Logo
FULL SITE MENU

1981 J5 compression test results.

D

Dr. Kailey

Member
Seems to be down on power, idle fine just soft at WOT.

Both cylinders were about 90 psi, shot some oil in and the top came up to 120 and the bottom 110.

Time for a re-build?
 
Your cylinders are within 10% of each other. The 5 hp motors are not high compression motors your 90 is fine. The basic design of your motor is straight out of the 60s.
 
Don't have a spark tester but there seems to be a good spark on the plugs when I pull it over. I'm quite sure it's running on both cylinders.

Any other ideas? Last year it would plane me and my little 10' Jon boat no problem, this year it won't at all. And no, I'm still 160 lbs. :p
 
The prop had a few dings so I cleaned it up and sharpened it a bit but it didn't make any difference. Any other ideas?


prop.jpg
 
johnnygjr said:
Butterflies should be horizontal at wide open throttle in relation to the carbs
Click to expand...

Butterfly is wide open at full throttle.

I really can't find anything wrong with this motor, I've just run two tanks with Sea Foam and didn't notice a difference either.

I did get an OMC service manual for it and they state the cylinders should be within 15 psi of each other but don't give a range. I'm starting to think I need a rebuild with fresh pistons and rings.
 
The plugs should be a toasty brown you said they are dark it may be running too rich? What elevation are you running? How big is the boat and how much weight in the boat? What is the pitch of the prop?
 
Way back there somewhere, somebody said to do a cylinder drop test. Have you?

As you found out, the mfr did not specify a compression reading.

A 5hp is borderline for planing any boat. Lots of factors can influence that borderline.
 
kimcrwbr1 said:
The plugs should be a toasty brown you said they are dark it may be running too rich? What elevation are you running? How big is the boat and how much weight in the boat? What is the pitch of the prop?
Click to expand...



These plugs were run quite a bit, look OK to me, no?




DSCN6073.JPG




DSCN6074.JPG



DSCN6074.JPG




As mentioned, it's the same small 10' Jon boat I had last year and I'm still 160 lbs. Am running at about 700' in good air so that's not a factor. Last year it would get right up on plane and go pretty quick, this year it will plane with some weight in the front of the boat but when it does it's noticeably slower and just doesn't sound like it's winding up like it used to, same stock prop. I'm going to borrow a buddy's tach, manual says it should turn 4500-5500 rpm.

fdrgator said:
Way back there somewhere, somebody said to do a cylinder drop test. Have you?

As you found out, the mfr did not specify a compression reading.

A 5hp is borderline for planing any boat. Lots of factors can influence that borderline.
Click to expand...


Did a drop test, it's definitively running on both cylinders. I run only ethanol free gas with OMC XD50 at 50:1 ?and a little Stabil. I don't think the carb is gummed up but a buddy said I should pull and clean it.

Any other suggestions?
 
Last edited:
Check fuel system from pickup tube,inside of tank to carb. Bulb for firmness while running, Have you pumped primer bulb while she runs? pulled fuel hose from carb and pumped primer,inspected flow? Poured some fuel into clear container,sit overnight,inspect?
 
johnnygjr said:
Check fuel system from pickup tube,inside of tank to carb. Bulb for firmness while running, Have you pumped primer bulb while she runs? pulled fuel hose from carb and pumped primer,inspected flow? Poured some fuel into clear container,sit overnight,inspect?
Click to expand...

Thanks. It's got the internal tank, I pulled the canister style fuel filter and back flushed it into a clean paper towel, it was clean.
 
Get a prop with less pitch and go fishing your not that far off. By all means go through the carb and make sure the sync is correct after you mount the carb!
 
kimcrwbr1 said:
Get a prop with less pitch and go fishing your not that far off. By all means go through the carb and make sure the sync is correct after you mount the carb!
Click to expand...

It doesn't plane my small 10' Jon boat like it used to...

kimcrwbr1 said:
Also replace the fuel lines so you dont stuff up the rebuilt carb.
Click to expand...

Good idea.

fdrgator said:
Crankshaft main bearing seals
Click to expand...

Was wondering about that also, there's a bit of fuel under the flywheel...
 
Oughta give it a prize for a 4.5hp planing a boat. But if it did it before, I suppose it should still do it now. Any possible change to the boat?
 
fdrgator said:
Oughta give it a prize for a 4.5hp planing a boat. But if it did it before, I suppose it should still do it now. Any possible change to the boat?
Click to expand...

Thanks, thought about that also. No changes to the boat, my weight, tilt angle, air conditions etc. etc.

It was no screamer but it would get up on plane pretty easy...
 
Back
Top