2 stroke compression question
#1

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.
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.
Last edited by underpsi68; 07-23-2020 at 04:54 PM.
#2





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blownhammer2000 (07-23-2020)
#4
Registered
iTrader: (1)

+1 on the scoring, in my jet ski days minimum compression was around 120, 70psi will def foul your plugs, another question I have is the 100:1 mix? We always ran 40:1, maybe 50:1, that sounds like it could be the problem right there, but then you said you've ran it for the last 5 years tho
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SB (07-23-2020)
#5

That's not the answer I was hoping for, but was expecting.
Manual states 100:1. The Envirude engine this replaced was also 100:1. I wonder if I should be running it 50 or 75:1? Issue happened while running it hard. It has been wot quite a few times since I owned it with no issues up till now.
I have to look into rebuilding this engine. Was a fresh water engine when I bought it and always used salt away after each use since I owned it. It's a very clean engine. Seems the parts are pretty cheap, They sell stock and 2 different oversized pistons, .25mm and..5mm. Does anyone know how far these cylinders can typically be bored/honed?
Manual states 100:1. The Envirude engine this replaced was also 100:1. I wonder if I should be running it 50 or 75:1? Issue happened while running it hard. It has been wot quite a few times since I owned it with no issues up till now.
I have to look into rebuilding this engine. Was a fresh water engine when I bought it and always used salt away after each use since I owned it. It's a very clean engine. Seems the parts are pretty cheap, They sell stock and 2 different oversized pistons, .25mm and..5mm. Does anyone know how far these cylinders can typically be bored/honed?
Last edited by underpsi68; 07-23-2020 at 09:01 PM.
#9
Registered

Those Score marks aren’t 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 it’s 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?
what did the plugs look like, wet with fuel, black soot, ash, heavy oil?