1978 Mercury 140 low compression

[gmcgough]

I inherited a 1978 Mercury 140HP 2 stroke motor. (inline 6 cylinder) It will idle and run just fine when hooked up to a hose in my driveway but has zero power and will not stay running under load while in the water. All cylinders are showing 68-70 PSI. Is this purely due to low compression? Or is there perhaps another issue? I believe the motor was sitting for the last 15 years or so.......I dont have any other history. I did replace the plugs, cleaned all the connections and rebuilt the carbs. Fuel is new as are the fuel lines. Is the low compression due to stuck piston rings? Also.......It will only start if I prime the carbs with gas or starting fluid.......but runs and idles fine once its going. The choke works fine. Any suggestions would be greatly appreciated. Thanks.

 

[Chris1956]

It is very unlikely that the cylinders are all at 68-70 PSI. It is more likely your compression gauge is bad.

If the cylinders have the same compression (within 15%) the motor is good.

I would check the link and synch. That motor should be a distributor model with battery powered CDI ignition. Idle pickup timing should be 4-6* BTDC and max spark advance s/b 21*BTDC. Make sure the carbs are opening all the way, when the throttle is wide open.

Those motors need lots of choke to start when cold. I doubt you could flood it.

 

[jimmbo]

I also second the idea of a wonky Compression Tester Gauge. I don't think it would run at all with those compression numbers. I guess if someone ran it without oil in the Gas, all cylinders could be pooched, evenly.
A couple cans of Merc Power Tune, or BRP Engine Cleaner will help free up any stuck rings.
Mix TCW-3 at 50:1 for that engine.
FWIW, In 1984, I had a 90hp of that model yr.

 

[racerone]

Open low speed mixture screws 1/8th turn at a time and test at the lake.-----You MUST install a new impeller.