Don V., I certainly agree with your statements on cams and exhaust valve size. A little number crunching shows that for the 30 cubic inch engine (K301), the larger K321 exhaust valve installation is not worth the effort.
I'd like to share a carburetor modification that really brings out the best in a mildly hot rodded K301. I did this to both a Walbro and Kohler carburetor and they both run great on my engine, with the Kohler carb having the slight edge in good transient response.
The standard Kohler carb for the 12 horsepower engine is the #26, with a 1.067 throat diameter and .81 diameter venturi. The 16 horse carburetor is the #30, with a 1.197 diameter throat and .935 venturi. As I've mentioned before, my engine is a "street" mill that has to have a relatively broad torque band and good, crisp throttle response. It's used to mow and throw snow so going too wild is not good. When it comes to carburetion, bigger is not always better and in many cases, can be worse in the part throttle and transient ranges.
In putting a #30 on a 12 or 14 horse block, the carb throat is bigger than the inlet port. This means grinding the port to match the carb. In my opinion, it's a lot of unnecessary work for an engine that doesn't turn over 4000 rpm. Some calculations show that the velocity of the air through the venturi of the stock #26 carb at WOT on the inlet stroke at 3600 rpm will yield a Mean Mach Index of .503 (Mean Mach Index is defined as the calculated air velocity divided by the local sonic velocity). For high performance engines, you'd like to stay below .6 and the stock carburetor already is! But now we'd like to turn the souped up engine a little faster to extract more horsepower.
By boring the carb venturi of the #26 carburetor from the original .81 diameter to .875, the area increases from .515 to .601 sq. inches, or a 16.7% increase. Now recalculating the Mach Index at an increased 3900 rpm, we get a value of .467, clearly lower therefore less restrictive than the original at 3600 rpm. One could go bigger yet on diameter, and some remove the venturi all together. I think this is not wise for a working tractor for reasons mentioned above.
Proof is in how well it works and this modification seems to put the finishing touch on the engine. Throttle response is unaffected with crisp, clean acceleration when the throttle is slammed open. Full throttle operation is where one will really notice the power increase.
A picture showing a stock Walbro and the identical carb with venturi machined is shown below. It's hard to tell from the picture, but the bored one is on the right. The modification is done on a lathe after complete disassembly. Pilot off the throttle bore to keep venturi concentric with bore.
