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Discussion Starter · #1 ·
When a piston is at TDC I attach the gauge and it reads 50 or 60% which is pretty bad, but if I keep turning the engine past TDC, the reading gets better, like down to 20 or 15%. I thought the valves are all closed at TDC and that would give the best results. Is that incorrect? Compression is normal around 120psi.
 

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Have you turned engine 1 full turn to TDC for that cylinder again?
Also if you use the timing mark as reference for TDC that is not correct as white engines do not have a TDC mark but instead mark the assembly point which is not TDC.
TDC can be found easiest by using the leak down tester hose on the #1 cylinder and turn engine over until you feel compression out of the hose. Remove the hose and then use a rod or wooden dowel (wooden clothes hanger cross piece) and turn engine by hand until the rod comet the furthest and then begins to fall, then back up a little. Now you have TDC #1. When you finish with #1 then go in firing order using wooden dowel (1-2-4-5-3) as the next i firing order goes to TDC it will be on compression stroke.
 

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Discussion Starter · #3 ·
Yes I'm using a dowel to find TDC. Do these engine have some valve overlap where one or both valves as still open at TDC?
 

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Yes I'm using a dowel to find TDC. Do these engine have some valve overlap where one or both valves as still open at TDC?
Overlap occurs only after exhaust stroke and in the beginning of intake stroke.
 
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