As discussed in part 2, it’s all about air. If you want more power, you need more air. Logic dictates that a larger carburetor will allow more airflow into the engine, and that additional air should result in more power. There are limits. Get carried away with the carburetor and you end up with an engine that only makes power at very high rpm. A 650cc single-cylinder engine should work good with carburetors around 34mm to 40mm. A 42mm might work OK, but it would probably sacrifice a lot of low-end power for high rpm power.
The stock carburetor is billed as a 40mm carb, but it’s venturi cross section is more on the order of 35.4mm. It’s a constant velocity (CV) carb, so it has a butterfly and throttle shaft in addition to the typical slide. It probably flows about the same as a 34mm variable venturi carb (Mikuni VM, Keihin PWK, DelOrto, Amal, etc.).
One of the goals of this project is to build our hotrod on a tight budget. With that in mind, I had been searching the internet for a bargain basement carburetor. About nine months ago, I found a deal on Amazon that was just too good to pass up. It was a Keihin PWK40 for $28 bucks. I had no idea what to expect, but hey, $28 bucks???? I figured I could take the risk. With shipping and tax, the whole shebang came out to $36 dollars.
https://www.amazon.com/PWK40-40mm-PWK-Carburetor-Carb/dp/B07M824BB2/ref=psdc_...