THE FUEL SYSTEM AND GOVERNOR
29
Primers
MTD engines use a dry bulb primer. This means
that there is no fuel in the primer bulb. The primer
works by pushing air into the float chamber of the car
-
buretor when the primer bulb is depressed. This will
force fuel to be sprayed out of the main nozzle into the
throat of the carburetor.
To test the primer:
1. Remove the engine shroud by following the
steps described in Chapter 3: Air Intake Sys
-
tems.
NOTE: Do not disconnect the primer hose.
2. Press the primer bulb while looking down the
carburetor throat. If there is fuel squirting into the
carburetor throat, the primer is working properly.
If not, replace the primer and hose.
NOTE: The primer and primer hose come as an
assembly so there is no need to determine
which part is bad.
To replace the primer:
3. If the primer is bad, disconnect hose from the
carburetor.
4. Remove the hose camp at the rear of the primer
bulb.
See Figure 4.10.
5. The primer is held to the shroud by a pair of split,
barbed posts. Squeeze the posts to release the
barbs.
See Figure 4.10.
NOTE: The primer bulb and hose will slide out
as an assembly.
6. Install the new primer by following the above
steps in reverse order.
Figure 4.10
Clamp
Barbed posts
Carburetors
If diagnosis indicates a fuel problem, inspect the
carburetor. This is important even if problems are iden
-
tified elsewhere in the fuel system.
IMPORTANT: the fuel must be tested for alcohol
content before diagnosing anything else on the
engine.
NOTE: It is important to perform a compression
or leak down test before condemning a carbure
-
tor. An engine can have a borderline compres-
sion reading and not create enough of a vacuum
to draw in a sufficient fuel/air charge.
NOTE: To determine if border-line compression
is the problem; remove the spark plug. Squirt a
little bit of oil into the combustion chamber to
seal the rings. Reinstall the spark plug. If the
engine starts and runs ok, then that was the
problem. If it does not start, move on to the car
-
buretor.
Inspecting the carburetor
1. Remove the float bowl and check for dirt and/or
varnish.
2. Inspect the needle valve and needle valve seat
for dirt and/or damage.
3. Inspect the gaskets and O-rings for damage.
4. Inspect the vents and orifices, verify that they
are free of debris.
NOTE: If a little cleaning and new gaskets will fix
the carburetor, do it. If the carburetor requires
extensive cleaning; it is better to replace the car
-
buretor.
IMPORTANT: Never try to mechanically clean
orifices. That will damage them and ruin the car
-
buretor.
NOTE: The jet markings (if present) may be
used for identification purposes, but the techni
-
cian should not attempt to infer orifice sizes from
the identification numbers.
NOTE: Installing the wrong main jet, or a carbu-
retor with the wrong main jet will produce perfor-
mance and emissions issues.
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