The 2nd series of the 91FX has given people fits with air bubbles from the needle valve area to the carb. This was cured with a different remote needle valve assembly.

The 3rd version of the 91 FX had a black plastic body on the remote needle valve. These have been OK so Far.

If the temperature has been in the 40 to 70 degree range, You could possibly be getting carb ice though it would be more common around the low 40's? I say this as you lead me to believe that it will run at high speed OK at times?

If the engine wouldn't run fast ever, I would say your LS needle was too lean?

I would try opening your LS mixture screw in the middle of the throttle arm pivot about 1/4 of a turn and see what results you get?

The 14-6 is correct.

Enjoy,

Jim