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I seem to be having a problem with #4 cyl on my 38 120 coupe. It doesn't seem to be firing, If I pull the plug wire off it seems to make no difference in the engine performance- RPMs don't drop at all where as all the others do. I have spark (changed both wire and plug) and the compression is within 5 lbs of all the other cylinders. I sprayed some carb cleaner around the intake manifolds and no change. I pulled the valve cover and everything looks to be OK while running. I am now running short of things to try next


ANY IDEAS?

Try plugging the drain line from the intake manifold for a minute and see if it picks up.

Drat - no difference

A thought. IIRC, a few years ago someone posted a photo of a manifold where a hole had developed in the area of intake manifold below the carb where exhaust is directed when the riser valve is closed. On that manifold exhaust was able to get into the intake side. Don't remember the symptoms or know if a leak like that would only affect one cylinder unless maybe it was directed into one runner.

You say the compression is good, the other 7 cylinders are obviously receiving fuel/air mixture and firing normally so that would make me suspect an ignition failure - bad spark plug, distributor cap problem, etc.

Have you checked the intake manifold vacuum.

Vacuum is good - changed wire ,plug, cap and even a known good dist. Very Very strange.

As you listen to the tailpipe while the engine is idling, is it obvious that 1 cylinder is not firing?

It does make a little puff puff sound but it always did that. I'm stumped as is a mechanic friend of mine

It would seem to me that the fuel charge is seeping away before it is fired by the spark plug. So I would be inclined to bring #4 up on the compression stroke and hold it to see how long the compression holds. If it dissipates faster than another cylinder, than a valve, head gasket, or intake gasket...Not Fun! I would check the intake manifold and gasket first. As odd as it sounds, a part of me wonders if there is a piece of junk in the intake manifold!Ernie

One more thing I noticed that I forgot to mention. While looking at the valve lifters with the engine running one of the valves and it lifter (on #4) were rotating a lot faster than the all the others. Does that mean anything to anybody?

I really really don't want to pull the head on my beast.