1975 Johnson 115 high compression on one cylinder

[FMP]

Hoping for him, he clips a 1/4" off each plug lead , sets the clips and that solves the ign issue.
Rings are fine just stuck from years of dried storage fog and it's a low hr jewel.

[Gilera]

Hoping for him, he clips a 1/4" off each plug lead , sets the clips and that solves the ign issue.
Rings are fine just stuck from years of dried storage fog and it's a low hr jewel.

Ha - here's to hoping!

The motor does generally look very clean for its age which was part of the reason I bought it with known ignition problems. It looks like someone has owned it though who does not really know motors and it has maybe had some neglect. I just dropped the lower unit and found a very new looking impellor, however no woodruff key in the keyway. I found the key buried in some debris in one of the passages. Would the impellor even spin without the key? Can't see any obvious signs of overheating.

I have been through the CDI troubleshooting guide but cannot pinpoint the ignition issue. I am getting a weak yellow spark on all 4 cylinders. I have a separate thread about it here - https://www.screamandfly.com/showthr...nson-115/page2

I was considering trying to fit a later ignition system just because I have one that looks in good condition but am running into a few compatibility issues. The original stator has a few black melted spots but tests ok on resistance and DVA. Timing base has had a sensor replaced at some point which makes it look suspect but seems to test ok on DVA. Resistance is a bit low across the 2 pairs of sensor wires. I haven't tried adjusting the sensor gap yet. Coils test ok but have some cracking. I figure it unlikely to be anything with the coils, leads and plugs because all 4 cylinders are affected. Plugs are new. Unsure if the power pack might have an issue as it seems to be a process of elimination. New battery, good connections, earth points all cleaned. I thought it would have been more clear cut doing the tests.

Prices for these parts over here in Australia are much higher than in the US and shipping costs to Australia are high if I buy them from the US so would make sense to buy all that I need at once if I can. But nothing has come up as the obvious cause. I absolutely know I will be spending a bit of money on the motor to get it running but if I go ahead and buy a stator + timing base + coils + power pack it would be roughly the cost of buying a later model running motor. Second hand parts too expensive just for troubleshooting purposes and not many 1975 model parts motors come up where I am.

At least if I sort this one out I'll have the peace of mind that i've thoroughly gone through it. Plan is to also change gear oil, rebuild carbs, check reeds, change thermostat.

[Gilera]

I have a can of Quicksilver Power Tune spray. Perhaps before I take off the bypass covers I should spray some into each cylinder through the plug holes, turn it over and and let it sit for a few days while the new compression tester arrives and see if any different? It won't hurt for it to be in there while i'm cranking the motor over a bit while i'm troubleshooting will it? Could be a couple of months before I get this running again.

[FMP]

You will not get an accurate reading without first running and burning off all the oils.
Physically check the rings as described to rule out being broken.
If sticking gummed up , soak and work them loose with same procedure.
If the pump key is gone , yes it could have overheated and no soaking will fix that scoring.
Knackered, I think is what you'd say. I wouldn't go any further with the ign until you check the bores and rings.

[Gilera]

Ok. I've got a few questions because I've never done this before. Photos below are from my seized parts motor for reference.

solid but gentle pressure straight on the ring

So I am rotating the flywheel until the ring is visible through these transfer port holes and then pushing it back into the housing slot in the piston to see if there is some give right?

Can I use a flat blade screwdriver or will it have to be plastic to avoid damage to the ring?



Can the same thing be done for all four cylinders through exhaust cover? Wondering if it might be easier to take that off rather than the 4 covers but maybe not.



Will it be obvious if they are broken?
If gummed up am I spraying them also through these ports to soak or into the cylinder through the plugs holes and turning the motor over?
To check the cylinders for scoring I will have to either use a borescope or remove the heads right? There is no other way?

[FMP]

A ring if not sticking or broken will yield into the ring groove and follow you back out as you release the pressure. If it doesn't give , already bottomed out or doesn't follow back out it's broken or sticking.
There's not a lot of travel, just push it in and watch it.
If it's broken it's obvious, no tension. A drive will do it with care but a piece of hardwood dowel or hard plastic works well. You want flat contact to the ring.
The spray will get around the rings , just spray and rotate the assembly. Through the plug and tranfers.
How well can you see through the plug hole and tranfers with a light to check, you decide.
Just in case...
https://maxrules.com/phkits_omc/100-110-C1x.php

[James45157]

i could be wrong but it looks like the motor may have over heated at sometime

[Gilera]

i could be wrong but it looks like the motor may have over heated at sometime

Mate that motor in the photos is stuffed! It had water go through it I was told. Inside is totally rusted up and frozen. I bought it for a starter motor and stainless prop mainly. The motor cost $200 and the wreckers over here wanted $150 just for a 40 year old used starter motor! Would be good if I can work out a way to use the ignition parts from it.

[Gilera]

Ok so tonight I took off the bypass covers. Checked all 8 rings and definitely none broken. I checked what i could see of fhe pistons. There appears to be what id consider to be some minor scoring which I am guessing is from some previous overheating possibly related to someone forgetting to put the key in to hold the impeller in place. The first picture is the worst bit I could find. I'd appreciate some opinions - does this mean a rebuild is in order? Not really what I want to hear of course. Further inspection of the cylinders?

[FMP]

Are they highring, top ring just below piston crown or like the the ones in your parts motor?
From the pics I'd run it if your comfortable with lower compression knowing it'll be weaker and possibly prone to failure. It could still come up some with a soak, did it look clean ?
Other option , try to find new rings and a quick hone if things aren't too lose, but then it's 3/4 the way and work to a proper rebuild which makes more sense. Finding those rings could be difficult?

Are the piston tops deflector without any pitting detonation damage?

Run it 40:1

[Gilera]

Yes it looks quite clean to me. I couldn't see any pitting. The rings are quite close to the top but they look the same in the 1980 model to me. One of the pistons in the parts motor seized in a position where you can see the ring - see below. Distance from the top looks about the same ?


1980 model

[bigbore]

Just spray some -great stuff in there-or fix a flat-.
JK sounds like your on the right track.

[FMP]

It's not highring

[Gilera]

Does this mean its been rebuilt at some point?
Does it also mean 120psi is reasonable compression?

[FMP]

No and no, I think. That particular year , HP may not have had those. Racer mentioned it's low at 120, with oiling giving a higher read it's lower still. If the 140psi cyl is oiled it's also not 140.
Once fired and burned clean, you'll have a better true value. A lot of different two strokes with the same pistons will have psi values for the particular HP model. Folks then think it'll be fine because the chambers or port heights are different which will change the psi and that 140hp tired has 125psi instead of 160+ and they think it's good when it's leaking down considerably and all four still near equal.
It'll be down on power, efficiency and life expectancy.
Like mentioned previous, your decision. Might as well pull the carbs off and remove the reed cages and check them out too.

And assume you meant all rings have tension and rebound after checking, with none broken.