Rotary Carburetors vs Butterfly Carburetors

[Vintage Engine Repairs]

It’s hard to dislike rotary barrel carburetors, they have no main nozzles to fail, don’t need accelerator pumps, they have far better / linear fuel delivery, no welch plugs, full adjustability, cheaper to make, no flat spots, can be run at part throttle without leaning out and don’t need to be set on the rich side at idle to for good acceleration.

Can anyone shed light on why they’re not used on chainsaws? It must be either engine capacity related or no need for sustained, part load throttle? Walbro only makes them for engines up to 50cc but I can’t understand why.

 

[Old2stroke]

I have often wondered the same thing. I have a Toro trimmer with a rotary throttle that runs great, it's been awhile since I had it apart but I think the throttle lifts a tapered needle in the main jet like the British SU carbs and all the Mikuni carbs on Japanese bikes and snowmobiles but I don't remember how the fuel got to the main jet. Couldn't be from a float bowl like the other carbs in order to be an all-position carb, maybe this all-position requirement for saws could be an issue?

 

[Vintage Engine Repairs]

Yep they’re an all position carburetor too, That’s right, they have a fuel pump and metering chamber in one, just on opposite sides. As you pull the throttle, the barrel opens on a cam to let more air in and at the same time the needle is raised from the fuel supply tube.

I did a video on one:

 

[sean donato]

The ramp that opens and closes the throttle wears out pretty quick in dirty environments. Other then that, and possibly fule/air flow issues idk why they couldn't be used. I do know they wear out quick in the little rotatillers. Easy enough to covert to a butterfly carb.