If your engine has a carburetor, the hard starting
problem is most likely choke related. If the
engine cranks normally and the spark plugs are
not worn or dirty, the choke probably needs to
be repaired or adjusted.
The choke may be set too rich or too lean.
Either way can cause hard starting. Inside the
choke housing is a bimetal spring that regulates
the tension on the choke according to
temperature. If this spring is broken, the choke
will not operate. If the choke linkage is rusty or
jammed with varnish deposits or dirt, it may
stick or not operate smoothly. Cleaning with
carburetor spray or solvent may help alleviate a
sticking problem.
On older fuel injected engines, a separate "cold
start" injector is used to spray additional fuel
into the intake manifold when a cold engine is
first started. If this injector is not working, the
engine can be hard to start. The injector is
controlled by a timer and relay, so if either of
these components is defective it can prevent the
cold start injector from doing its job, too.
If your engine cranks slowly, your cold starting
problem is not fuel related, but may be due to a
weak battery, loose or corroded battery cables,
or a weak starter.
Check for obvious problems first. Remove,
clean and inspect the battery cables (both ends).
Then check battery charge and condition. (More
information on the condition of your battery and
battery recharging is available.) If the battery
charge is low, recharging the battery may
temporarily solve your problem. But there's a
reason why your battery is low. You probably
have a charging system problem that requires
further diagnosis. (More information on
alternators is also available.) If your battery is
more than four or five years old, it is probably
near the end of its service life and needs to be
replaced. A "load test" will tell you if it still has
sufficient cranking capacity to provide reliable
cold starting.
Slow cranking during cold weather (below
freezing) can also be caused by oil that is too
thick. A high viscosity oil such as straight 30 or
40 weight oil in the crankcase can make an
engine very difficult to crank when the
temperature drops. Switching to a lighter
multiviscosity oil such as 10W-30 or 10W-40
should solve this problem.