Johnson 70HP 1978 Triple - Reasonable Compression?

[mainer4]

Hi, can anyone tell me if 105PSI would be considered a reasonable compression for this motor. It sounds low to me, my current motor is a Johnson 1970 40HP Big Twin and has 145PSI compression for both cylinders... it is a low hours motor, obviously. I have a line on this 1978 engine for my 1974 16' Starcraft. I know it is a gamble to buy such an old engine, but then again, my previous engines were a 1963 28HP Evinrude and a 1968 33HP Evinrude. Both of those big twins also had compression values in the 130PSI range. So hoping someone out there has a triple and has checked their compression recently

Is there anything I should be looking out for for this year motor - no power lift on this motor.

 

[lmuss53]

Re: Johnson 70HP 1978 Triple - Reasonable Compression?

I know the twins run higher, my 55 has about 150 in both cylinders. The 1985 70 on my Sea Nymph had about 120 across the board when I bought it. I don't think 105 is terribly low if it is consistant across all three cylinders, especially if the motor has not run in some time. It will come up a little if it hasn't been being used. If every thing else looks good the compression at 105 wouldn't scare me off of this engine. Others may have other opinions though.

 

[watergecko]

Re: Johnson 70HP 1978 Triple - Reasonable Compression?

105 is fine, as long as all 3 are within 10 percent.

 

[5150abf]

Re: Johnson 70HP 1978 Triple - Reasonable Compression?

If that was cold it is a good number especially if it hasn't run for awhile and as others have said, even is the rule, if it si 105 cold it shoudl get to 120+ hot, do a decarb and it my go higher than that.

 

[eavega]

Re: Johnson 70HP 1978 Triple - Reasonable Compression?

My 76 70 HP runs 110, 110, 105 cold. A decarb did nothing to bring those numbers up. I've never tried compression after the engine is warmed up so it may increase somewhat. What I can tell you is that it will run at 5200 RPM WOT and push my 15' tri-hull (weight is in the neighborhood of 800 Lbs I believe) at 32 MPH on flat water with no wind. As a previous poster said, the main thing is that all three cylinders read within 10%. There is too much variation on techniques, and even gauges to say definitively "this is good" or "this is bad". Best you can do is to use the same gauge and be consistent in the way you take measurements, and have all cylinders within 10%.

Rgds

Rgds

 

[Chinewalker]

Re: Johnson 70HP 1978 Triple - Reasonable Compression?

Also, are those the numbers you got from the seller or are they with your gauge. Not all gauges are created equal, so unless you measure both motors with the same gauge, I wouldn't put too much stock in the dissimilarities in readings. Ideally, I'd like to see an OMC triple punching quite a bit more than 100-105. More like what your Big Twin was reading...

 

[sutor623]

Re: Johnson 70HP 1978 Triple - Reasonable Compression?

Also, are those the numbers you got from the seller or are they with your gauge. Not all gauges are created equal, so unless you measure both motors with the same gauge, I wouldn't put too much stock in the dissimilarities in readings. Ideally, I'd like to see an OMC triple punching quite a bit more than 100-105. More like what your Big Twin was reading...

Right! I have had compression testers be 20-30PSI off from each other....

 

[mainer4]

Re: Johnson 70HP 1978 Triple - Reasonable Compression?

Thanks to everyone who replied, gives me a confidence boost to move forward with purchasing this $200.00 motor. My main motivation in purchasing it is to save fuel $$ and Don Kellog assures me that with the triple I will definitely save money over my thirsty 40HP Big Twin. Too bad though, that twin ran and idled the best among the three that I have owned, although the 33 I had was pretty good, but this one idles so nicely.