I ran a compression check on my 84 D150 slant, and got 100-110 on all cylinders. My book states 100 psi is MINIMUM pressure. Is this engine tired?

Try putting some oil in the cylinders...if the pressure goes up any, your rings are on the way out. Either way, I wouldn't loose any sleep over the readings.

Yeah, Eric's right. There are too many variables to rely on the actual numerical readings. Gauge quality/precision, whether you remembered to block the throttle and choke fully open before doing the test, battery, cable and starter condition (cranking speed), oil viscosity, etc. Look for consistency across all cylinders.

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