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-   -   2 stroke compression question (https://www.offshoreonly.com/forums/general-q/367651-2-stroke-compression-question.html)

kvogt 07-24-2020 07:25 AM

That cylinder looks normal to me. If you can't catch your finger nail on the scratch it really isn't a problem.

Your low reading on the compression may be your gauge setup. A two cylinder 15hp motor has a small combustion volume and the added volume of the hose on your compression gauge may reducing your readings.

resurrected 07-24-2020 07:28 AM

Agreed on the scoring. That's not enough to hurt performance. I had a sled with a ring welded to the piston and would run fine over idle and still lift the skis high in the air, but the only way it would start was with a primer bottle. Compression testing on a two stroke is difficult to gauge by numbers, especially when they indicate the same. A leak down would provide more useful information. The 100:1 ratio would be to satisfy emissions even though you may prematurely wear out the rotating assembly, by going against the manufacturers recommendation of 100:1 and switching to 50:1 re jetting may be required because you will be leaning out the amount of fuel in each charge.
I don't know but the carbon pattern on the heads and piston looks strange to me

sutphen 30 07-24-2020 07:42 AM

I'd look at the reed valves.little 2 stroke engines don't like scoring.doesn't take to much to stick the rings.btdt

mcollinstn 07-24-2020 07:44 AM

You should be seeing 115 - 130 psi.
I would run the cheapest TCW3 oil you can find at 50:1. Pennzoil at Autozone or Walmart is a good bargain.

Not sure why your compression is down unless your rings got too hot and lost tension.
Did you verify that the water pump was pumping properly when it was running poorly?
Might wanna double check yhe impeller.

oconnor marine 07-24-2020 07:59 AM


Originally Posted by underpsi68 (Post 4749742)
I have a 2004 15hp 2 stroke Yamaha. Bought it about 5 years ago and never had any issue. A couple of weeks ago I took it out on my inflatable. I was wot for a mile or so and went to an idle and shut it off. When I restarted it, it wasn't running right, down a little on power. When I got home I pulled the plugs and found both plugs fouled (never happened before).
Last week I changed the plugs and tried to use it. As soon as it tried to go 1/3-1/2 throttle I knew it was down on power. Put 4 new plugs in and would foul right away.
I thought it might have been the gas i mixed (100:1 with yamalube) but haven't been able to try it with fresh gas because of a torn seam on the inflatable.
The other day i did a compression test. Both cylinders were exactly 70psi. Does that seem low? It's 6.8:1 compression.
I pulled the head to look at the cylinders. Nothing jumped out to me but I'm not a 2 stroke guy.

head gasket junk. Sh0ws water steaming tge head clean. Also you need to mix it at 50:1

oconnor marine 07-24-2020 10:13 AM


Originally Posted by underpsi68 (Post 4749742)
I have a 2004 15hp 2 stroke Yamaha. Bought it about 5 years ago and never had any issue. A couple of weeks ago I took it out on my inflatable. I was wot for a mile or so and went to an idle and shut it off. When I restarted it, it wasn't running right, down a little on power. When I got home I pulled the plugs and found both plugs fouled (never happened before).
Last week I changed the plugs and tried to use it. As soon as it tried to go 1/3-1/2 throttle I knew it was down on power. Put 4 new plugs in and would foul right away.
I thought it might have been the gas i mixed (100:1 with yamalube) but haven't been able to try it with fresh gas because of a torn seam on the inflatable.
The other day i did a compression test. Both cylinders were exactly 70psi. Does that seem low? It's 6.8:1 compression.
I pulled the head to look at the cylinders. Nothing jumped out to me but I'm not a 2 stroke guy.

head gasket junk. Sh0ws water steaming tge head clean. Also you need to mix it at 50:1


underpsi68 07-24-2020 11:57 AM


Originally Posted by sailtexas186548 (Post 4749783)
Those Score marks arenít that bad for an two stroke, I would try Try a leak down at TDC before you start pulling pistons, better yet try running some ring free and check ur ignition itís probably been low comps for a while and ur spark has gotten weak

what did the plugs look like, wet with fuel, black soot, ash, heavy oil?

Picture with 2 plugs is day that problem started. Picture with single plug has 3 minutes of run time. Changed both plugs twice and fouled right away. I have to find other plugs. Top cylinder was definitely worse than bottom.
https://cimg2.ibsrv.net/gimg/www.off...61f0fb8012.jpg
https://cimg3.ibsrv.net/gimg/www.off...85f0463f57.jpg

I was thinking issue with spark as well? It does spark but how do I test if the coils are working correctly?

underpsi68 07-24-2020 11:58 AM


Originally Posted by kvogt (Post 4749801)
That cylinder looks normal to me. If you can't catch your finger nail on the scratch it really isn't a problem.

Your low reading on the compression may be your gauge setup. A two cylinder 15hp motor has a small combustion volume and the added volume of the hose on your compression gauge may reducing your readings.

I can't catch the score marks with my finger nail.

underpsi68 07-24-2020 12:03 PM


Originally Posted by mcollinstn (Post 4749804)
You should be seeing 115 - 130 psi.
I would run the cheapest TCW3 oil you can find at 50:1. Pennzoil at Autozone or Walmart is a good bargain.

Not sure why your compression is down unless your rings got too hot and lost tension.
Did you verify that the water pump was pumping properly when it was running poorly?
Might wanna double check yhe impeller.

The day and time it happened as I was wot I noticed the water wasn't spraying out the little water flow fitting on the side of the engine. That was the reason for me shutting it down. It is not uncommon for something to get stuck in the little hose/ fitting. When i removed the hose from the fitting it was pumping as it should. Blew out the fitting, reinstalled hose and all was good. Didn't run right after that. Water still flows through the engine when this happens.

underpsi68 07-24-2020 12:10 PM


Originally Posted by oconnor marine (Post 4749826)
head gasket junk. Sh0ws water steaming tge head clean. Also you need to mix it at 50:1

I was wondering what those spots where on the head. I didn't see any leak on the hg.
The last engine I had had a leaking seal which was causing water to get inside the cylinder. I wonder if a seal/s are leaking?
https://cimg3.ibsrv.net/gimg/www.off...1d1b8a505e.jpg
https://cimg7.ibsrv.net/gimg/www.off...5ebef6feab.jpg


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