2 stroke compression question
07-24-2020, 07:25 AM
#11
Registered
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.
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.
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PA.WOODCHUCK (07-24-2020)
07-24-2020, 07:28 AM
#12
Platinum Member
Platinum Member
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 don't know but the carbon pattern on the heads and piston looks strange to me
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PA.WOODCHUCK (07-24-2020)
07-24-2020, 07:42 AM
#13
Registered
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
07-24-2020, 07:44 AM
#14
Platinum Member
Platinum Member
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.
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.
07-24-2020, 07:59 AM
#15
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.
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Twin O/B Sonic (07-27-2020)
07-24-2020, 10:13 AM
#16
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.
07-24-2020, 11:57 AM
#17
Registered
Thread Starter
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?
I was thinking issue with spark as well? It does spark but how do I test if the coils are working correctly?
07-24-2020, 11:58 AM
#18
Registered
Thread Starter
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.
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.
07-24-2020, 12:03 PM
#19
Registered
Thread Starter
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.
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.
07-24-2020, 12:10 PM
#20
Registered
Thread Starter
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?
