You are correct on the WT-610. I looked and it has 32 in the venturi which is 32/64th or 1/2 or a little over 13mm. I have not tried this carb yet. Only the WT-600 which has 28 stamped on the venturi. The Ryobi in the plane idles good at 2300 with a stock carb and 18 inch prop. I guess I'll try a WT-600 on it as I know it will idle ok. I have 7 of these motors and will try another reed assembly on the test motor and see if that stops the engine from dying at idle. If I can keep this engine from dying at idle, the 18 inch prop may get the idle RPMs to a useable level. Thanks for you reply. Pat P.S. Bye the way, I talked to Aero Nut last night on the phone and he is building me a Charly Boy. I believe I read somewhere where you had one.