by pumpguy » Tue Nov 15, 2022 12:30 pm
Back in the day, my brother and I were running a '55 125 Hummer in scrambles racing and enduros.
I managed to find a flange mount carburetor that was one of a pair that came from a 2 cylinder Yamaha 250 YDS1 (or maybe YDS2) scrambler. I believe that carb was around 20MM bore size.
I cut off the original cast iron carb mount from the cylinder, then fabricated a flange for the Yamaha carb and brazed it on. Before the brazing, I opened up the passage to match the bore of the carb and adaptor flange. It worked great. Didn't even have to make any jetting changes.
With this illustration, I'm trying to make 2 points.
1) Unless you open up the passage in the cylinder to match the bore of the larger carburetor, you're not going to realize the gain the larger bore carburetor can provide.
2) Unless you feel you must keep the round clip type carb mount your cylinder has, cut the darn thing off and braze on an adaptor that will allow you to mount a bigger and better carburetor.