Chainsaw Repair

Generally speaking a rotary valve engine takes its' intake through the crankcase by means of a rotating disc .I think the advantage being a higher torque engine  than piston port without the nuisance of reed flutter,broken reeds etc of a reed engine .

I have no idea if or why the carb should be different .

Vespa motor scooters used them and some Brit 2 cycle motorcyles . I think one of my old boat anchor Homelite gear drives might be rotary .

The rotary valve Walbro carbs I have work differently from a butterfly carb.

The rotary valve is controlled by a cam and when the air part is closed, the barrel rotor is all the in and when the air part is twisted open, the barrel rotor is all the way out.

The fuel enters the center of the barrel and as the barrel twist in and out it varies the fuel flow.

The idle needle is in the center of the barrel.

The HS needle varies the over all fuel flow.

On mine, to prime the engine, you close the carb and pull out on the rotor. The vacuum of the engine pulls fuel in through the fully open fuel metering system.

I had it over to your shop when I brought the engines and cylinders but didn't know you were interested in the barrel type Walbro carbs.

The rotary or barrel type carbs still have the pump and metering diaphragms in most cases. Some times they are piggy backed on top of each other. I think there are some where the strength of the pump's volume is biased by engine speed instead of actually having a metering diaphragm and needle/seat as we know it.

It is just that the fuel is metered out differently in the throttle itself.

These rotary throttles are typically black plastic.

On a butterfly carb, the one side of the butterfly uncovers more holes in the low speed circuit until you are running from the high speed nozzle in the venturi.

On a barrel rotor carb the main fuel comes out a hollow spray bar that is in the center of the barrel. The low speed screw that is in the center of the barrel from the throttle lever side telescopes in and out of the spray bar according to how open the carb is. They use the tolerance of the O.D. of the low speed needle relative to the i.d. of the high speed spray bar and how far "IN" the low speed needle is inside the spray bar to adjust the fuel flow for the different throttle openings.

  The diaphrams, gaskets, needle, etc. are all ordered separately (no kit)around $60 just for parts (got them @ Jacks). It's not worth putting a $111.00 carb. on it.

Thanks for that info right there. I will be passing on any backpack blower with rotary valve carb. Which is the first half of the husky line up.
Time to start looking deeper into stihl blowers now and anyone with  diaphragm carbs I am used to.

Ratioparts and Blount have repair and membrane kits for these.
Others too I bet..

The rotary valve carb is used on trimmers and hedge cutters in various brand as well.
I don't think I seen it in any saw....