1985 120 Johnson VRO V4 - Compression

[NDIPGUY]

Recently purchased an oversized Johnson 120 that was supposed to have been rebuilt 4 years ago, VRO removed. Mounted on a 17' bow rider.

First thing replaced complete impeller housing and hoses. Compression is not within the 10-15% range. Marine Mechanic reading was 121 on both left side and 139 on both right side yet motor still runs very well. Would carb cleaning help out at all?

What would it typically take to correct this? New ring kit? What risk am I taking by running it this summer as is?

Side story: Lost stainless prop on test run (MAX 40 MPH) before I was able to take an RPM reading. Replaced with Aluminum prop with same 23 pitch but now I'm at (MAX 34 MPH) at 4,500 RPM. Considering a replacement to 21 pitch as it is much slow to come off plane now. Not sure of the standard RPM range for this engine...5,500 ?

Am I correct in assuming that I should be able to attiain 50 + MPH with this engine if compression and propulsion were corrected?

[Monte1961]

Two cycles engines make horse power with rpms. You motor should turn 5000 to5500 rpms @ wide open throttle. I would double check compression when engine is warm. If no change, I would remove head and inspect head and cylinders. Good luck and post back.

[wilde1j]

5800 @ WOT for that motor. I had an '85 140 (same block) for years.

[archcycle]

139 x 0.15 = 20.85 and 139 - 20.85 = 118.85 so you are within 15%. Or you can go the other way with 121 * 1.15 = 139.15. You're right there on the edge of 15% but it's not like 15% is some magic rebuild trigger number.

Try running some Seafoam cleaner through it and see if they even out a little more. There's a good decarboning writeup in the top secret files.

Carb cleaning won't help your compression. If both cylinders in that bank are exactly 121 I'd hesitate to blame the head gasket, but it's only a $30 part if you feel like throwing money at it and hoping.

[NDIPGUY]

Thanks all. I will SeaFoam the heck out of it as per Sticky and reply back. If it doesnt give me those RPM's at WOT I'll change the prop pitch and see the difference. Still if not I suppose I'll replace the head gasket. Part is cheap but is it heavy labor?