below describe the fuel
metering system and show 2 stages of dis-assembly:
Walbro has a gage for setting metering lever height, as shown below.
You can buy the gage from Walbro, or you can make your own.
you download WalbroGage.pdf
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
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
this side the gasket goes on first, then the diaphragm
(this is the opposite of the pump side). This is the newer
diaphragm, which does not have the interlocking tang for the metering
The following image shows the difference in the two diaphragm
If you have the style with the interlocking tang, be sure it is
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)
two diaphragms have different button heights, which will affect the
setting of the metering lever. The height I measured is
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.
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
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
this up, please contact me. But be aware that this is
unresolved. Having said that, this is my current Best Guess at the
- 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
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
- 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:
If you find information which
is more authoritative than this, use it instead.
On the pump side the installation order between diaphragm and
reversed - that is,
diaphgram portion goes
on first, then the gasket
. Note that the
material for the
pump diaphragm has changed. The original (black) one is at
left, the newer version is translucent with an embedded mesh visible.
The figure below shows the throttle plate modification to improve idle
and mid-range operation. This is discussed in detail by Gerry
The blue arrows show the notch before and after enlarging.