Have a 37 flathead I want to do a compression test on .Will spinning the engine with 12 volts give me a false reading ?? Also what should the compression reading be ??engine is stock and was rebuilt at some point .
You are really looking for how much they vary, rather than a particular number, so as long as you don't burn up the starter you'll be fine.
It won't give you a false reading, but you only need 3 or 4 revolutions. By your post, assuming you have the stock 6 volt starter, there's no need to crank on it for a long time and possibly hurting the starter. And I agree with above, you're looking for inconsistency.
ball park numbers 100 on all is good 90 on all is good 100 on some 90 on some is still good 90 on some and 50 on another is bad
I had a 53 Merc that had around 70 on all cylinders and it ran great but took a quart of oil about every 500 miles. I agree with ActionYobbo on the range between cylinders being close is important as the number up to a point.
There is a proper way to get an accurate compression reading test. Start engine letting it at least get some warmth/ does NOT need to be HOT-just more warm than cold. Disconnect accelerator pump and block or hold throttle bores of carb wide open to make maximum air gets inside engine. Take and record readings——closer to each other is better than scattered up and down differences—-could be wrong about this next statement, but I seem to remember that 10% deviation from cylinder is the max difference allowed. To see if the readings increase,squirt a little oil in each plug hole and note any difference. An elevated readings using the addition of oil generally means the rings are sealing better than the readings using no additional oil.
All plugs removed, throttle wide open when testing. Compression does indeed increase with cranking speed. It shouldn't be very much, though. You can slow it down by using a run down 12 volt battery and a little trial and error..
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