1984 johnson 75 hp compression

[flyingscott]

Sounds ok Definitely do the link and sync do the idle after it is warm. I think your idle is always going to be higher than a prop model because of the jet. As far as gas mileage it will be bad just the nature of a jet.

 

[JoshKeller84]

Believe i found the cause of the high idle. One of the carbs wasnt completely screwed on. Here it is now in the tank.

https://youtu.be/Ok6lUfnjaBU

 

[flyingscott]

that sounds good.

 

[Bosunsmate]

yeah thats much better, lucky you found that as otherwise you would be running lean and thus hot and possible:flame::flame:
Do your final adjustment on the water when its under proper exhaust back pressure. I think it can probably drop a bit more.
Make sure in your lync and sync that the butterflys are all properly closed

 

[JoshKeller84]

I ran it for a half hour and let it come up to operating temp. Took compression and got 118, 118, 109. Is that to be concerned with?

 

[Bosunsmate]

I ran it for a half hour and let it come up to operating temp. Took compression and got 118, 118, 109. Is that to be concerned with?

Nope thats fine. Just retorque the head and enjoy. (Dont over torque whatever you do)

 

[JoshKeller84]

Any ideas what would cause the loss after warmup? Is this motor still going to produce full power will the compression a little lower on that cylinder?

 

[Bosunsmate]

Any ideas what would cause the loss after warmup? Is this motor still going to produce full power will the compression a little lower on that cylinder?

Did you retorque it?
Your power will be almost the same as if they were all 120psi. That slight difference is negligible.
Your power loss will come about in your jets impellers condition and whatever inefficiencies you have in your hull/ wind resistance and loading

 

[JoshKeller84]

yessir, i did retorque them. Just seems weird that they were all 120 when i first put the head on, now its down on the bottom one again. All the other motors ive worked on have been pretty much dead on even. I have 3 ring sets, im debating honing and reringing all 3 while i still have the trim pieces off the motor. Or would I be just as well to run it as it is?

 

[flyingscott]

I don't think you need to worry about the compression it's fine it's within 10%. The loss of compression in the bottom cyl is probably because of the wear in that cyl, ring gaps could be a little large. The only way to fix that would be to bore it out and a complete rebuild and i don't think you need that.

 

[JoshKeller84]

so you wouldnt recommend honing and reringing all the cylinders before i fully put it back together, and just run it as is?

 

[flyingscott]

I would run it as it is You are not going to gain much by doing it. But if you are bored and have nothing else to do go nuts. One word of caution make sure all your pistons are from the same manufacturer as the rings. If they are not they not don't do it you also don't need to re torque the head until you run for at least 5 hrs.

 

[JoshKeller84]

sounds good, i'll just take it to the river and run it a bit before i decide. if I decide to play some more with it, i'll likely just have it bored .020 over and start fresh.

 

[Bosunsmate]

If you want more power you aint going to get much more by reboring this you would be better off doing it by selling and upsizing to a 90 or 100hp.
I retorque after first heat up and cool down use always, thats when i find the biggest difference is and is what is recommended in most rebuild manuals

 

[JoshKeller84]

Not looking to get more power than what its rated per say, i just want a strong, and morely a reliable motor for my river boat

 

[JoshKeller84]

She screams! Took it for a test drive on the river. Let it run on the trailer for 10 minutes, then pushed off. Picked up approx 5 mph over my 50/35, and the hole shot is night and day. 28 upriver, and 33 downriver. Im thrilled. Anything else i should check and do as i run it more?


https://youtu.be/nHV0qnkdrp0

 

[JoshKeller84]

Motor ran good after running an hour. One observation - my twch wasnt working, and the voltage on my gps never came up from 11.9 volts. Rectifier problem?

 

[flyingscott]

sounds like a rectifier problem good job enjoy your motor