Thread: Yamaha 115 2 stroke fouling plugs at idle

No. I put a new OEM fuel pump on it last summer. I know that doesn't mean it doesn't have a hole in it, but it has fouled plugs at idle since I've had it.

Originally Posted by FurFlyin

No. I put a new OEM fuel pump on it last summer. I know that doesn't mean it doesn't have a hole in it, but it has fouled plugs at idle since I've had it.

Having pilot jets that meter air and not fuel is not the 'norm' with most outboards. My best guess would be that at some point, the carbs were cleaned and set to a standard typical of most carbs. Which left your motor set too rich.

I just swap one problem for another.....

Easy fix would be to throw the motor in the dumpster, but I'm hard headed...

Went on a 3 day fishing trip. First evening on the water, the motor ran great at idle, and great at WOT. Fished a while and then went scanning for fish. Idled quite a bit. Headed back to the marina and boat started dying at WOT. Pulled back off the throttle and RPM's picked back up. I could run around 4000 RPM's w/o issues but when I tried anything over that, it acted like it was starving for gas. Ran it that way the next day, then yesterday it did the same exact thing it did a month ago. No throttle response. When I'd give it throttle at "blast off" it would just bog down. Did that a couple of times, then next attempt it jumped up on plane and ran fine. Still wouldn't run over 4K. Buddy could pump the ball on the fuel line at WOT and it would run like a scalded dog. If he pumped the bulb at 5-7 mph it would choke out the engine. Bulb is half empty or more while running.

Here's my thinking. I think I have multiple issues. I figure I'm sucking air or a hose is collapsing, somewhere causing WOT starving. I just replaced all OEM fuel hoses with 5/16 Gates hose last summer and an OEM primer bulb and a new OEM fuel pump. Maybe some of the hoses I made, where they bend at the fuel pump and where the two carbs are connected, are collapsing on themselves. I think I'll replace them with new OEM hoses, check fuel pump diaphragms for pinholes.

Pumping the ball at idle speed and choking the engine out sounds to me like needles are sticking open? If they were working properly, would pumping the ball force fuel past them? I just cleaned the carb and I am running a small micron water separating filter. Nothing should have gotten in the carb through that filter. Planning to replace needles and seats on carbs. There is no apparent distortion or wear on the needle tips, but they are rubbery feeling. I don't know if they're supposed to be pliable, but most carbs I've dealt with, the needle tips are hard plastic.

Then I'm going to pull all electrical components and check the resistance on each coil, plug wire, CDI box, etc... Even though spark jumps the gap on spark tester, I'm not certain I'm getting enough fire to burn all gas which is still causing the plugs to foul.

Opinions and ideas appreciated.

What did you end up setting the pilot jets to that gave you good idle?

Pumping bulb and getting positive feedback at WOT tells you it's a fuel delivery issue. Since it started later in the trip, I'd suspect pickup tube. Fill tank to full and retest. If the problem goes away until the tank level goes back down, it's most likely a problem with the pickup tube. If this is a built in tank, the anti-siphon valve could be causing this problem too. But fuel level will not change the problem.

Priming bulb at idle and engine loading up suggest either a leaking needle valve or incorrect float level. If enough pressure is applied to the bulb, you can unseat an otherwise good needle. The needle tips will either be a rubber or metal.

If the pump diaphragm has a hole in it, fuel will bypass directly into that cylinder and enter the bleed system which will carry it to other cylinders resulting in a flooding condition at idle. It will also put air into the fuel stream which can lean out wide open operation.

Leave the electrical alone. You've proven the issue is fuel related.

Fuel lines located between the pump and tank are prone to collapse in areas of a tight bend and are only pressurized by the primer bulb. Otherwise it's a low pressure (vacuum) system. Everything from the pump to the carbs are pressurized and will resist collapsing.

Did you clean carbs and put new rebuild kits in them? Ethanol is eating these carbs. You can't just clean one and put it back together with old components.Did any of the factory fuel lines have factory bends in them and you replaced them with a straight line and bend it?

The Pilot screws are set at 3/4 turn out. That is actually top of the range in my manual. I had been running them more open than that.

Originally Posted by FurFlyin

The Pilot screws are set at 3/4 turn out. That is actually top of the range in my manual. I had been running them more open than that.

That's not making sense in that if the motor was running rich earlier with a more liberal setting, it should be super rich at 3/4 turn out with an air adjusting mixture screw. Almost sounds like the carbs have been swapped with another model yamaha.

That could be. I don't know how to tell that though. Honestly there isn't much difference in how it runs at idle, regardless of how the pilot screws are set. Someone may have put 130 carbs on it in the past but they didn't change the timing to match a 130 if they did. I'm gonna order a rebuild kit for the carbs today. I don't know that they need it, but I'm tired of taking the dang things off.