Yamaha 15 two stroke running on one cylinder

[Chuck Hawley]

After you solve what I think is a carburetion problem, you should install an Attwood Fuel Demand Valve in the fuel line to the engine. This will hold the pressure in the fuel tank and allow only as much fuel as the engine is demanding.

https://www.westmarine.com/buy/west-...oaAiRrEALw_wcB

(Sorry, everytime I try to insert a link, the dialog box goes away before I can finish copying...)

Here's a video on the subject starring yours truly.

https://www.youtube.com/watch?v=TQi0qiF6tjs

Chuck

[Mike Banks]

Vent the tank. It it is not a fuel injected motor, you are probably flooding it. I had a two stroke with a pressurised tank--it was fuel injected. It would not run without tank pressure, and one had to give it a few pulls to pressurise the tank before switching it on--then it would start and run just fine.

[Siberian Sea]

“Only running on one cylinder “ damn..... that’s how I feel half the time anymore.

[keepondancin]

Originally Posted by Chuck Hawley

After you solve what I think is a carburetion problem, you should install an Attwood Fuel Demand Valve in the fuel line to the engine. This will hold the pressure in the fuel tank and allow only as much fuel as the engine is demanding.

Chuck

I am going to try one. When you read reviews on them, they don't necessarily hold up in the sun. I am quite confident it is a carb problem caused by the non venting tank pressure. I also don't believe they vent at 5 psi, it is really only a crappy black cap, and in the sun the tolerances have to be effected. They are a real accident ready to happen.

[meridian28]

To permanently vent new tanks, take off the cap and turn it over. There is a small button the size of a pencil eraser. Push it in with a small screwdriver until it pops in. Job done.

[a64pilot]

Originally Posted by 44'cruisingcat

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.

Also four strokes, it’s called “wasted spark ignition” it keeps from having to have a distributor or other sophisticated system, four strokes just fire on exhaust stroke too.
I’d bet ever one of our two cylinder outboards are wasted spark.

https://en.m.wikipedia.org/wiki/Wasted_spark

[model 10]

I can't believe that I've always got the plug wires back on right, now I find it didn't matter anyway!

[a64pilot]

The real tell is look to see if there is just one coil that has two wires coming from it.
Even four cylinder motorcycles used to have only two coils, and two sets of points, that was much less complex than a distributor.
New bikes may have individual coils directly on the plugs? My Suzuki outboard doesn’t though it still has plug wires

[Sailing Drummer]

About the new tanks - IMO - it's best not to leave them vented when not using them. They will accumulate moisture in the tank. Instead, and this is a real pain in the a$$, disconnect the fuel line when not using the motor and keepnthe cap tight. Then, before plugging it back in loosen the tank cap to vent and relieve pressure. It does the opposite of what the tank is supposed to do, but in reality it's better for the environment because you won't be getting gas forced out of the engine or hose, spilling all over the boat or in the water. Also less chance of spilled gas igniting.

[44'cruisingcat]

Originally Posted by Mistress Sirena

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.

This definitely sounds like a possibility.

I think I'd organise some way of venting the tank too.

[s/v Jedi]

Fix it with these: https://www.amazon.com/Black-Univers...gateway&sr=8-3

[keepondancin]

I just received the carb rebuild kit, and I was assured it included the diaphragms for the fuel pump, but didn't, so I have to wait for them to arrive. Working on land, rather than in the cockpit, looking at one of the diaphragms, there is a slight tear where it looks like it was stretched out. I am convinced it was the pressure from the tank when I hooked the fuel line to the engine. It is very easy to forget to vent the tank before making the connections. I did add the Attwood demand check valve. Playing with it, on the tank side, there is no flow when under pressure. On the engine side, it only allows flow when there is a vacuum, so it should work as designed. I really believe these non venting tanks are a disaster ready to happen. You can't convince me that these tanks have reduces VOC's being released, when the have to be manually vented, and if not, spray liquids all over. Hope to get my parts early next week and see if it solves the issue.

[keepondancin]

Put the engine back together, used a carb rebuild kit, changed the diaphragms and gasket in the fuel pump, I found a very small tear in one, and new plugs. Also installed the Attwood flow valve. Started on second pull, runs on both cylinders. Ran it in a garbage can, so could put it in gear, but not really rev it up. Tried, but it sent a rooster tail 5' in the air. Didn't totally warm it up, but seems to run quite well. There is a very slight hesitation and vibration, but again, it was in a garbage can and not warmed up. Looking back, I really think it was the tank pressure, and an aging fuel pump, that caused the issue, and the tank pressure was too much for the float valve. I am hoping the fuel flow valve will work as designed. I also hope no one gets hurt releasing the tank pressure when it expands to football shape. I am going to search for an old style tank in good shape.

[s/v Jedi]

Originally Posted by keepondancin

Put the engine back together, used a carb rebuild kit, changed the diaphragms and gasket in the fuel pump, I found a very small tear in one, and new plugs. Also installed the Attwood flow valve. Started on second pull, runs on both cylinders. Ran it in a garbage can, so could put it in gear, but not really rev it up. Tried, but it sent a rooster tail 5' in the air. Didn't totally warm it up, but seems to run quite well. There is a very slight hesitation and vibration, but again, it was in a garbage can and not warmed up. Looking back, I really think it was the tank pressure, and an aging fuel pump, that caused the issue, and the tank pressure was too much for the float valve. I am hoping the fuel flow valve will work as designed. I also hope no one gets hurt releasing the tank pressure when it expands to football shape. I am going to search for an old style tank in good shape.

Did you see my comment above? You can just add a vent to your tank.

[keepondancin]

Saw the post. My concern would be water intrusion, not just from condensation, but also leaving it open in the rain, or tied down in the dinghy and a heavy rain rain filling the dinghy. I didn't remember to open the tank, I probably wouldn't remember to close the vent.