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Low Compression and the Mercury 7.5 h.p. Still Starts and Idles??!!

B

boatdokker

Regular Contributor
This is a 1976 Mercury 7.5 h.p. 2 cyl. outboard that read about 50 to 55 p.s.i. on each cylinder. Seems to start and idle o.k. Eventually, I guess it will ultimately fail, and I’ll have to repair or overhaul the motor. Thinking about going to a 1:24 or 1:16 oil to fuel ratio for this motor.

Here’s the video I posted online.

1976 Mercury 7.5 h.p. Outboard Video at…https://youtu.be/FG1udcJa-wI
 
Your compression gage may be lying to you--those are tiny cylinders and auto gages are designed for MUCH larger ones. It sounds like she runs great, so have fun!

Need a winter project? You might consider hone and re-ringing her some time.

Jeff

PS: Those 7.5/ 9.9 hp Mercs are damn good motors that have never been fully appreciated.
 
Remove plate on side of block.--Look at pistons and rings.-----Just 3 screws.-----Cost you the effort to make a gasket from a cereal box as well.------How old is the impeller ?-----And you need water about 6" to 8" above the impeller in your drum.
 
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Stick with 50:1 - more oil will make the engine run worse and cause excessive carboning and ring sticking. If you want to clean the internals buy some PowerTune from your Mercury dealer and follow the instructions on the can.
 
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