User Manual

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intake/choke.
(2) Both lowend AND highend needles feed the topend fuel supply.
Let's tune up a Walbro!
Set the lowend & highend needles to about 1 3/4 to 2 turns each. Choke the carb or prime it, until the carb is
wet. Fire up the engine and let it warm up. Let's set the topend first since it's the easier of the two. Go to full
throttle. Adjust the topend needle for peak RPM. Leave it wide open for about a minute to see if it changes
any. Should the engine go lean, open the lowend needle slightly, if this dosen't work... you will have to adjust
the needle valve inside the carb.( I will explain this later) If the topend runs OK, then slowly pull the throttle
down until the engine begins to "four cycle" hold the throttle there. Adjust the lowend needle until the "four
cycling" barely stops. Now lower the throttle more until it "four cycles" again, and adjust the lowend again.
Keep doing this until you reach full idle. Now, from full idle begin to throttle up until the engine starts to bog
or hesitate. Open up the lowend needle just enough to eliminate the bog or hesitation.
When this is done right, you will be able to set the throttle in any position and it won't four
cycle, plus you will
be able to transition from idle to full power without any hesitation at all. Now, for easy starting it's best to have
the lowend a little rich and it will four-stroke a little.
TYPICAL PROBLEMS encountered in Walbro carbs:
The float diaphragm will go bad occasionally and reap havoc on tuning, especially the lowend and idle.
The internal needle valve WILL leak when they get old or worn out. If you notice gas dripping from your carb,
or if the idle gets unreliable, replace the needle valve and adjust the lever even with (or slightly below) the
carb housing if you don't have a gauge.
The fuel pump membrane gets stretched and/or sometimes hardens from the gas. It needs to be replaced
occasionally. If your pump doesn't pump properly, (1) it will be hard to start (2) it will tend to run lean, and
opening the needles won't help much if any.
Erratic idle, or no idle is often traced to a bad internal needle valve, bad float diaphragm, bad pump, and most
common... crap in the carb. You may also have an air leak!
Things to know...
There are three types of fuel pump membranes available. The black one is the rubberized fiber, the Beige one
is fiberglass based, and the blue one is Acetate. They all work well, but the black one moves more gas than the
others but it wears out the fastest. The Beige one works best if you use methanol fuel and is reasonably
durable. The Blue Acetate is the most durable but pumps the least fuel.
All Walbro carbs will run in any position, but they tune "best" as a sidedraft carb. The down draft position
tends to run a little rich at idle, and the updraft tends to run a little lean at idle. No big deal though, it's easily
tuned none the less.
This photo depicts one of Walbro's premier carbs, having a large bore and equipped with a highspeed check
valve and external fuel pump pulse inlet.