9.9hp 2 stroke runs poor at idle to 1/4 throttle

[Yamahataos]

I have a 2011 yamaha 2 stroke 9.9 hp that runs poor at low rpm idle to 1/4 throttle , it seems like it's running rich and if left running at 1/4 throttle for a couple of minutes or more it seems like it's starting to load up the plugs , I then run it at higher rpm and it clears up , I don't run it a lot a slower rpm as I don't want to foul the plugs , it did it from new , I have changed the plugs no differences ,I cleaned the carb full dissasembly ,cleaned the jets ,fuel pump ,lines .fuel filter, no dirt notice , all orifices are visibly clean , carb cleaner, and compressed air, no holes in fuel pump diaphrams , did a compression check 10 compression strokes 135 psi in both cylinders, cold engine ,performed a spark test 28000 volts at both coil wires, I'm running oil mix at 75 to 1 , I was told these engines must run at 100 to 1 or it will cause this kind of problem ( not sure if this is true , the plugs look a little dark that is there not a nice brown ,tan color ,but not black , the engine runs perfect at about 1/3 throttle to full and lots of power ,no hesitation , or bog. This engine has very little hours maybe 2 tank of gas, oh also I'm using regular gas 87 octane with 10% ethanol, I'm thinking of using premium 93 octane as there is no ethanol in our premium gas, I also always run engine out of fuel before storing it, due to ethanol leaving a varnish film in jets of the carb as I have experienced this on another small engine .
Thanks
Don

[99yam40]

Did it do this before you modified it to a 15 ?

[Yamahataos]

Yes it was the same ,I ran it with the original reed valves for about five hours as I didn't want to push it durring break in , no difference after the new reed valves for the 15 hp were installed. And the only part that is different between the 9.9 hp and 15 hp is the reed valves ,on the 15 hp they open wider .i checked every part down to the cotter pin on the prop. And of course the decals are different.

Don