Carb Diaphragm Materials

[ChillyB]

Rebuilt a carb for a cheap Craftsman saw. Kit came with the skeletonized gasket later and two different materials for the valve side. One layer was black, the other layer was the color of a bandaid. I installed the black layer. What is the purpose of the bandaid colored diaphragm?

 

[buzz sawyer]

If it's what I think it is, that holds up better with alcohol-based fuels. Can you post a photo?

 

[ChillyB]

 

[ChillyB]

If one does not use gas with ethanol which material is better? Black glossy stuff, or bandaid looking material?

 

[buzz sawyer]

I'm assuming the black glossy stuff is a rubberized (neoprene) material. Either one will work. Frankly, I am old school and like the black material better. I use non-eth/Rec90.

 

[ChillyB]

Thanks.

 

[heimannm]

The black glossy one is acetate or Mylar.

Mark

 

[KASH]

Thanks Heimannm great informative post.
Kash

 

[heimannm]

Not my photo, I don't remember who originally posted it but it comes in quite useful at times.

Mark

 

[KASH]

I dont use any gas with ethanol but I took a carb off a Partner P70 tonight and the pump diaphram was the Teflon type in the past I would have changed it to a rubber one but after I read your post it is definetly staying in the carb.
Thanks for teaching an old dog a new trick.
Kash

 

[dboyd351]

I learned something too. Didn't know the "bandaid" material was teflon or that is was rated better than rubber. I also always had an old school bias toward the rubber one.

After re-reading Mark's post, I do see it says the teflon materiel may pump too much gas for many carbs. Since I only use non ethanol fuel, maybe I'll stick with rubber!

 

[Okie]

older thread but:

If the teflon has higher pump CAPACITY the needle on the metering side of the carb should regulate the amount of fuel being used by the engine?
I had a 2 cycle walbro kit for a small tiller awhile back and the little carb adjusted and ran lots better with the Acetate pump material instead of the black. The kit did not have the teflon pump material.
I just happened to test the carb with the Acetate type after it did not adjust correctly with the black material. The carb passed the needle pressure test at 5-7 lbs.


What am I missing here? (is the teflon pump capacity more than 7-10 lbs?)

 

[Old2stroke]

The more flexible the pump diaphragm material is, the more it will deflect in the pump chamber and the more fuel it will be capable of pumping. The amount of pressure the pump delivers is related to the strength of the crankcase impulses as well as the stiffness of the diaphragm. The teflon dia. is the best compromise between longevity of the dia. and the volume of fuel delivery. The "pop-off" pressure that will force the needle valve open should always be above the fuel pressure from the pump and the volume of fuel the pump will deliver should always be enough to supply the volume of fuel required for the engine to operate at full power. The main problem that can occur with diaphragms is the acetate one can be too stiff to deliver enough fuel for some applications. The interaction of the main fuel chamber dia. with the inlet valve is indeed what regulates the fuel used by the engine and the choice of dia. should not affect the tuning as long as it will supply enough fuel.

 

[Okie]

The interaction of the main fuel chamber dia. with the inlet valve is indeed what regulates the fuel used by the engine and the choice of dia. should not affect the tuning as long as it will supply enough fuel.

My thoughts also but I got a pleasant surprise when I swapped the pump materials and the itty bitty carb came alive. If I see the kit today I'll post up the model of the little carb.

Posted later today:
The carb is a Walbro 827B that preferred the Blue plastic pump diaphragm for a good tune and run and the kit is a K20.

 

[heimannm]

The capacity of the pump refers to volume, not pressure. They will all create more or less the same pressure, and never enough to unseat the needle unless there is another issue (worn or dirty needle, etc.) The more flexible the material the more it flexes with the impulse signal and thus more flow. I'm not sure what the OP was referring to when they noted the Teflon may be too much for some carburetors since the metering system will regulate the flow based on demand.

Some kits also include a Teflon fuel pump "diaphragm" with a hole where the pumping portion of the diaphragm would be. These are to be used with the Nitrile (rubber) diaphragm as a back up to insure proper sealing.



Mark

 

[heimannm]

 

[Okie]

The capacity of the pump refers to volume, not pressure.

Very good.

and the info about the teflon diaphragm to help insure sealing of the Nitrile (rubber) diaphragm.

Thanks