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Originally Posted by imagineer
I suspect I will have to rejet. It is running too lean and needs almost full choke to go to wide open without sputtering. I haven't looked at the carb yet, are there any mixture screws?
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Yes, there's a mixture screw that trims the air / fuel mixture, but it's for idle and the transition from idle to main. Remove the top cap from the carb, carefully compress the throttle spring with one hand and remove the throttle cable from the slider with the other.
Take the slider assembly to a work surface and remove the spring plate that holds the needle jet in the slider, then remove the needle jet. This is the needle that mates with the main jet, and it is secured by a small circlip in one of five grooves. Move the clip to the bottom groove, and that will max out the fuel flow. Reassemble in reverse order and see how it runs.
If it's still too lean, remove the carb and pull the bowl off of the bottom. The main jet will likely unscrew from a brass hex holder. Keihin and Mikuni style carbs use an intelligent system that references millimetres, rather than some arbitrary number. If the main jet is stamped as 100, that means 1.00mm. A jump to 1.04 mm would be the next logical step.
I pick up jets at my local Yamaha dealer for a couple of dollars each.