300/440/550 650sx won't hold tune (new to skis)

[j0hnny]

okay so i searched and reading through all i can find is superjet stuff which while it helps some its not exact.

scenario:

friend has a 650sx, all stock as far as we know. carb is for sure a stock 38mm mikuni unit. only thing i have ever tuned only had 2 needles and idle adjustment i am not familure with pop off and high speed adjusters so maybe that is where i need to start. ski starts well and runs decent. but sputters and surges at top speed. i started by tuning just top end (2 1/4 turns out slightly leaned it from there till it ran amazing) then started tuning low speed and idle (2 1/4 out and richened it a little) ski was running amazing but every time we come into the dock to change riders the speed and consistency changes. after we let it cool down and go to get gas it makes plenty of power to stand up and ride but goosing the throttle barely changes acceleration. and no matter how i mess with it i can't get it to run flat out again like it was.

i'm thinking carb needs a rebuild. because that is my experience when a 2 stroke is this inconsistent minute to minute.

any help is greatly appreciated. if u need more info lmk.

thanks johnny

 

[Layne]

Carb would definitely be good to take apart and clean, especially if you don't know when the last time way.

When you are tuning though, make sure you do the low speed first and have that set just right, then leave it alone and move to the high speed.

 

[j0hnny]

okay that may be part of the problem i thought i should be doing it the other way around. just went out and looked at the plugs now. compression is good between the 2 cyl's but i think i may have week spark on the rear cyl. i cut down the wires and it made a huge difference thanks man i will go tune low speed first, see if that helps. any recommendation on the rebuild kit to go with?