compression leak ?

[turtles11756]

hello, when doing a compression test on an old 1968' merc 500 the compression in all four cylinders is going up on my guage to about 115 psi then it quickly drops off to 50 psi. when i stop turnig the motor over. the guage holds good on all the other motors i have tested but this motor the compression must be leaking back past the rings ?. it runs so at $75..00 i'm gonna haul the big black anchor home. any ideas what this might be and try to get it to hold ? thanks, i know this is the place for the best help

 

[lckstckn2smknbrls]

Re: compression leak ?

IIRC On my gauge it will hold the reading until I press the release button.

 

[sschefer]

Re: compression leak ?

The amount of pressure you keep on the gauge depends on where the pistion stops when you quit cranking the engine. If the piston is down when you stop your compression is going to go out the exhaust port.

Use a compression gauge that will store and record the maximum pressure in the gauge and hold it until you press the button on it's relief valve.

In a round about way, what you're experiencing is called leak down but that test is done in a completely different manner so the results your getting right now are not accurate as far as a leakdown test would be concerned.

 

[j_martin]

Re: compression leak ?

Long story short. The check valve in the gauge is failing. <--(That's a period)

hope it helps
john

 

[turtles11756]

Re: compression leak ?

just brought the motor home so i'm going to go thru this again. but it's only this motor that the compression leaks off on the gauge so the relief valve must be good .it will leak down when i stop turning the engine and the piston is down on my other motors and then will hold tight when i turn motor over again and piston is right. this motor drops off the compression every time. thanks i will keep updated

 

[sschefer]

Re: compression leak ?

Turtles, your comparing a two stroke to a four stroke. You ain't got no flipping valves in a two stroke!

 

[lckstckn2smknbrls]

Re: compression leak ?

Turtles, your comparing a two stroke to a four stroke. You ain't got no flipping valves in a two stroke!

The relief valve is part of the compression gauge.

 

[turtles11756]

Re: compression leak ?

well, turns out j_martin was right it was the gauge. used the same gauge when i got the motor home and it held tight at 130 ,132, 130, 112 psi.