Walbro Carburetor Troubleshooting & Tuning
Adjustment and correction of problems found on
2013-10-17 Added HE Engine Pop Off Pressures
2014-11-13 Added reference to a source article and notes.
See also a
carburetor overview, Buyers guide for Walbro
Carbs and Rebuild Kits
Please visit the original source of this information
http://tech.flygsw.org/walbro_tuneup.htm for even more detail.
The most relevant parts have been included here in case the original
Some basic terms (see diagrams). The lowend
needle refers to the one that trims idle mixture mostly and affects
mid-range mixture somewhat. The highend needle trims mixture at
mid-range through full power. Unscrewing the needles on this carb
increases fuel flow which richens the fuel/air mixture. Screwing them in
reduces fuel flow which leans the fuel/air mixture. Don't confuse fuel/air
mixture with fuel/oil mixture which refers to how much oil gets
poured into each gallon of gas.
First, screw in both lowend & highend needles all the
way then unscrew to about 1 3/4 to 2 turns out each. Choke or prime the
motor until the carb is wet. Start 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.
Walbro carburetors should have a pop-off
pressure of between 10 and 14 psi. Here are some recommended values based
on the carb/engine combination ±1 PSI:
Walbro 32 on Black Devil=10.5
Walbro 32 on Solo 210=12.0
Walbro WG8 on Top 80=14.0
Walbro WG8 on H&E 90 & 125
12.5 to 13 (Thanks Larry Koral)
For more information on how to change pop
off pressure, visit aerocorsair.com.
Typical Walbro Problems
They're really quite reliable, but when problems arise,
they are frequently one of these.
The small filter on the fuel-intake side clogs. Clean it
out by removing and spraying carb cleaner through in the opposite
direction of normal fuel flow. Re-insert using a pencil eraser.
The float diaphragm degrades and reap havoc on tuning,
especially the low end 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!
The pop-off pressure is out of the normal range. About
12 PSI is normal. Pop off pressure is how much pressure is required to
pop the main-jet needle off of its seat. The fuel pump pushes fuel
against a spring. Either having the wrong spring, a cut spring or
stretched spring can affect this pressure.
All Walbro carbs will run in any position, but they tune
"best" as a side draft carb meaning the airflow intake is horizontal. The
down draft position tends to run a little rich at idle, and the updraft
tends to run a little lean at idle although it's easily tuned regardless.
This carb is equipped with a high speed
check valve and external fuel pump pulse inlet. Diagrams courtesy
Your ENGINE will determine which pulse inlet type you
need. If the engine carb mounting surface has a hole drilled into the
crankcase you will use the STANDARD pulse port and the optional (if you
have it) port must be closed off. If
there's no hole drilled, you will find a fitting located somewhere on your
crankcase. Use a piece of fuel line to connect the crankcase fitting to
the fitting on the optional pulse inlet. There's no need to block off the
standard port, as it's already blocked off by the engine mounting.
The carb must get a pulse signal from the engine!
This signal "pushes and pulls" on the pump diaphragm which feeds the carb
Float, Needle & Seat Setting
This critical setting is the primary way to adjust mixture
on some Walbro carbs such as the WG8 used on Top 80 motors. Apparently
Walbro offers a setting gauge to properly set the height of the
lever for each model. Without that gauge you'll have to use trial & error
which is a pain since you must open up the carb for each adjustment. In
general, the lever will be parallel to the carb base. If the lever is too
high, the mixture will be rich, too low and it will idle ok but be lean at
midrange and above. It may also run the float cavity dry at full throttle
and die, regardless of your highspeed / lowspeed needle settings.
The needle valve seat is pressed into the carb base. Don't
remove it without having the correct tools and setting gauges.
|The engine stalls when accelerated
||Highend needle way too lean, or lowend needle
|Four strokes as fast idle, mid RPM
||Lowend needle too rich, float diaphragm needle lever
slightly too high
|Engine goes lean in flight
||Highend needle slightly lean AND lowend needle is
rich, float needle lever may be set too low
|Engine goes rich in flight
||This is a special problem with cowled in engines --
the float diaphragm cover vent will need work (SEE NOTE 1)
|Engine runs good, but no idle at all
||There's crap in the idle jets, the carb will have to
be removed and cleaned. You may also have an air leak at the base of
the carb. The throttle butterfly could be damaged or worn out.
|carb leaks fuel when not running
||Float needle is bad or has crap stuck in it, or the
float lever setting may be too high, or the float diaphragm is bad.
|My engine four cycles momentarily when I back off
the throttle, then runs normal
||This is perfectly normal for carbs NOT equipped with
a "check valve" highspeed jet. If you do have the check valve, then
your float needle setting is slightly too high, or your float needle
is leaking a little.(SEE NOTE 2)
|the fuel leaks back into the gas tank when it isn't
||Bad fuel pump membrane, or an air leak in the fuel
line at the carb
Cowled engines may suffer from air pressure changes in
flight causing changes to the "natural" pressure on the float diaphragm
making the engine run rich in flight. There are several possible fixes
available. Most of the time you can simply tune your engine for flight
by trial & error. However, the easiest fix is to open up the cowling
around the carb area to lower the air pressure. You may also rotate the
cover to different positions to see if that works. The "BEST" fix is to
solder a piece of brass tubing where the vent is, and route the vent
line to a better location. I normally route it into the fuse going
through the firewall. It works perfectly every time! Plus, your ground
tune doesn't change in flight!
Carbs equipped with the highspeed check valve are
greatly superior for flying aerobatics, or flying whereby the throttle
will be used extensively. The check valve prevents jet dripping when you
back off the throttle. That's all it does...
Straight through (non-check valved) jets always drip a little
fuel while the throttle is being backed off, and causes a momentary four
stroking of the engine until the jet stabilizes to the new air flow
rate. This is completely normal.