Yamaha delayed acceleration or stalling

[kartoffel]

I have a 115HP,2stroke, 1995 yamaha. Ran great for the last 2 years and the first trip this year.

Engine starts and idles perfectly and runs up to 20RPM just right. Last week when pushing the throttle to 30-45rpm the engine would hesitate about 5-15 sec (but not drop rpms) but then accelerate all at once and run perfectly.

Today, It did the same but after a fishing drift and restart it would run great to 20rpm (throttle was in the expected position). When pushing past that the engine would drop rpms and stall. Restarted fine and the behavior repeated (after about 5 tries it hesitated and got to high rpms). Then after another drift, I had the stalling behavior, but it idled enough to get me home at 20rpm

I put a fresh parker fuel water separator each year and the fuel is fresh from portable tanks. (and since it idles just fine I doubt its fuel).

Any ideas? I'm thinking fuel pump but would like to cover all bases before ordering parts.

Thanks
JP

[kartoffel]

by the way, compression was good last year. I haven't checked this year.

[kenneth80]

sounds like a clogged high speed jet in the carburator or a bad fuel pump diaphram.
Order a new carb kit and completely tear down the carburator, soak and blow with compressed air. Easy job if you have the manual.
Try squeezing the bulb. If you see gas coming out of the tiny hole in front of the fuel pump youre looking at buying a new fuel pump kit or better, buy a new fuel pump. It cost around 40 bucks.

[kartoffel]

Try squeezing the bulb. If you see gas coming out of the tiny hole in front of the fuel pump youre looking at buying a new fuel pump kit or better, buy a new fuel pump. It cost around 40 bucks.

squeezing the bulb with engine off or running? Which tiny hole? do I have to remove the cover off the fuel pump? I was thinking to buy a new fuel pump but would like to be sure.

Thanks

[mickyb]

squeezing the bulb with engine off or running? Which tiny hole? do I have to remove the cover off the fuel pump? I was thinking to buy a new fuel pump but would like to be sure.

Thanks

Yup, fuel pump. I have the same problem with one of my engines and went yesterday to the boat and did that same test and the pump's diaphragm is bad. I'm waiting for the new one.
Remove the 2 bolts that hold the pump to the block, they are 10mm, then looking at the pump from the side that goes to the block squeeze the bulb. If fuel squeezes out from a small hole the pump is shot.

[kartoffel]

So I removed the fuel pump from the engine squeezed the bulb and fuel came out of the back. I took the cover off the fuel pump to inspect the membrane and it looked fine after I peeled it off. I thought the fuel came out of the screw hole and not the center hole. Just to check for fun, I reinstalled and reprimed and checked for fuel leakage and there was none, so I reattached to the motor.

It's odd that opening and closing the pump solved the leak but that's life.

Ran at the dock and things seemed normal. Parts place is closed on Sunday, but I'll pick up another pump and gasket tomorrow to be safe, since I don't think the pumps been changed in 5 years or more.

Thanks

[kartoffel]

So I'm back with the same problem. I replaced to a new fuel pump and the problem persists.There is no leakage when squeezing the ball with the pump unattached.

What confused me was the motor works fine in neutral. In Neutral the RPMs track the throttle immediately (from 8-35 rpm). Only in gear it doesn't work above 20 rpm. I took it out today and the problem was there (delayed acceleration or even stalling above 20rpm). It also does it when I drive the boat against the dock (dies above 13 rpm). Sometimes it kicks in after 30 seconds to 4 minutes of holding the throttle right at the stall point. When it kicks in it runs fine at 30-42rpm, but as soon as you go below 27 or so it goes back to idle and starts over

Any ideas? I may call a mobile mechanic but want some idea of what could be wrong.

Thanks
JP

[kartoffel]

I just went and checked a few more things. When running against the dock it stalls at 13-14 rpm, if I hit the choke switch it revs up to the low 30's. It must be something with the fuel (not electrical - faulty rev limiter, etc). the ball feels about half fuel but squeezing it doesn't improve the situation while running. I squeezed the ball while checking for leaks but don't see any.

The fuel lines and ball are 2 years old and good quality. I tightened all the hose clamps to be sure. New Parker water separator this season.

Any more ideas? I don't think it's the carburetors, since I use external tanks and this is fresh fuel. I rebuilt the carbs last year (worked fine) All 4 coils and wires were changed 3 years ago.

Any more ideas?

JP