Flying the Mosquito
Wind Drifter
Technical Info

Walbro WG-8 Carb Disassembly

This shows the disassembly of Walbro WG-8 carb that was supplied with the Radne Aero 120 on a Mosquito NRG.  Note the purge button on the diaphram cover - this has apparently been discontinued on later models, but is very useful for bleeding air from the fuel lines.  Apparently some units are now shipping with the WG-10 carb.  A comparison of part numbers on Walbro's website shows that they are nearly identical.  The only differences I could find were:  
So presumably the information on this page will apply to the WG-10 in most respects. There is also a WG-12, which uses the same repair kit, which implies a great deal of similarily for that model as well.
Walbro WG-8

This exploded parts diagram is for the WG-8, but applies to the WG-10 (with slight exceptions noted above). Thanks to Greg Dewenter for the edited form of the parts diagram below (which is better than the one currently on Walbro's parts page):
Exploded view and parts list
Parts are available on Walbro's Website

There are two repair kits available.  The K12-WG has almost all the parts, including needle, fuel strainer screens, etc.  Note that it does not have a metering spring.  The D12-WG has just the gaskets and diaphragms (metering and pump).
Repair Kits

The figures below show the operation of the fuel pump and the fuel pump diaphram on the WG-8. Air impulse pressure on one side of the diaphram serves to pump the fuel on the other side.  The blue arrow points to the final fuel filter screen (which has collected some debris in the photo).  The operation diagrams and explanations on this page are excerpted from theWalbro Service Manual:
Fuel Pump Operation
Fuel Pump Side

The figures below describe the fuel metering system and show 2 stages of dis-assembly:
Fuel Metering Description
Metering lever and spring

Walbro has a gage for setting metering lever height, as shown below.

Setting Metering Level Height

You can buy the gage from Walbro, or you can make your own.  If you download WalbroGage.pdf  you can print out an exact scale image of the gage (be sure to turn off page scaling when you print it).  If it does not print correctly, there is a ruler in the picture you can use to scale it.  The dimensions are also included.  NEW INFO - see discussion below on different diaphragms.

The figure below shows the installation of the metering diaphragm and cover.  Note that on this side the gasket goes on first, then the diaphragm.  (this is the opposite of the pump side).  This is the newer style diaphragm, which does not have the interlocking tang for the metering lever.
Installing the Diaphragm

The following image shows the difference in the two diaphragm styles Diaphragm Differences
If you have the style with the interlocking tang, be sure it is properly engaged in the metering lever slot.  When I bought the K12-WG repair kit from a Walbro distributor, it did not have the tang (on the right in the above image).  The replacement kit I ordered from Radne (part #32155) had the diaphragm on the left (in a bag dated Mar '05 - so perhaps it was old stock)  

Important:  The two diaphragms have different button heights, which will affect the setting of the metering lever.  The height I measured is nearly 1 mm (0.94 mm is what I measured being as precise as I could). The button height on the interlocking button (not counting the interlocking tang) is about 2 mm, while on the plain button it is only about 1 mm.  The question is, which diaphram is the gage designed for?  I think perhaps the button version (on the left), but that is only a guess.  Responses from Walbro (US and Australia) were less than totally enlightening.  Yes, there is a difference, yes, it should be compensated for, no, it's not that critical, they are trying to standardize settings, and so on.  If you have information to clear this up, please contact me.  But be aware that this is currently unresolved.  Having said that, this is my current Best Guess at the correct settings:
  • Diaphragm with tang - set using the Walbro Gage WG tang (1.7 mm below the body surface). This is approximately flush to the carb body surface
  • Diaphragm without tang - my logic says the lever should be ~0.95mm higher to operate the same, therefore the lever should be set ~0.75mm below the body surface.
  • The displacement of the metering valve is limited by the metal plate on the diaphram. When this contacts the carb body, no further movement is possible. The WB gage sets the tang approximately even with the carb body. That means there is about 2mm of movement possible with the interlocking tang. With the 1 mm high plain tang, only 1 mm of movement is possible. Maybe that is enough, but erring on a low setting for the lever certainly risks fuel starvation.

This image further clarifies the travel limiting effect of the metal plate on the carb body: Lever Travel Limit
If you find information which is more authoritative than this, use it instead.
On the pump side the installation order between diaphragm and gasket is reversed - that is, the pump diaphgram portion goes on first, then the gasket.  Note that the material for the pump diaphragm has changed.  The original (black) one is at the left, the newer version is translucent with an embedded mesh visible.
Pump side installation

The figure below shows the throttle plate modification to improve idle and mid-range operation.  This is discussed in detail by Gerry Farell:   The blue arrows show the notch before and after enlarging.
Throttle plate modification

The newer carbs have done away with the purge valve (see the figure at the top of this page).  I have found it very useful for bleeding the fuel line, and I'm glad to have it.  I understand the newer ones just have a hole in the metering diaphragm cover.  It probably wouldn't be too hard to modify a cover to add a purge valve.  The pics below show how the original works.  It needs to be able to press on the metering diaphragm/lever, without damaging it.

Purge Valve Operation
The fuel line on many units has a 1/4" line, which can allow bubbles to collect, especially in hot weather (fuel percolating).  Even at full throttle the fuel flow is not sufficient to move the bubbles from a high spot in the line, where they eventually collect and slow fuel flow enough to cause performance to decrease.  A solution with which many have reported success is to use a smaller 1/8" (~3mm) line, which also involves removing the white plastic barb on the carb fuel inlet.  Before (top) and after(bottom) are shown below:
Small fuel line conversion
(The silver braid is/was a failed attempt to suppress radio noise from the ignition - resistor spark plugs are a much better solution).

I have discovered that the nylon TyWraps work well for securing the fuel line, however, be sure to take two wraps around the hose, as a single wrap will not apply uniform pressure all the way around. Using TyWrap to secure fuel line
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(updated December 23, 2023)