselves!!!!
Maybe that sticky intake valve had a chance to wear off some of the stickum while running and will soon recur?
It will be back. At just the wrong time.
I may be wrong on this, but this is how I see it.
1. If it is a leaking valve---it will still cough.
2. If it is a leaking intake valve---it will still cough.
3. If it is a sticking valve---the cough will be intermittent, depending on the deposits at the valve guide & stem.