Yamaha 15hp Enduro misfiring

[Panache5000]

I have a 2009 Yamaha 15Hp Enduro 2stroke, well maintained. When operating it just above idle speed, the engine mis-fires (runs rough). Upon opening the throttle to get on the plane, the engine hesitates/mis-fires acouple of times, and then takes off.
When operating at 1/2 to Full throttle, it performs 100% and no mis-firing.

I also notice a lot of wet oily deposit at the rear outlet opening on the outside of the upper drive casing. The manual states 50:1 oil mixture for this Enduro model, and 100:1 for the more "civilised" 15Hp model, which are identical in specification. I have reduced the oil to 75:1, but no improvement.

New spark plugs (acc to manuf specs) made no diffirence.

Any suggestions/similar problems?

[colemj]

Take a look at the fuel pump membrane on the carburator - probably small leak in it or stretched beyond specs. Cheap to replace just to see if it was the problem.

Mark

[Vasco]

I would start by cleaning the carb. As for oil/gas mix, the only reason Yamaha recommends 100:1 is for the EPA. (when 2 strokes were still being sold in US). Elsewhere they recommend 50:1 for the same engine.

[MarkJ]

It sounds like you have a mis-adjusted slow speed mixture and high speed mixture. (air mix, not oil/fuel mix)
On the Mercury they are two screws to fix it. I dont know about the Yamaha.

[Panache5000]

Originally Posted by colemj

Take a look at the fuel pump membrane on the carburator - probably small leak in it or stretched beyond specs. Cheap to replace just to see if it was the problem.

Mark

Hi Mark. Thanks for advice. I have replaced ALL the rubbers/membranes on the fuel pump. NO change, still the same.

[TOM]

Having owned one of these motors i would say that you almost certainly have a carburettor problem . You need to purchase a couple of cans of carbi cleaner ,take the carbi off and give it a good spray. Make sure you clean all the jets it may look clean but you only need a microscopic piece of dirt to stuff things up. It is not difficult to take off the carbi but i would recomend you take the motor off the dingy first .

Good luck.

byamee1@hotmail.com

[perchance]

Start by examining the spark plugs to see if one or both are oiled up. The oil is from incompletely burned fuel.
The float level may be set slightly high causing flooding at low speed.