Re: 1938 Super 8 1605 - adventures with a newbie

Posted by TxGoat On 2024/6/18 16:46:04

If the intake manifold is leaking into the exhaust heat jacket that surrounds it below the carburetor flange, spraying solvent may not affect it.

Another possibility is that the check valve at the bottom of the intake manifold drain pipe (if present) may not be sealing tight and allowing an air leak.

A dead miss on one cylinder on a engine that shows good on a compression check [usually] points to an ignition issue.

An exhaust valve that seats well when the engine is cold can begin to leak enough to cause a miss as the engine warms up if the tappet clearance is too small. That might allow a good compression check reading, yet still have a valve-related miss once the engine is running. An exhaust valve that is not seating fully will get very hot very quickly and expand, thus aggravating a low clearance problem.

My car recently developed a miss at idle and low speeds. I figured I had a burned valve or cracked seat. In fact, one exhaust valve was set too tight. Adding a few thousandths clearance corrected the problem, and it has not re-occurred. I don't know how the valve clearance got tighter, but it did. Perhaps when the valves were adjusted, there was carbon on the valve face and seat, and me driving the car frequently burned off the carbon, thus reducing the tappet clearance enough to make the valve overheat.

**If your car has a vacuum booster built into the fuel pump, it could have a leaking diaphragm, which could allow air and possibly crankcase oil into the intake manifold. This would probably affect 2 or four of the cylinders, and could cause missing and fouled plugs.