99 15 merc, isolated rough spot?

[Wee Hooker]

My buddy just bought a 99' 15 merc 2 stroke ( I owned one myself for 13 years and consider this design to be one of the best small engines out there. That shift in throttle feature is awsome!)
Anyway, the motor starts great and runs super accross the entire RPM range EXCEPT in one small spot, 1/3 up from idle, it runs rough. It stubles as you twist up thrugh this spot, stubles as you come back down and continues to run rough if you leave the throttle it in that spot. Move the throttle just a tad in either direction and purrs again.
Anybody ever run into anything like this? Scratching my head as to what the root cause could be since the motor runs so good at all other throttle settings both above and below.
Any thoughts greatly appreciated!

Dave

 

[DamianJP]

Re: 99 15 merc, isolated rough spot?

Hey Wee Hooker

Sound like a fuel/air mix or delivery problem.

Check to see if all the adjusting screws are in spec. Sometimes when they are
out of spec when you goto throttle up it will get too much or too little fuel depending on which way the screw needs to be adjusted. I forgot which one does that though, I think the air mixture screw. I'll try and dig up my manual and take a look on how many turns they need to be out from seated.

All fuel filters clean in any installed?

Does the carb have a fuel pump intergrated in it or on the engine block?


Lemme know what ya find or think,
DamianJP

 

[rg2]

Re: 99 15 merc, isolated rough spot?

Wee Hooker

I have a 1999 9.9-2S that does the exact same thing. I've posted here about it in the past with no response.

I felt it was an idle mixture crew adjustment so I reset the screw to 1 1/2 turns out and made the final adjustments while idling at slow idle in the water. It ran fine that day and all of the next but after sitting for a week and then being used again it did the same thing. I talked to a mechanic that I trust and he told me it sounded as though it was a lean idle condition. I set the screw slightly on the rich side so I don't believe that is the case. I was wondering if it was occurring at the point where the low idle mixture switches over to the main jet as far as fuel supply to the carb. Mine will make a very soft, somewhat muffled backfire sound, then rev up slightly, puff, rev, etc.

Once you get beyond that point, which is a very small range on the throttle, it is fine again. I've had my carb apart twice to clean it but to no avail. If you determine the cause please let me know so I can correct mine as well. I will do the same for you if I can figure it out.

All I know is these small outboards can be very fickle sometimes

 

[rg2]

Re: 99 15 merc, isolated rough spot?

One other question - Do you have the owners manual on your motor? If so, I was wondering if you could check something for me.