Troubles @ >1/2 throttle

[ERRM]

Hello,

This summer past I purchased a yamaha 2-stroke 15HP, model number 15MSHC. Although it had been sitting in a garage for a at least 2 years, it started right up after a 1/2 dozen pulls and seem to work fine. I put it on a 9.5ft (100lbs) inflatable tender (high pressure floor and inflatable keel). Before i put it in the water, I changed the gear oil, cleaned the carb(including jet), cleaned the fuel filter, and changed the plugs. When the stars, sea conditions, and wind all aligned (i am exaggerating slightly), the boat carrying my dog, fuel tank, and myself gets up on plane in about 100 feet and the thing screams at full rpm (The boat's top speed is 19.7Knots/~30MPH over land once on plane). The problem that i am seeing is that before i get on plane, the thing seems to start to bog and sputtering after about 1/2 throttle. When i have another person in the boat the motor wont get us "out of the hole" at all. Again, it seems to sound fine until about 1/2 throttle, after that it just bogs, sputters, and occasionally backfires until (if) we get onto plane and then the motor revs up to what seems to be full rpm and everything is good. I found it got worst after the motor had been running for awhile. I spoke to boat dealer to see what his experience was with various motors on that model boat, he said a good working 9.9HP 2-stroke should pop the boat (carrying a couple of people) onto plane within seconds. This indicates to me that there is a problem with my outboard. I ran through probably 4 tanks of fuel since i bought it and have used Yahama oil @ 100:1 (i tried varying this a bit), didn't get better nor worst. At the beginning i did adjust "low speed jet" for idle, but was told that there was no "high speed jet" to adjust- it was fixed.



I am hoping someone could offer advice on where i should start to troubleshooting.



Thanks in advance

ERRM

 

[WillyBWright]

Re: Troubles @ >1/2 throttle

Adjust the idle so it's a bit rough. It needs to be a little richer than a nice smooth idle for acceleration. Basically you find smoooth idle and richen 1/8 to 1/4 turn. 100:1 always makes me nervous.

 

[Ray Neudecker]

Re: Troubles @ >1/2 throttle

Signs are of a fuel restriction. What did you do about the fuel pump?

 

[JB]

Re: Troubles @ >1/2 throttle

Howdy, ERRM.

Welcome to iboats.

I am no expert on your engine, but I have meddled with small engines for a long time and fixed many a carb, including many that had just been "cleaned".

In the throat of the carb there are small orifices that feed fuel into the airstream. There are often seperate orifices for low, medium and high speed operation. They come after the jets that meter the amount of fuel.

Your description sounds exactly like a carb that has a restricted or plugged medium speed orifice. If the backfire you describe is a "cough" then that confirms a lean condition.

Take the carb off again and inspect the throat for those tiny openings. They can be cleared with a piece of wire bent into an "L" shape, but the best way to clear them is to remove the plug opposite them and run a wire through them or blow them clear with high pressure air.

Good luck, and let us know what you find.