1973 Johnson 115 Compression question

[wv_transplant]

I have a 1973 johnson 115 outboard motor. Im getting 90 lbs compression on all 4 cylinders. Is this good? What should the compression be for this year motor? The motor is clean and looks in excellent condition. It starts up quick. I was told to check the compression with all the plugs out and with the carbs wide open. I would really like some advice from some of you local pros here on ihub. Thanks for reading and have a great day

 

[adoseth]

Re: 1973 Johnson 115 Compression question

My 1981 johnson 115 is around 120 (+-) 5 so that is kinda low, perhaps someone else knows the spec limit, i am still waiting on my repair manual to come in

 

[boobie]

Re: 1973 Johnson 115 Compression question

There are really no specs on compression. The OMC service manuals say no more than 15 psi between cyls. You may want to do a good decarb on it to see if you can bring it up. Other than that, how's it run ??

 

[wv_transplant]

Re: 1973 Johnson 115 Compression question

There are really no specs on compression. The OMC service manuals say no more than 15 psi between cyls. You may want to do a good decarb on it to see if you can bring it up. Other than that, how's it run ??

I have not ran it very long yet. I just got it and want to check it out real good before i put the boat in the water. I may try the decarb tomorrow. The motor has not been run for about 5 years

 

[JB]

Re: 1973 Johnson 115 Compression question

Relax, Transplant. There are plenty of things that could cause you to get slightly low readings that are not indicators of engine trouble.

It is about as unlikely as a blizzard in Miami that 4 cylinders would all go bad by exactly the same amount. What is most likely is that there was a flaw in the gauge or technique that repeated itself 4 times. Refer to the instructions in the Top Secret File ^^^.

If that were my 115, I would consider the test successful and move on.

 

[jbjennings]

Re: 1973 Johnson 115 Compression question

I also agree that your gauge is probably off. The 2nd best tell-tale to me is whether or not the motor idles nice and smoothly, and nice and smoothly LOW. Low compression is much more easily noticed at low speed. If it had low, but even compression, you likley wouldn't even know it at full throttle unless you'd driven the same hp and model of outboard recently. BTW, whether the throttle is open won't affect compression readings on a 2-stroke---a weak starter or starter battery would.
JMO,
JBJ

 

[wv_transplant]

Re: 1973 Johnson 115 Compression question

Thanks for the advice guys. My gauges are fine. Turns out that all the motor needed was to run a little after being alseep for about 5 years to loosen up the rings. I decarbed it and the compression is up around 120 across the board. Now that i know the motor is good i can replace the water pump and lower seals. I may go ahead and re ring it just for kicks. She is gonna push a 1964 21-ft starcraft holiday

 

[emdsapmgr]

Re: 1973 Johnson 115 Compression question

Those older crossflows ran fairly high compression. I would expect that 125 was normal for a fresh factory engine of that vintage. You are really close to that now, why re-ring at this point with those numbers?

 

[wv_transplant]

Re: 1973 Johnson 115 Compression question

Im restoring the boat and motor from top to bottom so i want everything new