It cranks but won't start?
Fuel filter OK?
If so, the carburetor is suspect.
Carburetors have chokes - a flapper plate at their air intake. When the
engine is cold that flap ought to be closed to restrict the air volume
and so enrich the air / fuel mixture for starting and initial warm up.
As the engine warms the choke opens until it's wide open at the engine's
normal operating temperature. Choke opening and closing is usually done
(on most carbs) by a bi-metallic spring that gets heated by the engine.
If the choke plate is not shut then you need to find out why not. The
linkage could be dirty and binding. If it's shut then you need to check
if fuel is being supplied to the engine.
You can check if fuel is supplied to the engine by looking into the
carburetor - push the choke open and pump the accelerator.
Don't crank the engine, use a flashlight to see NOT a match!!
You should see gasoline being squirted into the venturi by the carb's
fuel jets. If not, then either the carb is dry or the accelerator pump
The carb would be dry if the needle valve that regulates the fuel flow
into the carb is stuck shut by gum or dirt. Try tapping, NOT rapping,
the side of the carb while cranking the engine - it may loosen the
needle. If not, the carb must be cleaned ideally by adding cleaner to
the fuel or (aaarrrrhhhh) disassembly, cleaning and reassembly using new
gaskets (usually sold as a kit). If you've never rebuilt a carb number
every piece as it comes off or out and bag them so all the pieces go
back in. It's not hard but must be precise.
The float's level inside the carb must be correct to allow the carb to
supply the correct air / fuel mixture. A kit includes a float gauge
(templet) to check the float's tang position (that controls the needle
(This is somewhat similar to the workings inside a toilet tank, simple
once you know the concept.)