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Old 27-07-2019, 09:47   #1
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Yamaha 15 two stroke running on one cylinder

My Yamaha 15 2 stroke, ran very well. I needed a new fuel tank. Ran great on the old tank, had purchased a new hose last year. The new tank is the new non venting tank. When i went to hook up to the new tank to the motor, I didn't vent it, and a huge spray of gas came out the connection at the engine. It was 100 degrees, and the tank was swollen like a beachball, which the new ones do. The engine started, but runs really bad. Trouble shooting, the bottom cylinder isn't firing. It will start with just the top cylinder plug connected, and the bottom one not, but not in the reverse order. I had to run the engine to the dinghy dock, and when I returned, the engine was flooded, and wouldn't turn over. Took the bottom plug out, and pulled the cord, and gas shot out the plug hole. So i disconnect the hose each time I shut down. Cleaned the carb twice, was clean to begin with. My thought is the needle valve needs replacing, so I am going to order one, no one local has one in stock. I also switched the coils, cylinder for cylinder, and same results, bottom cylinder not firing. It was hard to do searches with a cell phone, now back where I have internet, I am searching. What I understand, is if the CDI is bad, it wouldn't fire on either cylinder. I read one thread, somewhere, that the flooding will cause the bottom cylinder to not fire, and that it is the fuel system creating the problem. Somewhere also, I read if the fuel pump gets stressed, fuel can bypass it into the carb. I am wondering if the diaphragms in the pump are compromised some how, by the tank pressure. I also switched plugs, changed plugs, so I can almost rule out electrical, unless I am wrong about the CDI allowing one cylinder to fire. Anybody have something similar happen? It all seems to extend back to the fuel tank, and the pressure that builds up in it, being non venting.
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Old 27-07-2019, 09:54   #2
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Re: Yamaha 15 two stroke running on one cylinder

I must add, this new tank and hose requirement is really stupid. The instructions on the tank tell you to vent the tank when hooking up the fuel line. If you don't, as soon as the little schrader valve is released, you get a bath of gas. Certainly well thought out. I bet they cause more hydrocarbon vapors released than the old style. Let's keep that to ourselves, because the next generation of tanks will be even worse.
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Old 27-07-2019, 10:08   #3
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Re: Yamaha 15 two stroke running on one cylinder

There are 2 plug wires from the cdi unit and there must be some sort of circuit that fires 2 different plugs.
I think if the fuel pump had a problem, both cylinders would fail.
I saw one of those new tanks in the sun the other day, freaked me out.
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Old 27-07-2019, 10:23   #4
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Re: Yamaha 15 two stroke running on one cylinder

Geez, there are times Im very thankful not to live in the 1st world...those new tanks in the USA are absurd (Ive come across then running other boats).

Your educated guesses are all good I think. Now, just need to isolate which one.

Spark. I would start here 'cus its easy. Test for spark on at each plug. No spark on bottom cylinder plug then there is your problem. If its got spark, then move on to the other possibilities.
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Old 27-07-2019, 10:34   #5
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Re: Yamaha 15 two stroke running on one cylinder

I think the new gas tanks are an explosion waiting to happen although I haven't of any. If they sit in the hot sun the pressure gets so high that they become shaped like a football. Just loosen the cap and it works like the old tanks.
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Old 27-07-2019, 11:07   #6
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Re: Yamaha 15 two stroke running on one cylinder

I have spark to the lower cylinder. Just learned not to start a twin cylinder two stroke with one plug out, it is like a flame thrower. Took a spare spark plug and put the plug wire on it, in the shade, and there is definitely spark, both on the plug, and plug to ground. So that really leaves fuel. It started with the new tank under pressure, so I need to figure out is it just float and/or needle valve, or is the fuel pump check valves and diaphragms a problem. Also don't know if running it with one cylinder not combusting is hurting anything. Being two stroke, it is still being lubricated, and no crankcase for the gas to fill. And yes, those tanks are a problem. the other day, half filled 6 gallon tank, went to vent, unscrewed the cap almost out, and the pressure blew it past the last thread and almost out of my hand
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Old 27-07-2019, 11:14   #7
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Re: Yamaha 15 two stroke running on one cylinder

Pull the plugs - how to they look?

I have a 70 HP 3 cyl that was not firing on one cyl. Turns out that it had a blown head gasket. That cyl's plug was wet but of course it was water not gas.

Being a 2 stroke there are no valves to make a head gasket replacement into a fairly big job. It may be worth pulling the head to check it out....

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Old 27-07-2019, 11:15   #8
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Re: Yamaha 15 two stroke running on one cylinder

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Originally Posted by keepondancin View Post
I have spark to the lower cylinder. Just learned not to start a twin cylinder two stroke with one plug out, it is like a flame thrower. Took a spare spark plug and put the plug wire on it, in the shade, and there is definitely spark, both on the plug, and plug to ground. So that really leaves fuel. It started with the new tank under pressure, so I need to figure out is it just float and/or needle valve, or is the fuel pump check valves and diaphragms a problem. Also don't know if running it with one cylinder not combusting is hurting anything. Being two stroke, it is still being lubricated, and no crankcase for the gas to fill. And yes, those tanks are a problem. the other day, half filled 6 gallon tank, went to vent, unscrewed the cap almost out, and the pressure blew it past the last thread and almost out of my hand
Yahoo! Thats one way to verify spark!
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Old 27-07-2019, 12:57   #9
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Re: Yamaha 15 two stroke running on one cylinder

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Originally Posted by Ecos View Post
There are 2 plug wires from the cdi unit and there must be some sort of circuit that fires 2 different plugs.
I think if the fuel pump had a problem, both cylinders would fail.
I saw one of those new tanks in the sun the other day, freaked me out.
It's not uncommon on 2cyl 2 strokes for both plugs to share the same coil, and spark at the same time. When one cylinder is at TDC, the other is at BDC, so the spark has no effect.

It sounds like the pressure in the fuel tank is forcing fuel into the carburetor float bowl even though the float needle valve is shut. Excess fuel accumulating in the lower part of the intake manifold, flooding the lower cylinder.

Could be a faulty needle valve, or maybe it just wasn't designed to handle the pressure from a non vented tank.

Why not test run with the vented tank?
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Old 27-07-2019, 18:43   #10
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Re: Yamaha 15 two stroke running on one cylinder

Check to see if the 2 stroke has the "safe mode" as in get home safe without damaging the engine. In the 4 stroke model the engine goes into "safe mode" - not sure what it's really called - by only firing one cylinder. Usually that's from overheating or bad oil pressure. Not sure if it would apply to starting though.
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Old 27-07-2019, 18:50   #11
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Re: Yamaha 15 two stroke running on one cylinder

That was the first thing I did, and the damage was already done. I do think the gas is bypassing the fuel pump and the needle valve and flooding the bottom cylinder. I will try to pick a dealer's brain. I can't find if there is a total carb rebuild kit, or individual parts. Gas doesn't combust as a liquid, so I am guessing that the flooding is stopping internal combustion.
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Old 27-07-2019, 18:52   #12
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Re: Yamaha 15 two stroke running on one cylinder

I am pretty sure there is no "safe mode" on a two stroke. No oil, so no oil pressure.
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Old 27-07-2019, 19:55   #13
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Re: Yamaha 15 two stroke running on one cylinder

On the Enduro is a temp sensor that shuts the engine down for a few min and then you can go again, like if you have no water pump. Totally limp home mode.
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Old 28-07-2019, 09:44   #14
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Re: Yamaha 15 two stroke running on one cylinder

Same happened to my 1976 Evinrude 4 hp 2 stroke new tank old motor blew out diaphragm in fuel pump flooded lower cyl through vacuum port for fuel pump replaced fuel pump leave tank cap lose problem solved.New tanks are a nightmare very dangerous.
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Old 28-07-2019, 09:57   #15
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Re: Yamaha 15 two stroke running on one cylinder

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Originally Posted by Mistress Sirena View Post
Same happened to my 1976 Evinrude 4 hp 2 stroke new tank old motor blew out diaphragm in fuel pump flooded lower cyl through vacuum port for fuel pump replaced fuel pump leave tank cap lose problem solved.New tanks are a nightmare very dangerous.
post script...
we had left the dinghy in the water over night with the new fuel tank hooked up
and found we lost about 2 gallons over board through the blown fuel pump
so much for environmental protection these new tanks with the new gas cans
have made a messy dangerous situation.
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