Carb kit question?

[Arkie]

Several small engine carb kits now days come with nylon flapper reed valves diaphragms AND the black rubberized type reed valve diaphragms. The black rubberized type is usually what I find in
EOM Walbro carbs.

For example the Walbro K20wat carb kit has both types in the kit. (for a Walbro WT827B carb)


When should the lighter colored nylon type be used instead of the black rubber type?

[KE4AVB]

As far as I know the plastic looking piece is a mylar piece.

I just use which ever one that came with the carburetor myself. Now I have ran into where someone have install both pieces which causes problems later.

Now the mylar is more ethanol resistant and there are metering side mylar diaphragms too but those kits are expensive and they are just the metering side.

[Arkie]

As far as I know the plastic looking piece is a mylar piece.

I just use which ever one that came with the carburetor myself. Now I have ran into where someone have install both pieces which causes problems later.

Now the mylar is more ethanol resistant and there are metering side mylar diaphragms too but those kits are expensive and they are just the metering side.

I also just use whichever type that came with the carb, but I have a Walbro WT827B carb that will not run with the rubberized type flapper reed type that is normally in these carb's. I can install the nylon looking one and the carb works really good and the jets adjust correctly. This nylon looking one is not the stiff plastic, it looks more like nylon cloth and more flexible than the mylar plastic type. I cannot see any difference in them other than the type material???????
Just cannot figure out why the carb runs really good with the nylon type and no go with the OEM type rubberized??????? The rubberized reed type diaphragm that looks like the OEM one severely floods or causes the engine to be over rich.
The carb would not operate the little 2 cycle engine and had old hard reed and pump diaphragms when I took it apart for theK20wat kit.

I''ve also heard of guys installing both of the reed flapper type and having issues also.

[Skywatcher]

Hi Arkie

On these carburetors, there are three different materials used for the pump diaphragm, neoprene, Mylar and fiberglass. Mylar can be blue, black or clear, neoprene is always black and the fiberglass diaphragms are brown with a weave pattern. If the face of the carburetor is machined to a mirror finish, then the Mylar or fiberglass diaphragm should be used. If the face of the carburetor is as cast with no machining, then the soft neoprene diaphragm must be used in order to form a seal against the cast face. Hope this points you in the right direction. All the best,

Sky

[Arkie]

Hi Arkie

On these carburetors, there are three different materials used for the pump diaphragm, neoprene, Mylar and fiberglass. Mylar can be blue, black or clear, neoprene is always black and the fiberglass diaphragms are brown with a weave pattern. If the face of the carburetor is machined to a mirror finish, then the Mylar or fiberglass diaphragm should be used. If the face of the carburetor is as cast with no machining, then the soft neoprene diaphragm must be used in order to form a seal against the cast face. Hope this points you in the right direction. All the best,

Sky

This carb does have the polished very smooth surface.
I just happened to accidently stumble onto this and the carb would have went into my carb salvage bucket if the nylon type had not been in the kit. I had put a china clone carb on the unit and got a good idle but a rough full throttle and the H jet no adjust (bad clone) but the new looking engine & Walbro would not even get a start due to flooding immediately.
The pump side of the carb tested good using vac/pressure and the nylon reed flapper just happened to be handy from the kit when I removed the reed side to inspect so as a one last attempt I installed it and was surprised when I got a run. I took the carb back apart and carefully inspected the reed flapper holes and the rubberized reed and same thing again. Re-installed the nylon one and A-ok. If the carb had not been really user friendly to remove/re-install and test it would have went to my in my carb salvage bucket fast.

I searched on-line for any info about using the nylon or mylar and no info but did see a video of a guy installing a kit in a WT827 carb and his Walbro K20WAT kit DID NOT have the nylon type diaphragms, just the neoprene type.

Maybe Walbro just started adding the nylon type pieces to their later kits??

Will have to keep a heads up on this one when doing future carbs.

I'm wondering now if a guy could use a flat surface such as a piece of glass and 1200 wet/dry and lightly break the polished surface and get a good run using the neophrene rubberized type when the mylar or nylon is not in a kit as a last step attempt at getting a good run?

[SUKI]

If haven't already, you need to get the W tools for setting the metering lever. And yes there is two versions of them as new carbs requires the new W tool. But the Zama still uses the same Z tool.

If metering is set too low it will cause lean out at full throttle and when set too high will cause flooding at idle. Also note that the metering needle due develop ringing and that can cause flooding too. You need a strong loupe to see this ringing or at least I do. I recently had one carburetor to pass the pump side pressure test but still was flooding because of this.

[bgsengine]

I'm wondering now if a guy could use a flat surface such as a piece of glass and 1200 wet/dry and lightly break the polished surface and get a good run using the neophrene rubberized type when the mylar or nylon is not in a kit as a last step attempt at getting a good run?

I'd consider inspecting the rubber diaphragm especially around the flappers (though they should not matter if the metering needle itself is seating properly, since metering needle controls fuel flow to jets from fuel pump side) But as you say engine floods when you try to start it, then you have a problem with fuel leaking past metering needle , that or a crack in the carburetor body allowing fuel to flow from pump side to metering side. Choice of diaphragms should not affect it nor should it matter really - both should work equally well, chasing that for a flooding issue is just a red herring..

[Arkie]

I'm wondering now if a guy could use a flat surface such as a piece of glass and 1200 wet/dry and lightly break the polished surface and get a good run using the neophrene rubberized type when the mylar or nylon is not in a kit as a last step attempt at getting a good run?

I'd consider inspecting the rubber diaphragm especially around the flappers (though they should not matter if the metering needle itself is seating properly, since metering needle controls fuel flow to jets from fuel pump side) But as you say engine floods when you try to start it, then you have a problem with fuel leaking past metering needle , that or a crack in the carburetor body allowing fuel to flow from pump side to metering side. Choice of diaphragms should not affect it nor should it matter really - both should work equally well, chasing that for a flooding issue is just a red herring..

The pump side is also what I suspected and tested the new needle and pump diaphragm with the mityvac and could not make it test leakage.

Also when when it failed again I went to the reed side ONLY, did not even bother the pump side and did not find anything wrong and just happened to try the nylon reed when comparing it to the neoprene visually and got the good run.
Also inspected very carefully the new neoprene reed diaphragm at the flaps and the carb body under the magnifier after the nylon reeds ran ok.
could not detect anything amiss so I had to try the neoprene AGAIN and no go. another head scratcher brain teaser.

I be putting this rig to long term performance test before long and report back if the carb goes south. Keeping my fingers crossed.

[Arkie]

In reference to my question about carb pump materials.

Seen this guy asks about such:

Note post #7 about differences, etc.
https://www.arboristsite.com/threads/ca ... st-7718654