Military-Vehicles: Re: [MV] M-43 troubles

Re: [MV] M-43 troubles

Alan Bowes (alan_bowes@phast.com)
Wed, 06 Aug 1997 21:24:56 -0600

A few more thoughts on your M43 problems:

First of all, when you said the timing mark was jumping back and forth under the (timing)
light, you mentioned that you had a hard time getting a reading. I'd like a little
clarification. Was it actually flashing intermittently, or were you getting a regular
flash but getting different varying timing readings? Or both?

But rather than "dwell" on that, let's consider a couple of possible problems that could
result in any or all of the above.

1) Let's consider the simplest possibility of all: Improper point gap. Improper gap will
result in improper dwell time or dwell angle. This is the angle between the point when the
points close to when they open again. A smaller point gap results in a longer dwell and
vice-versa. As a rough rule of thumb, if the points are set too wide, the dwell will be
too small, which means that the primary winding of the ignition coil will not have time to
become saturated at higher engine speeds, resulting in rough performance at high RPM due
to a weak spark. On the other hand, if the point gap is set too small, you will get arcing
across the points that will mess up your starting and your low-speed performance. It will
also cause the timing to "jump around." Improper point gap is by FAR the most common
ignition problem. Back in '68 when I graduated from high school, I worked in a garage at
Mt. McKinley National Park. It was the only service center in over 100 miles in any
direction. After the long drive to Alaska, several tourists per day would drive their
sputtering cars in, complaining about their rough-running engines. My guestimate is that
in over 80% of the cases, readusting the points (the gap was nearly always too small)
cured the problem. The long drive up from the lower 48 was enough to wear down the rubbing
block a bit (especially if it wasn't lubricated) so that any car with marginal point
settings when they set out on their journey ended up with too small of a gap by the time
they put 3,000 to 5,000 miles on their cars.

Other rules: Always set your timing AFTER setting your points, since point setting changes
will retard or advance the spark. Be careful to hold the feeler guage parallel to the
points when setting them, and hold it very lightly. If the points are worn and pitted, you
can't get an accurate reading with a feeler guage. You can file points (don't use
sandpaper...it leaves grit), but it's not worth it. Buy new ones.

2) A faulty condenser can cause an erratic spark (misfiring), though usually it won't
cause erratic timing of the spark. A condenser of improper value can also cause problems,
but these are usually related to premature point wear from metal being transferred from
one point to the other, the direction being determined by whether it is an over- or
under-capacity condenser.

3) A worn distributor shaft bushings (or even the shaft itself) can cause the above
problems, due to the point gap constantly changing as the shaft and cam rattle around.
This can cause both misfiring and erratic timing. I believe that someone already mentioned
this. Wiggle the shaft around and see how much side-to-side play there is.

4) As someone also mentioned, a weak breaker arm spring can cause "point hop," which can
cause erratic spark and timing. Unless the spring is VERY loose, its effect will be more
pronounced at higher RPM. Also, you said you had new points. I'm assuming that your points
came with a new breaker spring. On some point sets, the spring tension is adjustable via a
slot on the end of the spring. You can loosen a screw and slide the end of the spring in
or out as necessary to adjust the tension.

5) Wear or lack of lubrication in the advance mechanism can cause erratic timing changes.
It can also cause misfiring, typically under load. A centrifugal advance mechanism is
fairly simple, but it must be clean and lightly lubricated. On some distributors, you must
remove the cam and yoke from the distributor shaft to lubricate it, but on others, you can
just remove the rotor and put a drop of oil down the center of the shaft. Put a small drop
of oil on the advance weight pivots as well, and make sure the springs aren't too rusty.
By the way, if the two weight springs don't match, don't panic. Most distributors use a
heavy and a light spring to give a more even advance curve over a wide RPM range. Some
also employ a tiny flat spring at the spring attach points. This is a very light spring
that provides a small but rapid advance with a slight change in engine RPM. A vacuum
advance mechanism is also quite simple. The usual cause for problems with vacuum advance
systems are leaky tubes or vacuum diaphragm, or a stuck backing plate (where the points
are mounted) that needs lubrication so that it can rotate when a vacuum is applied.

6) Grease, oil, or other crud on the points can cause erratic ignition behavior. Put only
a very light film of grease on the distributor cam (and a tiny dab behind the rubbing
block) when you install the points.

7) On engines with vacuum advance, you are usually instructed to disconnect the advance
prior to setting the timing. If you don't, you will get false readings.

8) A loose backing plate can cause misfiring and erratic timing.

9) Cracks or carbon trails in the distributor cap (or rotor) can cause all kinds of
erratic behavior.

10) Bad spark plug cables can cause a weak spark and sometimes cross firing, though it
shouldn't cause erratic timing.

11) A bad ignition coil can cause misfiring, but it shouldn't cause erratic timing.

12) A loose or corroded connection in any of the ignition wires, especially in the primary
circuit, can cause misfiring and sometimes erratic timing.

13) A bad ignition resistor can result in a weak spark or misfiring, but should not cause
erratic timing.

14) Check your primary ignition wiring for internal breaks. You can do this with a meter
or a light bulb, shaking the wires to see if there is any break in continuity.

Let's see, what else...

Well, as has already been mentioned, you could remove the distributor and take it to a
shop that has a testing unit that will simulate an engine and measure a few parameters. If
you do this, before you remove it, mark the position of both the distributor body relative
to the block, and the position of the rotor relative to the distributor, so that you get
it back on correctly.

I guess that's more than enough for now.

Regards,

Alan Bowes
(Salt Lake City, Utah)

Hayes-Holgate, Shaun wrote:

> this past weekend i took the advice from the list and insulated my fuel
> lines to prevent vaporlock
> and golly would'nt you know it but i think it worked. maybe it was just
> me being too hopeful but i'm sure
> it was easier to start after a run in the hot sun. two other things i
> did was a compression test and set
> the timing.the compression test gave me 90 psi on all cylinders so i
> figure thats fine. i got the timing light
> going with the clamp on a car plug cable but i had a hard time getting a
> good marker reading. it was
> jumping back and forth under the light so i set it as close as i could
> to the pointer. my timing light does'nt
> have an adjustable dial on it.do i need that to get an accurate reading?
> this is all a vain attempt to restore
> the engine to full power.i get a good idle but as soon as i rev high it
> coughs and sputters and backfires.
> i've got new plugs,new points,condensor,good batteries,rebuilt carb,new
> regulator,a steady fuel flow after
> the fuel filter,air filter ok,fuel cap off, idle screw set,but still i
> get weak accelerator response. next i'll test the
> coil but then i'm out of ideas. the plugs have a slight black sooty
> deposit but nothing like the caked on soot
> photos you'll see in manuals so i'm thinking their ok. i'm scratching
> my head. any thoughts?
>
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