Correct compression for 1983 Johnson 75hp

[Spivy30]

I am looking at a 1980 16 foot Starcraft with a 1983 Johnson 75hp the boats in good shape and the motor looks clean and runs good but I just want to check the compression just to be on the safe side, so I am wondering what the correct compression should be. Also I am looking at the identical boat with a 1980 or 1981 Evinrude 100hp V4 which also runs good and is clean. Which is a better motor? I think the 100hp is only a few mph faster

 

[tashasdaddy]

Re: Correct compression for 1983 Johnson 75hp

i would go with the the 100hp if all else is equal. it is the proven reliable v4,

BUYING A USED MOTOR

overall condition, condition of the place you are buying it from, usually tells you how they care for things. compression 100 + psi, and within 10% of each other, good strong blue spark, no water in the lower unit oil. look for discolored paint, (sign of over heat at
one time, also scarred up bolts, as why. have them start it. if they fail to put it in a barrell or tank, or on muffs, or in the water, to start it they don't know what they are doing. best yet is to take it for a 30 min to 1 hour test run. also if remote controlled motor, the controls should come with it.

 

[kenmyfam]

Re: Correct compression for 1983 Johnson 75hp

As TD, V4 is a great motor.

 

[JB]

Re: Correct compression for 1983 Johnson 75hp

Well, I once replaced a crossflow V4 100HP with a nice little triple looper 70HP.

WOT was a bit slower than the 100, but fuel usage was about half.

In my opinion the 75 is a much better engine.

Compression: The purpose of a compression test in a 2 stroke is not completely the same as in a 4 stroke (car). 4 strokes have valve sealing issues that 2 strokes don't have. In the 4 stroke psi numbers are important, in a 2 stroke they mean less.

In 2 strokes the goal is to see if any cylinder is far below the other(s). The importance of psi numbers is in the comparison, not in high numbers.

In either of the above engines you want all cylinders to match within 10-15%. If that match is below about 80-90psi, try another guage and improved technique preparing the engine for the test.

Good luck, and again, the 75 is the better engine.

 

[Spivy30]

Re: Correct compression for 1983 Johnson 75hp

Thank you everyone for your imput I think I am going to go with the 75hp since the seller is more willing to go for a test run out on the lake.