Hi Rollmyown,
It sound as though there is some confusion about carb size. Carbs are sized by the venturi diameter. Walbro carbs have a number cast into the inlet side of the venturi. This number is in 64th's, so 28 equals 7/16ths or 11.11mm. 32 64ths equal 1/2" or 12.7mm and so on. Zama carbs don't have this and must be measured with a drill bit shank or caliper. I have seen the same Zama carb model with two different venturi sizes. The next thing is the throttle bore which I find to be 5/8" on all of my Walbro carbs 11mm and above. I don't have any large Zama carbs since all of mine are off trimmers. The throttle bore varies on the 10mm and below carbs from 7/16ths to 9/16ths depending on what the manufacture wants. When you told me that you were using a 14mm carb, you were talking about the throttle bore and I assumed that you were talking about the venturi size. Your carb is actually a 8,10 or 11mm which is fine for a 25cc engine. I guess the big test is to try the Toro with both 11mm and 12.7 and see if there is a gain. If there is no gain, the 11mm will give better endurance. Another interesting experiment would be to make a manifold in three or four sections to test fly and see if a shorter manifold is OK. This would make cowling easier.

Dave