TOPIC: Low Compression 1960 - 75 HP Johnson SuperSeahorse

So. Got my Dorsett all restored. Focusing on the engine now.

I replaced all of the fuel lines, they leaked on the test run.

Tried running it last week, starts, barely idles, runs rough, does not rev, cylinder 4 not firing.

Checked the manual. It says, first thing do a compression test. Here are the results:

Cylinder 1 = 82
Cylinder 2 = 80
Cylinder 3 = 82
Cylinder 4 = 75

Questions:
1) Is this acceptable compression?
2) What causes the lower compression in #4, rings? Scored cylinder? Reed valve?
3) Can the engine run OK with this lower compression in #4?
4) What else could cause the rough running condition?
5) If I do need to correct the compression problem, can I remove the piston re ring and hone just the one cylinder without doing the major overhaul?

Any other comments or recommendations?

HELP!

Thanks,

Mike

The numbers are low, but within 10% of each other. Using another compression gauge may give different numbers, as they vary greatly in quality, so you want CONSISTENCY in your readings. Cylinder four not firing would be one reason for its reading to be a bit lower. Solving the #4 cylinder issue may fix the running issues. Get it running on all four and recheck the compression readings.

Mike, I ran a '63 75 hp for Decades with those same numbers. My service manual states that number 4 will give trouble before the other cylinders for some reason. The old motor will idle a tad rough ,but will do satisfactory. If you go to the trouble to rebuild 4,you just as well do them all. Check the usual stuff carb, dist etc.That can help it a lot.

Mike,
How long has it been sitting since it ran last? It may be that the rings are stuck. I know my '67 80hp had lower readings but hadn't been run in forever so I sprayed Marvel mystery oil in the cylinders, hand turned it over a couple times and let it sit for a few days. Once I fired it up I made sure I used Seafoam in the first couple of tanks of gas and the compression readings went from the mid 80's to 95 across the board. I haven't checked it again so they may be closer to 100 now. My '59 Fat Fifty was in the same range as yours and it ran fine. I only changed it because I wanted more power.

I suspect you may have a bad wire to number 4 since the others are firing. Does the magneto cap look good inside at number 4?

Have you rebuilt the carbs and fuel pump? I almost always rebuild both if the motor has been sitting for many years and I have no info from the previous owner as to their condition.

Sorry don't know the whole back story on your motor so just some quick stuff off the top of my head.

Bob

Thanks everyone! All encouraging information. I'll start today with ignition system tune up and go down the list from there.

I've heard great things about the sea foam. Will definetly use it.

The fuel has been sitting since September 14, so, I'll change that out.

I'll check back.

Thanks, Mike

Use Seafoam spray "Deep Creep" High idle and start spaying the carbs a good soaking and then flood it out until it quits. Tilt up, pull the plugs, and soak the pistons best ya can. Let it sit over night, tilt down, spin it to blow out excess, plugs in and fire it up. Gonna be a lotta smoke, but thats OK. Carry some spare plugs and get it wet. Might foul out a plug. I de carbed my old motor and went from on plane at 3000, down to 2500 and still held plane.

Bob:

You nailed it! I tested the plug wires with a battery charger and a multi meter. wires 1,2&3 all passed 11 volts. #4 wire only passed 4 volts. So I remade the wire with the old ends and a new piece of wire and it passes 11 volts now.

Test firing later today.

Thanks,

Mike

Your compression test looks good to me all four are very even.

Excellent Mike, I hope the refire went well, let us know.

Bob

You ARE using copper core plug wire, right? Automotive graphite core wire doesn't work for long and should be avoided in old outboards.