Thread: Yamaha 115 2 stroke fouling plugs at idle

Update on the problems:

This motor has never kept a firm "bulb" on the fuel line. I could pump it up before headed to fish and I'd have to pump it up at the boat ramp. The Yamaha's have a mesh fuel filter installed on the side of the motor. The bowl for that would always get 3/4" of air in the top. That's what I had to pump back full with the bulb.

I checked the synch on the carbs. They weren't in synch. They weren't terribly out but the bottom carb was open a little. I had synch'd the carbs when I cleaned them after buying the motor, but... I set them and then my fishing buddy looked inside and told me that the bottom carb was still a little open. I told him just to dive in and fix it while I was replacing the kill switch. He set them. I had pumped the bulb up last night when we quit working on it. I went back to the shop today to do more work on the boat to get ready for a road trip and the bowl on that filter was still completely full. So that leads me to another question. Would the bottom carb being barely cracked open, allow the fuel to drain from the filter bowl when the motor is stored at an angle, as in on a motor toter?

I also put one number hotter plugs in it. I read that if you do a lot of idling in a boat to drop a number on the plugs. Lower is hotter, I had originally thought higher was hotter but I was wrong.

1) Issue with primer bulb is not related to motor loading up at idle. If it's loosing it's firmness during storage, it's most likely letting fuel backwash past the check valves toward the fuel tank. Once the carbs are filled, they cannot siphon back. The air noted in the filter is pretty common. (In short, if there was an air leak in the fuel circuit or a ruptured fuel pump diaphragm, you would be running out of fuel at idle or flooding the engine while priming it.) I wouldn't be concerned about this unless you experience a symptom that indicates a failure in the fuel feed circuit.

2) Carbs that are out of synch can give the user a fit trying to get a proper idle. If the difference is visible, it's enough to throw everything off. Also, the throttle shutter being cracked open has no bearing on fuel pressure in the fuel circuit. A leaking needle valve in the carb can effect system pressure and allow air to back track toward the pump and filter element. It will also allow over filling of the carb which leads to fuel dripping from the front of the engine while not running and a very rich running condition. Also noteworthy, if you are squeezing the primer bulb and holding pressure, a moderate decline in pressure in a matter of seconds indicates that you have a leak in the system. It can be a hose connection, failing needle valve, failing check valve in the primer bulb or a ruptured fuel pump diaphragm. Depending on what it is will determine to what extent the problem is and what symptom it will give.

3) Using a one number hotter plug for extended idling / trolling won't hurt anything. But if you have set the pilot screws too far open, the hotter plug will do little to help.

Yamaha outboards are very finicky about their idle mix setting. If memory serves, your setting should be 1 1/2 turns out from lightly seated. It could be 2 full turns and I've seen some 130hp Yamaha's have a factory spec of over 3 turns. I would recommend moving the settings back to their original positions if known or starting at 1 1/2 if unknown. It was mentioned earlier about a vacuum gauge. That is mandatory for 4 stroke carb'd engine. But due to the characteristics of a 2 stroke, it's not needed and not part of a factory trained procedure. You can set the initial idle settings and speed while hooked to a water hose at the house and get pretty close. But a final adjustment in the water and in gear will have you idling all day without loading up and give you the instant throttle response expected out of a 2 stroke.

Good luck with it, Sliderman

Thanks Sliderman. The fuel hose (new Gates hose with OEM Yamaha bulb as of last summer) won't leak down when unplugged from the motor, so that eliminates the check valve.

My pilot screws are set at 1 1/4 turns out.

What year model are we dealing with? This wouldn't happen to be a late 80s-early 90s model, would it?

98 C115 V4

It would appear that the pilot screw on that model is like the 130 and meters air and not fuel. Try backing it out to 3 1/4 turns and see if things don't improve. (at 1 1/4 out, the system is now too rich)
As for the primer bulb. When you disconnect the fuel fitting, you form a vapor lock of sorts that will not allow fuel between the connector and primer to migrate back toward the tank. Kind of like placing your finger over a straw and lifting out of a glass. With it connected, the end of the circuit (needles) can allow air to vent the system. If that makes sense.

Thanks for the info. Makes sense on the fuel line.

What year is your Yamaha as different years are different settings on the carbs?


If you really want to nail down a good idle with multi carb engines you do need a good vacuum gauge only way to tell where each carb is running rich or lean does not matter if 2 stroke or 4 stroke..