1975 Johnson 115 high compression on one cylinder

**Gilera** · 12-24-2021, 01:46 AM

I got round to doing a compression test on my outboard today. It has spark issues and Ive not had it running yet. Previous owner said it was 120psi across all 4 cylinders. I found 120psi on 3 cylinders and 140psi on the other. I cannot find much about this, only about low compression on one cylinder. Does anyone know what would be potential cause for this?

I did pit about 1mL of 2 stroke oil in each plug hole just to keep it lubed while I'm troubleshooting but guessing that wouldn't cause it.

**Gilera** · 12-24-2021, 03:39 AM

Maybe I should just push forward and see what its like after after i get it running for a while?

**racer** · 12-24-2021, 08:33 AM

The oil could effect compression but a 1975 in good condition would have more than 120 psi if cranking above 250 rpm on a good gauge. It is possible you have 1 that is good and 3 that are low.

**FMP** · 12-24-2021, 11:10 AM

If you're up to it , remove all the tranfer bypass covers and through the ports check each ring for rebound with solid but gentle pressure straight on the ring. They could be gummed up stuck from sitting, or you might find a broken one or three. Try a good soaking with your favorite seafoam type overnight or two, check rebound again hoping none are broken , then run it clean and check. Don't run it with broken rings.
You'll likely need new tranfers gaskets but if you're real careful with a knife you might be able to reuse them for quick fire up.

Should add, since you don't know the history of the motor I'd also pull the reeds.
With the transfer covers off you'll also have a look at the skirts at least from that side.
A borescope down the plug hole will help.
'75 was a while ago, good motors deserve it.

**James45157** · 12-24-2021, 12:27 PM

Run hi test fuel as those motors had hi ring pistons best to back timing back a couple degs

**Gilera** · 12-24-2021, 07:14 PM

Thanks. I didn't realise these were a higher compression motor. What pressure should a healthy motor be - 140-150psi? I don't want to spend a lot of time and money on the motor if it wont give me a good few years of service. I'll try with another meter to compare but i suspect it is correct.

**James45157** · 12-25-2021, 09:45 AM

I remember one time on a 1973 135 v4 the motor had 160 0n all 4

**FMP** · 12-25-2021, 11:48 AM

Do what we suggest, if you don't want to know the condition now you'll know the condition when you're paddling back to the dock.

What I get from your post is a motor with ign issues and sketchy compression but you want it to last a few years without spending some time and money.

Just assuming the rings aren't broken, oiled up to 140psi , all 4 are whipped and with three that low it's Wiseco time with all seals and everything else. JMO

**bigbore** · 12-25-2021, 02:10 PM

Plugged relief valve might cause that/pull 1/4in. Hose off cylinder orfice n spray carb n choke cleaner in it/then do the same at the other end of the hose at front of it.

**James45157** · 12-25-2021, 03:39 PM

Cross flow motors donot have idle relief holes

**Pulsare2400** · 12-25-2021, 11:21 PM

I am not trying to give you false hopes, but I would certainly try another compression gauge just to rule that out. I used to drag race and once changed the camshaft and the compression was much lower than it should have been (but at least it was almost the same on all 8). I borrowed my friends gauge and it jumped 30 psi on each cylinder. I have seen the same difference just by using a 4.4:1 gear reduction starter which turns slower and causes a larger drop in compression readings than you would expect. Of course you did not change the starter and likely used a fully charged battery.

**rhardwick** · 12-28-2021, 08:09 AM

Could be someone changed that piston with a high ring piston for the 1977 140. That would account for the higher compression. Those were great so long as it had 92+ octane fuel.

**Gilera** · 12-28-2021, 08:41 AM

I've ordered a new compression tester and I'll recheck once it arrives.

Originally Posted by FMP

Do what we suggest, if you don't want to know the condition now you'll know the condition when you're paddling back to the dock.

What I get from your post is a motor with ign issues and sketchy compression but you want it to last a few years without spending some time and money.

Just assuming the rings aren't broken, oiled up to 140psi , all 4 are whipped and with three that low it's Wiseco time with all seals and everything else. JMO

I don't mind spending money on sorting the ignition out, but it wouldn't make financial sense to do that and also rebuild the powerhead.

From this your telling me that even if they were all the same reading, 120psi is flogged out for one of these. The previous owner told me they were 120psi across all 4. I thought that was pretty good for a 2 stroke outboard. I just never knew these were supposed to be higher compression.

**FMP** · 12-28-2021, 09:43 AM

First thing would be checking the rings as mentioned above.
If not broken try the soaking if sticking.
If not sticking once the oil is burned off the next compression test will likely drop.
What's the ign issue, have you been through it flywheel to spark boot end and plugs?

**James45157** · 12-28-2021, 10:42 AM

the replacement pistons for the cross flow motors lowered the compression a lot they were very heavy with a very strong skirt i rebuilt a ton of them for omc so i dont think you would two difference pistons in your motor i remember a v6 150 hp lost 500 rpm with the new pistons very heavy