Expected Compression for a 1992 Johnson 120 after rebuild?

[Drewski Canuck]

Bought a rebuilt Johnson 120, 1992, which apparently has been rebored and new aftermarket pistons. Compression test cold, with all other plugs pulled, was 142 - 145 range across all 4 cylinders. I am told the motor had 10 hours on it after the rebuild, so it would seem to be broken in.

Is that about what to expect? i had never seen a compression test with all other plugs pulled, and done cold. I always thought you want a warm up to get the best comparison to operating conditions.

Drewski

 

[scott craven]

Sounds better than average for that motor, anything above 120 per cylinder
is good, with all being even the best.

 

[Gunsmith]

My information tells me that way gives the most accurate compression test. Some older two stroke racing motors had to be cranked over at a specified rpm to give a good number tho I have not seen that for outboard motors.
My thought really tell me that it is just easier to crank over.

Bought a rebuilt Johnson 120, 1992, which apparently has been rebored and new aftermarket pistons. Compression test cold, with all other plugs pulled, was 142 - 145 range across all 4 cylinders. I am told the motor had 10 hours on it after the rebuild, so it would seem to be broken in.

Is that about what to expect? i had never seen a compression test with all other plugs pulled, and done cold. I always thought you want a warm up to get the best comparison to operating conditions.

Drewski

 

[trendsetter]

Bought a rebuilt Johnson 120, 1992, which apparently has been rebored and new aftermarket pistons. Compression test cold, with all other plugs pulled, was 142 - 145 range across all 4 cylinders. I am told the motor had 10 hours on it after the rebuild, so it would seem to be broken in.

Is that about what to expect? i had never seen a compression test with all other plugs pulled, and done cold. I always thought you want a warm up to get the best comparison to operating conditions.

Drewski

That's a good result. Compression usually starts around 125psi on those after a rebuild and gradually builds up to 135-140 as the new rings become seated.

Remember to use double the oil mix 25:1 for the first 20 hours and then switch back to 50:1. Also the head bolts should be re-torqued and another compression check done at the 20 hour mark.

Note, if the oil injection system is working then the fuel should be 50:1 for the first 20 hours. 25:1 if the oil injection has been disabled.

 

[Island Fish Lifter]

When checking compression, the main thing to look for is differences between cylinders. For example, if one cyl. tests at 120psi, and another tests at 105, then you probably have a problem. THe rule of thumb is no more than 10psi difference between cylinders.