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2 stroke compression question
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. |
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Yep low, should be over 100 and looks like you found some scoring, but I'm no expert
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+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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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? |
I've always used the rule of thumb min 100psi compression for 2 stroke engines.
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First time ever i read about 100:1 ! That is more than twice leaner than what i ever saw a recommended mix!
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I've seen a full 1 to 2mm increase in bore size, there is more than enough material there to clean up the scoring, I would not continue the 100:1 ratio, minimum I'd do is 50:1, I'd also switch to amsoil over the yamalube, I've personally never been a fan of it
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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? |
Have used 100:1 synthetic for years in chain saws and weed wackers
Even the 50:1 oil injection dino oil motors runs from 100:1 at idle and go up as rpms increase |
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. |
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 |
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
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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. |
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. |
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. |
Originally Posted by sailtexas186548
(Post 4749783)
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? 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? |
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. |
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. |
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
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 |
Originally Posted by underpsi68
(Post 4749844)
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? |
Originally Posted by sailtexas186548
(Post 4749861)
bottom plug looks a little steam cleaned, maybe leaking HG. When they get old they can get tiny pin holes and start passing water
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I have a new hg and thermostat ordered. Hopefully they will be here the beginning of the week. I'll reassemble and do a leak down test.
I am also running an aftermarket set of reeds. I'm going to put the stock reeds back in as well. I'm just curious why the compression is low? Thanks for all the help everyone!!! I'll report back after I run it. |
I'm not sure why none of your photos show BOTH cylinders, but what little I can see shows one plug looking like it is brand new and the other looking pretty typical for a mid-hour 2 stroke plug.
Wish I could see both piston crowns and both combustion chambers. My guess is you overheated it and got water in the cylinders. The cheap route would be to run a flat file across the cylinder deck and the head, pit on a new head gasket, torque it in the correct sequence to the correct torque, and then with plugs out and throttle open - recheck compression. If its still under 115 you will be looking at total disassembly. If compression is back up, inspect/replace water pump impeller and make sure there is no debris in the thermostat area. If you don't run in cold weather, you can remove the thermostat completely. Check the water "pee stream" frequently Anniston for changes in the sound of the exhaust (low water flow usually changes the exhaust note). |
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I have to find the old plugs. After installing new plugs they would come out all black/glossy instantly. The bottom plug wasn't as bad, but still fouled. I'm thinking the water was cleaning the plug off.
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Well the plot thickens.
I removed the plate on the side of the cylinders to take a look. I found a piece of aluminum that corroded off the exhaust port wall that was blocking the flow. Water was leaking into one cylinder. Looks like it not worth repairing. The cylinder itself is $1200 to replace. I really don't want to go to a 4 stroke because of the weight. I don't have a trailer for the raft and carry the engine from the basement to the truck. This engine weighs about 80lbs which is about the limit of what I want to lug around. A 15-20hp 4 stroke is about a 100lbs. Problem is all the 2 strokes are getting older and could be the same issue. https://cimg7.ibsrv.net/gimg/www.off...049b090937.jpg https://cimg0.ibsrv.net/gimg/www.off...4597fad013.jpg https://cimg2.ibsrv.net/gimg/www.off...545d93aff4.jpg |
Yes, you are right.. Nobody makes ultralight 2 stroke outboards anymore. It's a shame. For what you describe, I would look into a powerhead or block.
Complete powerhead - https://www.ebay.com/itm/Yamaha-Outb...-/254663384100 Above motor may have salt rot in it. I suppose you could ask the guy to pull off the exhaust chest cover and send pictures... Or, here is a very pretty bare block. You might want pictures of the cylinders.. https://www.ebay.com/itm/15HP-63V-15...-/273383360024 |
Originally Posted by underpsi68
(Post 4750049)
Well the plot thickens.
I removed the plate on the side of the cylinders to take a look. I found a piece of aluminum that corroded off the exhaust port wall that was blocking the flow. Water was leaking into one cylinder. Looks like it not worth repairing. The cylinder itself is $1200 to replace. I really don't want to go to a 4 stroke because of the weight. I don't have a trailer for the raft and carry the engine from the basement to the truck. This engine weighs about 80lbs which is about the limit of what I want to lug around. A 15-20hp 4 stroke is about a 100lbs. Problem is all the 2 strokes are getting older and could be the same issue. https://cimg7.ibsrv.net/gimg/www.off...049b090937.jpg https://cimg0.ibsrv.net/gimg/www.off...4597fad013.jpg https://cimg2.ibsrv.net/gimg/www.off...545d93aff4.jpg |
Originally Posted by mcollinstn
(Post 4750068)
Yes, you are right.. Nobody makes ultralight 2 stroke outboards anymore. It's a shame. For what you describe, I would look into a powerhead or block.
Complete powerhead - https://www.ebay.com/itm/Yamaha-Outb...-/254663384100 Above motor may have salt rot in it. I suppose you could ask the guy to pull off the exhaust chest cover and send pictures... Or, here is a very pretty bare block. You might want pictures of the cylinders.. https://www.ebay.com/itm/15HP-63V-15...-/273383360024 I found a new Yamaha case for just over 1000. I seen the China version. Much cheaper but I worry about the quality. The machining tolerances and also the metal quality. Thanks |
Originally Posted by oconnor marine
(Post 4750086)
clean real good with brass brush and apply marinetex. Let cure and flat file. Reasemble with new gaskets
Thanks for the idea!! |
Originally Posted by underpsi68
(Post 4750135)
Hummm I never thought of that. I'm trying to salvage the summer and I could worry about building a new power head over the winter.
Thanks for the idea!! |
Originally Posted by oconnor marine
(Post 4750136)
ive done it many times
I'm also concerned the compression is low. I won't know till I put it back together. |
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