The Stovebolt Site Forums The Shop Area Engine Shop Rough running when cold

This is a new issue. My 1954 235 runs pretty good. I recently adjusted the valves. It has a newly rebuilt carburetor. I pulled all the plugs to make sure they are not fowled. I double checked the distributor cap for corrasion. Wires are good. It suddenly developed a miss when the engine is cold. Once it gets to operating temp it smooths out and runs well. Choke on or off makes no difference in the miss. Any ideas what I should be looking at.

Can you isolate the cylinder that's misfiring? Or is it multiple cylinders causing the missing and rough running?

Pull one spark plug wire at a time (carefully) and listen for the reduction in RPM. The cylinder which does NOT produce any change when the wire is pulled...is your miss.

Given what you've described here, I'd go looking for a vacuum leak. Could be that something is leaking, running way lean, when the engine is cold. Leak closes up once parts heat up and expand. Spray some carb cleaner (or flammable liquid of choice) around the manifold, carb base, wiper connection, etc. Listen for it to smooth out. Some folks prefer an unlit propane torch, but you have to disconnect the fan belt for that to work very well.

Having said all that, I would expect closing the choke to improve the miss....if a vac leak were to blame. It's an easy and logical place to start, anyway.

How cold out was it when this happened? Is your exhaust/intake flapper valve functioning properly.

How do you know the wires are good? Don't assume that because they're brand new that they're good.

After you do jw51's test, and find a cylinder that doesn't cause a change in the way the engine runs after you pull the plug wire off, replace that wire with a spare or switch the wire with one from another cylinder and see what happens.

I have seen perfect looking plug wires be the source of a misfire due to an internal crack or some other invisible problem

What was the last thing you did to it prior to it mis-firing? Re-check the valve adjustment if that was the most recent change that was made.

X2

For your 1954 235’s cold miss issue:

1. Recheck Valve Adjustment: Ensure they aren’t too tight.
2. Check for Intake Manifold Leak: A minor vacuum leak can seal up when warm.
3. Confirm Ignition Timing: Small timing deviations can cause a miss when cold.
4. Verify Spark Plug Heat Range: Make sure they’re the correct type for cold starts.

These should help pinpoint the source of the cold miss.

Check for vacuum leaks around the carburetor base and intake manifold to cylinder head flanges by using gas from an unlit Propane torch, particularly during warmup. If the engine smooths out or speeds up when you apply the gas to a specific area, you've found your leak. Don't "fowled" spark plugs usually have feathers on them?
Jerry