this may be a long shot. but engines that have sat for a long time can have issues with the rings not holing as tightly as they could. the trick and old motorcycle mechanic tough me is to pour Marvel Mystery (MM) oil in the cylinders and let the rings soak overnight. this will loosen any old oil that may prevent the rings form making good contact with the cylinder wall.

The engine I bought for my SD sat for years and had poor compression when I first tested it. after a night of MM oil it had much better compression.

ATF can be used if you have no MM oil. I've used ATF mixed with Diesel Purge before as well.

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