A vacuum leak or compression leak are other common possibilities.
You should see what kind of gap you can jump a spark across while cranking the
engine. This isn't very scientific, since it doesn't indicate what kind of
current the ignition is putting out, but the ignition ought to develop at least
enough voltage to jump a half-inch gap while cranking the starter motor.
Sometimes a faulty or misaligned starter motor can draw much more than the
usual amount of current. This could result in a weak spark and difficult
starting. You said it was "turning over nicely" which tends to suggest a good
starter, though it's possible that it might still be drawing too much current
or cranking too slowly. You might check it with an ammeter to see how much
current the starter draws. You could also check the voltage at the coil primary
while cranking the engine with the starter. Don't expect to get full system
voltage, but it shouldn't be too many volts below normal.
A particularly weak battery (or batteries) could also contribute to a weak
spark, but if they are cranking the starter at a normal rate and generating a
good high-voltage spark, then they can usually be eliminated as a cause,
especially if they are capable of cranking the engine for a sustained period of
time without slowing appreciably.
Low system voltage could be a contributing factor. What is the system voltage
while the engine is running? You might want to adjust the regulator to a
slightly higher voltage.
You said it ran fine once it was started, so I'm assuming that it develops
normal power across the usable RPM range, without any missing, hesitation,
backfiring, pinging, etc.
Sometimes low compression can be a factor. When cranking the engine slowly, you
might be only getting part of its normal compression, due to leaky valves,
rings, etc. When you coast the vehicle and pop the clutch, it's turning the
engine over at a higher rate and because of the proportionally lower percentage
of compression loss, it would be developing something closer to the normal
amount of compression, making the mixture easier to ignite. However, if the
problem appeared suddenly, this is not a likely cause. If the problem has
developed over a period of time, it is a better possibility. If you have a
compression tester, it might be interesting to see what the compression is on
all six cylinders.
A vacuum leak is actually QUITE a common problem that typically affects an
engine more when cranked slowly than when turned over quickly, such as when
popping the clutch on a moving vehicle. Loose or cracked hoses or bad intake
manifold gaskets are typical causes of vacuum leaks. A stuck PCV is another
related factor.
Anyway, a weak spark could result from:
Bad coil.
Corroded primary ignition wiring.
Improper point gap or badly burned points.
Faulty starter motor drawing too much current or cranking slowly.
Bad condensor (not typically a problem at lower speeds, unless shorted).
Incorrect spark timing.
Moisture or carbon tracks in the distributor cap.
Many breaks in spark plug wiring.
Shorting/fouled spark plugs.
Some other possibilities might include:
Old gasoline.
Carburetion problems (especially problems in the idle and low-speed circuits).
Vacuum leak.
PCV stuck open.
Bad gasket on intake manifold.
Poltergeists.
One final thought: Very often, a problem like this results from a COMBINATION
of several causes, such as marginal batteries, questionable starter motor, weak
coil, slight vacuum leak, timing off a bit, etc. This is why it's always a good
idea to check everything. Even if fixing one thing makes it start OK, you might
still be on the cusp of another failure when another component gets just a
little too far gone.
I can probably think of a few more possible causes, but these should give you
some things to check and eliminate.
Have fun,
Alan
(Salt Lake City, Utah)
MJW71683@aol.com wrote:
> My M-37 starts fine when cold but won't when hot. It turns over nicely but
> won't fire up. I can "pop-start" by towing. Once started it runs fine. My
> guess is a weak ignition coil. Any other ideas ?
>
> ===
> To unsubscribe from the mil-veh mailing list, send the single word
> UNSUBSCRIBE in the body of a message to <mil-veh-request@skylee.com>.
===
To unsubscribe from the mil-veh mailing list, send the single word
UNSUBSCRIBE in the body of a message to <mil-veh-request@skylee.com>.