Cub Cadet MTD 70 Series - Manual (Page 61)

F UEL S YSTEM AND G OVERNOR
55
Carburetors
If diagnosis indicates a fuel problem, inspect the carburetor. This is important even if problems are identified else-
where 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 carburetor. An engine
can have a borderline compression reading and not create enough of a vacuum to draw in a sufficient fuel/air
charge.
NOTE: If the engine has border-line compression, a quick test to see if that is the problem is to 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 carburetor.
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 carburetor.
IMPORTANT: Never try to mechanically clean orifices. That will damage them and ruin the carburetor.
NOTE: The jet markings (if present) may be used for identification purposes, but the technician should not
attempt to infer orifice sizes from the identification numbers.
NOTE: Installing the wrong main jet, or a carburetor with the wrong main jet will produce performance and
emissions issues.
Disassembly and rebuilding the carburetor
1. Clamp-off the fuel line to prevent fuel spillage, then
disconnect the fuel line from the carburetor.
NOTE: If the carburetor is equipped with a primer,
disconnect the primer hose.
2. Remove the carburetor by following the steps
described in Chapter 3: Air Intake and Filter.
NOTE: An insulator separates the carburetor from
the cylinder head.
• A bowl vent port is in a recessed passage on
the end of the carburetor that faces the insula-
tor.
• A second passage in the insulator supple-
ments the passage on the carburetor.
• Gaskets separate the insulator from the cylin-
der head and the carburetor from the insulator.
• A port in the carburetor to insulator gasket ties the bowl vent passages together.
Figure 4.27
Bowl vent port
Bowl vent
channel
Primer port
(if equipped)
insulator block
gasket