HD 21 H and L screw o rings

[Tumleweed66]

Hi all... I put an HD 21 on the 039, the needle, gaskets and stuff look good but the o ring on the High screw was a little less than, the saw revved good but stumbled as I squeezed the throttle until the Low screw was a full 3 turns out and would idle erratic. Could it be sucking air past the high screw o ring causing it to act up?

I ordered a kit but I read here somewhere that the o rings don't come in the kit does anyone have the size for the o ring? ( it is a bit difficult to get an accurate measurement for size)

Also as I blasted the passages with the carb cleaner it appeared it has the joining fuel passages between the High and Low screws if so is this a less desirable carburetor.

thanks and any help is appreciated. Mike

 

[Vintage Engine Repairs]

Hi all... I put an HD 21 on the 039, the needle, gaskets and stuff look good but the o ring on the High screw was a little less than, the saw revved good but stumbled as I squeezed the throttle until the Low screw was a full 3 turns out and would idle erratic. Could it be sucking air past the high screw o ring causing it to act up?

I ordered a kit but I read here somewhere that the o rings don't come in the kit does anyone have the size for the o ring? ( it is a bit difficult to get an accurate measurement for size)

Also as I blasted the passages with the carb cleaner it appeared it has the joining fuel passages between the High and Low screws if so is this a less desirable carburetor.

thanks and any help is appreciated. Mike

Carbs with connected drillings are known as dependant carbs. The H circuit feeds the main jet whilst also the idle drillings. Thus you need to tune the H first, then the L. They are ok, they get the job done.

Regarding the o-rings, many saws don’t have them, some do. If they are present I’d keep them there / replace them. No idea of sizes I’m afraid. Treat yourself to an assortment pack of viton ones and use whatever fits. Note that o-rings should never be super tight or super squeezed.

They should only fit lightly snug - when they are in the drilling you will get the slight compression and sealing on them whilst still having full, unrestricted adjustment of the screws. If they are slightly too big they will cause more problems than you’ll solve.

If you don’t have o-rings to hand try smearing some thick grease around where the o-ring will go. If that solves your lean symptoms temporarily then you know your answer, if it doesn’t then it’s not your problem. Sometimes (not always) they are simply there to stop the needle backing out.

My guess is the o-ring isn’t your issue, any leak there if there even was one, would likely be very minor and easily adjusted out. You’d need a bigger leak than what you’d get through an adjustment screw to require 3 turns out and it still idle erratically. Don’t discount that it could be a lack of fuel rather than too much air too of course. Keep us updated

 

[Tumleweed66]

Carbs with connected drillings are known as dependant carbs. The H circuit feeds the main jet whilst also the idle drillings. Thus you need to tune the H first, then the L. They are ok, they get the job done.

Regarding the o-rings, many saws don’t have them, some do. If they are present I’d keep them there / replace them. No idea of sizes I’m afraid. Treat yourself to an assortment pack of viton ones and use whatever fits. Note that o-rings should never be super tight or super squeezed.

They should only fit lightly snug - when they are in the drilling you will get the slight compression and sealing on them whilst still having full, unrestricted adjustment of the screws. If they are slightly too big they will cause more problems than you’ll solve.

If you don’t have o-rings to hand try smearing some thick grease around where the o-ring will go. If that solves your lean symptoms temporarily then you know your answer, if it doesn’t then it’s not your problem. Sometimes (not always) they are simply there to stop the needle backing out.

My guess is the o-ring isn’t your issue, any leak there if there even was one, would likely be very minor and easily adjusted out. You’d need a bigger leak than what you’d get through an adjustment screw to require 3 turns out and it still idle erratically. Don’t discount that it could be a lack of fuel rather than too much air too of course. Keep us updated

thank you, the kit will be here today so I'll see how it acts then. thanks again Mike