Dan C. May: Fix the #30 for the customer for at least a couple of reasons.
1. IH fitted #26 to 10 HP and 12 HP motors, but used #30 for the 14 HP motors (I'm not sure about the 16 HP Kohlers).
2. Because . . .
I believe the "26" and "30" refer to the carb throat diameter in millimeters. A carburetor with a throat size of 30mm can flow a lot more than a carburetor with a throat size of 26mm.
Nic compares the difference to carburetors flowing 650 and 750 cubic feet per minute and states that the carb's suitability for each application depends on the state of the engine's tune. That is, you can "hop up" an engine designed for 650 cfm to handle, or even require, a 750 cfm carb; or, more to the point, de-tune a 14 HP motor to use a carburetor designed for a 12 HP motor.
Just focussing on the carb comparisons, I must note that the difference between 26mm and 30mm for the throat size is not the same as comparing flow rates of 650 and 750 CFM. At first glance, it might appear a valid comparison: 650 is 86% of 750 and 26 is 86% of 30, but remember the units. 650 & 750 are units of flow; 26 & 30 are throat diameters. If we compare the AREA of a 26mm carb throat to the area of a 30mm carb throat we see that the difference is more like 75% (pi*r*r). The fuel/air mix doesn't see 26mm as only 4mm less than 30mm, it sees it as a hole 175 square millimeters smaller than what it needs (531 mm2 for 26mm throat versus 706 mm2 for 30mm throat).
Bottom line, a #26 carb will <u>not</u> work on a 14 HP Kohler where a #30 will; and a #30 carb will work on a 14 HP Kohler where a #26 will not.
On the other hand, I think Nic could probably get a 12 HP up to where it needs a #30; but I bet he would have a hard time getting 14 HP down to where it was satisfied with a #26.
Note: A qualified engineer would look at more than just displacement, but also the crank's "throw" which affects the speed of the piston in the barrel. A bigger crank will have more torque, but will also suck more gas. I don't have the specs of the K301 and K321 in front of me, but any hot rodder (read "puller") worth his salt would be looking at the mechanicals before fitting the carb, recognizing each change's implications for carburetion.