The Stovebolt Site Forums The Shop Area Engine Shop Random dead cylinder?

Hey bolter's. Ive got a really strange issue going on, thats stumping me for the most part. I have a jobmaster 261 i6 engine, and for some reason cylinder 5 has decided it doesnt want to work anymore, ive tried testing about everything i could with the tools i had access too as well as some pm, set points, set valves hot, cab adjusted right. All vac leaks sealed up and fine, new cap & rotor. Plugs and plug wires. Gapped to .035 as the manual says. Doesnt run well unless its 8° advance in base idle timing. But other than that it runs really strong, i have spark, fuel and air, for cylinder 5, it just doesnt light off, ive tried swapping plugs with another cylinder, and swapping plug wires, still no change. Cyl 5 is still dead. Valves are properly set to spec. Could use some wisdom here or things i havent thought of i should try? Thanks .

Low compression,( worn out cylinder walls ), burnt valve ( leaking = low compression ) .

Have you performed a compression test on all cylinders and that one in particular? This should be your first test.

Since it doesn't appear to be a blown headgasket or cracked head, I'd suspect a broken piston ring(s) or even a hole in the piston top. It could even be a stuck or broken exhaust valve spring or a burned exhaust valve causing the cylinder to have little or no compression.

If you don't have a compression gauge, go to Auto Zone or Advance Auto Parts. Both stores will loan you one for free. (pay the deposit and return the tool when finished for full refund)

Check your oil for coolant and/or gasoline. Gasoline means its getting into the crankcase past the piston. Coolant means there is an internal headgasket leak.

Run a power balance test. If you aren't familiar with that one drop me a PM.
Jerry

I think a compression testor is one of the finest tools a back yard mechanic can have in his tool box. That and a vacuum guage. Do a compression test. Its easy, cheap and its low hanging fruit. If no compression, re do the test but this time, give a couple of good squirts of any kind of 30 wt oil and re do the test. Lets see if this is compression related or ignition related.

Is the #5 spark plug wet with gas?
A compression test will only tell you what you already know. No compression in cylinder #5.
My compression tester has a quick disconnect in the hose which just happens to have the same male air fitting as my air compressor hose.
I turn the pressure from my compressor way down with a regulator and apply air pressure to the spark plug hole.
With the piston at top dead center with the valves closed, listen for air escaping though the carburetor, exhaust pipe, or into the crankcase.
That will indicate bad intake valve, bad exhaust valve, or bad rings respectively.

I will try and get a compression tester, it was strange, thursday it ran like a top. Like i had more power out of this thing than i thought it would ever have. Ran like a sewing machine. Friday morning its got a dead cyl. Ill update once i get my hands on a compression tester thanks!

Good one. I forgot about that.

I found a hung up exhaust valve on a '64 Galaxie that way. Air was blowing out the tailpipe.

Be careful on testing a cylinder with a piston anywhere but bdc. Even better, drop fan belt so no fingers get sliced.