The carb linkage is the right way when you see the screw threads when looking into the carb from the choke side with the throttle closed. When the butterfly is closed, the screw head goes towards the engine. If this is incorrect, it can cause all kinds of idle and throttle up problems. Check yours. I use an 11mm carb on mine, and it works fine but I have it set up carefully. I use a long arm on the carb linkage to slow the movement down somewhat, and I use some exponential on my throttle to slow the movement down as well. I also tune my idle a touch rich to help with the throttle up transition. If everything is just right, an 11mm carb works ok.

A little less finicky and easy to set up, would be a bit smaller carb. Look for one that has either a "20" or "24" cast into the carb throat near the choke. Either should work fine on the .90. You can look up carb numbers and venturi sizes here: http://wem.walbro.com/walbro/partnum...anuItemNumber=

AV8TOR