Motorcycle Repair: watercraft carb adjustment, fuel mixture, bn series

Question
Any advice will be helpful. I have a 3 cyl 750 cc 2 stroke polaris watercraft. One piston was burnt from a lean carb. I have replaced it. It has three carbs so how do I adjust the mixture. It has 2 screws, one above throttle butterfly and one below. Should I use a fuel conditioner to clean carbs...John

Answer
John,

-not sure what carb you have, I thought maybe
a Mikuni BN series for watercraft.

-The lower screw is usually the low speed mixture
and if it is a BN model it would be under
where the throttle cable hooks up.

-The high speed screw is on the other side of the carb.

-The idle speed screw should be where the cable hooks up
as well.

-The low speed mixture is adjusted by setting the idle
speed low and then turning the mixture screw in or out to
obtain the highest or smoothest idle.

-high speed is best done by a fast run and kill the
throttle quickly, read the spark plugs,
black is rich fuel, light or white is too lean,
tan or light brown color is good.

-keep a close watch on your plug color as
this will warn you of lean mixtures which
do engine damage.

-other possible causes of lean mixtures
can be a leak around the intake or carb.
Also on two strokes, crankshaft seals
can leak and lean out the fuel mixture.

-if unsure you might want to run a slightly
cooler heat range spark plug. Higher
plug number is a cooler plug. a 6 or 7 is hot
and a 8 or 9 is cooler on NGKs.

-some people seem to like seafoam fuel additive
or conditioner. It seems to help in some cases.