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I have a new old stock sherpa "O" series petrol engine 1991 that I installed 3 years ago. I have a misfire on cylinder 1 (detaching spark plug lead makes virtually no difference to the running engine). I've have been around the block a few times switching plug leads, dizzy cap, plugs etc and cant find any issues on the ignition side of things. The ignition timing has been checked with a timing gun, the spark is consistent and very strong with a spark tester. The mechanical valve timing is correct according to the manual.

No.1 cylinder is definitely getting at least some fuel. I turned the engine over without ignition, whipped the spark plug out and applied a naked flame to spark plug port. A high pitched flame shot out.

So eventually I figured it must be the valves except that the compression is great at 150 psi and the same as the other cylinders. Am I right in thinking that the good compression means that there is negligible valve leakage and are generally working ok?

I should note that the van often is used for only 10 minutes at a time and the engine often therefore doesn't even reach running temperature. However when the engine is warm the van runs about just fine even on 3 cylinders though it does labour up hill and fuel economy is bad.

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  • To check valves, you need to do a leak down test which is quite different from a compression test. You can also apply air pressure to the cylinder, then listen at the intake and exhaust for the noise of air escaping. Some air will go by the rings, but not much. Commented Nov 22, 2023 at 13:46
  • @Pᴀᴜʟsᴛᴇʀ2 Thanks for your reply. As much as Id like to do a leak down test I don't have the equipment. Can you think of a single reason why air would pass in a leak down test if the compression is good. I mean - good compression means no leak , No? Commented Nov 22, 2023 at 15:08
  • Realistically, I don't think your problem has to do with compression or leaking valves. I think it will fall back to ignition considering everything you've stated. Commented Nov 22, 2023 at 15:16
  • Remove spark plugs, add a teaspoon to tablespoon of motor oil to each cylinder and run the starter to distribute oil into compression rings. Check compression again. All cylinders, in theory with oil helping with piston ring sealing, should have higher compression numbers if valves are not worn.
    – F Dryer
    Commented Nov 22, 2023 at 18:06

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