Yamaha 9.9 HT help

[Thunder21]

Hey I’m looking for a hand with getting my 9.9ht running correctly.

Its been sitting for a couple years so I cleaned the carb, serviced it and gave it a once over but I’m still having an issue . When firing it up it’ll idle great but as soon as I give it a small amount of fuel it’ll want to die. If I go from idle to heavy throttle it’ll rev up and run fine. It seems like maybe an accelerator pump problem because the issue seems to happen as soon as the pump moves. I’ve pulled it apart and I don’t see any tears in the rubber but it’s almost like it’s getting air from somewhere when that accelerator pump starts to move. When at idle if I push down on the pump with my finger it’ll bog down. I’m curious if anyone has encountered this or has any ideas.

Thanks

 

[seachicken]

I'd say the main jet in the carb is partially blocked but not knowing exactly the configuration of the carb I can't say for sure. Small carbs that I've worked on typically don't have accelerator pumps. If you're pushing on the accelerator pumps to add fuel without opening the butterfly then your making the mixture rich and it should bog down

 

[Thunder21]

I'd say the main jet in the carb is partially blocked but not knowing exactly the configuration of the carb I can't say for sure. Small carbs that I've worked on typically don't have accelerator pumps. If you're pushing on the accelerator pumps to add fuel without opening the butterfly then your making the mixture rich and it should bog down

I’m pretty confident the jets are clean

 

[ab1752]

Sure sound like a fuel pump, mine went out last season at the beginning. It's easy to replace and I was in a hurry so I just snagged one off amazon. Just be careful to swap the O ring too...don't ask haha

 

[Max123]

Sounds like you were on the right track with the accelerator pump. I had the same issue with my old Yamaha 9.9. If you pump the throttle it will rev up, but then dies? If so, accelerator pump.

New accelerator pump diaphragm solved the issue. Possibly its not seated very well?

 

[Sea Chaser]

Same issue with my 8hp, just happened I took the pump off to look at it and dropped and broke the damn thing real good so had to replace it. But fixed the problem.
Jets in the carb still plug up
If it sits for a long period of time .

 

[Thunder21]

Sounds like you were on the right track with the accelerator pump. I had the same issue with my old Yamaha 9.9. If you pump the throttle it will rev up, but then dies? If so, accelerator pump.

New accelerator pump diaphragm solved the issue. Possibly its not seated very well?

Yeah was just playing with it some more. If I keep the throttle high it’ll run continuously and it’ll also idle fine. It’s the bottom 1/4 to 1/2 of the throttle it stumbles a bit , runs good for 30 seconds or so and then will randomly loose rpm and die unless I drop the throttle to idle or give it more throttle.