Need some advise on GM 3.0- 140 Mercruiser Compression issue

[fastryker]

I have a question. I recently purchased a 1977 Marquis Riviera Tri Hull project boat. Powered by a 140 Mercruiser 3.0 GM Motor. The old motor was seized from not running for 15 years. I soaked the cylinders with marvels oil overnight, and it turned right over.Good new oil in motor. Compression test was front to back 120,120, 60, 40, thought maybe rings stuck. Then After replacing the fuel pump, points, rotor, cap, plugs ect. got it to run a few minutes, won't idle though, I'm sure carb needs rebuilt, Then the starter quit. Replaced with new style starter, ran a few minutes again and got motor warmed up. Checked compression again, 120,120,100,60.I have already put so much money into getting this thing going, I hate to give up on it, or should I. My question is do you think I should run with it and see if the rings and compression improves? Or go ahead and have the motor rebuilt while I have it out and I'm rebuilding transom. Money is tight for me but I hate to have to remove the motor again. Not sure what to do. Or maybe I'm hoping for a ring spring back miracle. Your opinions would be very helpful as I'm an A/C mechanic by trade, not a boat mechanic, lol. Thanks, Mark

 

[Bondo]

Re: Need some advise on GM 3.0- 140 Mercruiser Compression issue

I have a question. I recently purchased a 1977 Marquis Riviera Tri Hull project boat. Powered by a 140 Mercruiser 3.0 GM Motor. The old motor was seized from not running for 15 years. I soaked the cylinders with marvels oil overnight, and it turned right over.Good new oil in motor. Compression test was front to back 120,120, 60, 40, thought maybe rings stuck. Then After replacing the fuel pump, points, rotor, cap, plugs ect. got it to run a few minutes, won't idle though, I'm sure carb needs rebuilt, Then the starter quit. Replaced with new style starter, ran a few minutes again and got motor warmed up. Checked compression again, 120,120,100,60.I have already put so much money into getting this thing going, I hate to give up on it, or should I. My question is do you think I should run with it and see if the rings and compression improves? Or go ahead and have the motor rebuilt while I have it out and I'm rebuilding transom. Money is tight for me but I hate to have to remove the motor again. Not sure what to do. Or maybe I'm hoping for a ring spring back miracle. Your opinions would be very helpful as I'm an A/C mechanic by trade, not a boat mechanic, lol. Thanks, Mark

Ayuh,.... Welcome Aboard,... Odds are slim to none that the compression will come back, but, Hey,...
Try it...

A leak-down test might tell ya whether it's rings, or valves...

 

[fastryker]

Re: Need some advise on GM 3.0- 140 Mercruiser Compression issue

I'm not real sure how to do the leak down test. Could you please explain? Thanks Bond-o, Any help or good ideas are appreciated. as having this motor rebuilt will really set back my project by months. Unfortunately, it may have to be done.

 

[Bondo]

Re: Need some advise on GM 3.0- 140 Mercruiser Compression issue

I'm not real sure how to do the leak down test. Could you please explain? Thanks Bond-o, Any help or good ideas are appreciated. as having this motor rebuilt will really set back my project by months. Unfortunately, it may have to be done.

Ayuh,... I cheat,...

Insteada doin' the percentage, by gauges,.....
I use the tag end of my compression tester, which hooks to the gauge with an air quick-connect...
My shop air uses the same connectors,...
I hook an air hose to the connector, 'n turn on some air, the Listen....

Bring the cylinder to Tdc, 'n you can hear whether it's the rings, or which valve...

 

[fastryker]

Re: Need some advise on GM 3.0- 140 Mercruiser Compression issue

That's freaking brilliant! Yet so simple, I wonder why I never thought of that? You missed your calling Bond-O, you could be working for NASA bro. Thanks for the info. I'll give it a try

 

[Scott Danforth]

Re: Need some advise on GM 3.0- 140 Mercruiser Compression issue

Bond-o, dont you have to pull the schroeder valve on your compression tester? I do on my whip line when I use it for holding valves up while changing springs/seals with the heads on, or when doing a leak test.

 

[Bondo]

Re: Need some advise on GM 3.0- 140 Mercruiser Compression issue

Bond-o, dont you have to pull the schroeder valve on your compression tester? I do on my whip line when I use it for holding valves up while changing springs/seals with the heads on, or when doing a leak test.

Ayuh,... On my tester, the gauge holds the pressure, the whip end is an open hose, 'n connector....