1982 7.5 hp Johnson compression numbers?

[dfrants]

Guys, just picked up a 1982 7.5 Johnson 2-stroke. It starts and idles great. Trolls down soo slow its impressive, but it doesn't have have the full speed power it should. I did a compression test pulling the starter cord 5 times on each cylinder getting 90psi on top and 95psi on the bottom. Is this an acceptable amount?

 

[Crosbyman]

likely low a bit and equal which is desired ....but....it runs good, idles great...how much faster do you need to go and at what cost...to improve PSI's.

7.5s go way back to the 50s ... great fishing motor not so good for skiing

 

[racerone]

The 90 / 95 is very good.-----The 1980 and newer are a wonderful motor.-----Is it running on both cylinders ?-----Pull one lead off the sparkplug.---Try and start it .-----Repeat with other lead.----Anything broken on carburetor linkages ?

 

[saltchuckmatt]

Personally I stop using them just under 90 lbs....but what size boat you pushing?

Those 80's motors are great and worth rebuilding or finding a good used power head.

 

[racerone]

Compression is so confusing.----Reading of 90 is fine.-----Find the real reason for the lack of speed.

 

[ct1762@gmail.com]

Personally I stop using them just under 90 lbs....but what size boat you pushing?

Those 80's motors are great and worth rebuilding or finding a good used power head.

even a reading of 75-80psi wouldnt make much difference on the top end if everything else is in good shape

 

[Crosbyman]

well seems comp #s are ok but lots of variables to check.
how's the prop/boat match ..... linkage.....sparks..... etc... .
can you measure RPM's at wot ?

 

[ct1762@gmail.com]

is the throttle plate opening 90 degrees at WOT? no need to look down carb throat just observe roll pin on starboard side of throttle shaft. stick a toothpick in it if you really have to

 

[dfrants]

Spark is good, throttle linkage is ok and appears to open the carb wide open...but, checking the prop...with the motor in neutral and spinning the prop by hand it seems not to spin as freely as it should, it turns easy for 3/4ers of a turn then there's some friction. It appears as though someone replaced the prop shaft seals at sometime. I drained the gear lube, no metal shavings, and refilled with new lube.

 

[dfrants]

Checked the high speed jet and that was clean and clear.

 

[racerone]

Prop should spin with your little pinky finger.-----Perhaps lower unit was full of water at one time.-----Here I would take lower unit apart in seconds to inspect bearings and gears.----Perhaps lower unit is binding up , thus the lack of speed.

 

[dfrants]

Finally some positive results...I hooked up a tiny tach and discovered that the throttle linkage is set too far and going beyond wot. After finding wot if recorded 4700 rpms with the hood on...but gained 200-300 rpms with it off, I guess I need to find a GT 7.5 hood with louvers vents like a GT 175? Also, the more that I've been running it, the easier the prop has been spinning freely. Thanks guys for all the help!!!

 

[saltchuckmatt]

Your analogy doesn't make sense.

 

[dfrants]

Bad choice if words, my context was that I finally got a positive result from the motor after all of the wrenching I did on it...my bad.