Carb rebuild. Too many pump diaphragms in the rebuild kit

[jlmanatee]

Hi folks. I'm rebuilding a carb off of a Stihl MS362 saw. The kit I ordered has 2 pump diaphragms in it. (not the metering diaphragm)
One is black and is very flexible. The other is tan colored and is slightly stiffer. How does one know which to use? Last week I fixed an
older Homelite that had the same situation. I ended up using both, the black one under the tan one thinking the extra pump diaphragm
was some kind of later improvement to the design. The saw worked great afterward. If it had not, I'd have removed the tan colored one.
Does anyone have experience with this situation? The included image is from the Homelite kit

 

[hotshot]

Either one will work, the tan is for folks that can’t get ethanol free or pure gasoline. Used mostly in newer carbs, is yours a Walbro WTE series?

Never tried stacking two fuel pump diaphragms together like that, and never seen it recommended by any carb manufacturers but glad it worked out for you.

 

[heimannm]

 

[Jay Hansen]

View attachment 1161145

Mark, this says that the Teflon diaphragm's pump capability is too high for many carbs; does that mean it can pump too much pressure for the metering needle to hold?

 

[jlmanatee]

Thanks everyone for this great information. I'm amazed the Homelite is actually working with both installed.

 

[heimannm]

Jay - No, I think what it is trying to tell you is that it can flow a lot of fuel. My opinion is the rubber one likely pumps even more since it is more flexible. Any pressure generated by the fuel pump section is simply a reflection of the impulse signal minus the resistance of the diaphragm.

I didn't put that illustration together, I just snagged it and keep reposting when the subject comes up since it seems to do a nice job of presenting the information.

jl - With the above in mind, having both diaphragms installed may reduce the overall flow the pump can put out which could lead to a lean condition under high load conditions if the carburetor was marginal to begin with.

Many carburetor kits include a second Teflon fuel pump diaphragm with the pump portion cut out. The flaps on the Teflon diaphragm back up the rubber ones, and the cut out section allows the rubber to flex more freely.



Mark

 

[GMB74]

Interesting.
So would you substitute the Teflon diaphragm with the cutout for the normal paper gasket? Or would you stack all 3 of them in there?
What would be the expected result of doing this?

 

[jlmanatee]

Interesting.
So would you substitute the Teflon diaphragm with the cutout for the normal paper gasket? Or would you stack all 3 of them in there?
What would be the expected result of doing this?

I stacked all three and the saw seems to be working. At least my neighbor has not brought it back yet. But if he does, I'll correct my mistake and probably have to tweak the hi and low needles. No matter how often I tell folks to only use non-ethanol gas on these older engines, they still use the less expensive stuff and I'll be sure to use just the teflon diaphragm only when I have a choice.

 

[heimannm]

Most of the time the gasket will be needed since the parts are not necessarily made with a true, flat surface. It is quite common to use the Teflon back up (with the fuel pump cut out) with the rubber and the gasket. In fact, on any SDC carburetor I go through I always use the Teflon back up whether there was one in the originally of not.

I don't see too many Tillotson cube type carburetors so I don't know if the current kits commonly have the Teflon back ups for the HS type carburetors.

A quick Google search doesn't show any for the Tillotson like those used on the Walbro SDC's.



Mark

 

[huskihl]

I wouldn’t stack multiple diaphragms together. I always use tan because it has as good or better ratings than all of the other fuel pump diaphragms so it makes sense to use it. The pump diaphragm and one gasket goes on one side and the gasket and then the metering diaphragm go on the other side in the opposite order