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CP514-350i, C514-350, CP512-300i, and CP512-300 High Speed Carburetor Rebuild
Date – January, 23
rd
2015
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.
Some basic terms (see diagrams). The
low end
needle refers to the one that trims idle mixture mostly and affects mid-range mixture
somewhat. The
high end
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 low end & high end 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 top end first since it's the easier of the two.
Go to full
throttle. Adjust the top end 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 low end needle slightly, if this doesn’t work you will have to adjust the needle valve inside the
carb.( I will explain this later) If the top end runs OK, then
slowly pull the throttle down until the engine begins to "four cycle" hold
the throttle there. Adjust the low end needle until the "four cycling" barely stops.
Now
lower the throttle more until it "four
cycles" again, and adjust the low end 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 low end 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 low end a little rich and it will four-stroke a little.
Rebuild
a
Two
‐
Cycle
Engine
Carburetor
Before gasoline makers started adding ethanol to gasoline, rebuilding a carburetor was a repair that was rarely--if
ever--needed. Since the ethanol in gasoline breaks down the gaskets and other rubber components in the carburetor,
these days you will find it necessary to rebuild your carburetor when the parts become cracked or brittle.
