Re: Dwell/point gap

[jlandry@domain.elided (John A. Landry)]

The aliens commander decided Mark A Pepe <mpepe@domain.elided> would make a
perfect specimen for dissection, and he yelled...

>I've never been able to set the
>point gap well (using feeler guages etc.) on either of the scouts I have
>owned so..... I have been
>doing it by eye and just starting the truck and checking the dwell as I go
>until I get it where  it should be.

Mark,

Setting the gap is not only physically difficult when the distributor is on
the truck, but the lighting is usually not adequate to do it properly.  The
best way to accurately adjust and set the points is to remove the
distributor completely from the engine and clamp it in a soft jawed vice on
your well lit workbench.  This may sound extreme, but something you're
supposed to do (and most people don't) is not only adjust the gap, but
first adjust the point alignment.  It takes a strong light behind the
points as you examine them to accurately judge how precisely they are
aligned.  If they aren't (which is normally the case out of the box), you
carefully bend the base of the fixed contact point (never the moving
contact point) with a pair of needle nose pliers.  Once the alignment is
correct, then setting the gap with a feeler gauge is simple... with the
distributor still in the bench vice.  After setting the gap, the points
should be cleaned by dragging a light free cloth saturated with alcohol
between them.

>Sometimes I can do it it one or two
>tries but other times close has to do. (I always try to get it right at 29
>degrees as the motor seems to have the most spunk at the lowest possible
>degree setting).  Is this O.K. ?  I just assumed that the proper dwell
>setting corresponded with the correct point gap (I do have good points
>BTW)

Yes... the proper dwell corresponds with the correct point gap.  The two
are directly linked.  Theoretically, a higher dwell number (smaller point
gap) should be better for performance at higher engine rpms.  This is
because the dwell number represents the number of degrees of distributor
rotation that the coil's primary field is being saturated with
electricity... in preparation for field collapse and firing.  The higher
the dwell number, the longer the coil has a chance to be saturated by the
electricity.  As the engine rpm increases, the amount of time the coil has
to cycle and fire the spark plugs becomes shorter and shorter.  At very
high engine speeds, this time can become so short that the coil's primary
field cannot be fully saturated before it's fired.  This can result in a
less than optimal spark right when the engine needs it most.  But for a low
revving IH engine, this really isn't a concern.  In the days before
electronic ignitions, this problem was addressed by dual point distributors
which extended the dwell.

Like everything else, the point gap and dwell specifications represent a
reasonable compromise between point life and adequate performance across
the engine's entire operating range.  The gap has to be large enough to
break the electrical current cleanly and prevent excessive point burning,
yet be small enough to saturate the coil adequately at higher engine rpms.
If the IH dwell spec is 28 to 32 degrees, setting it closer to 32 degrees
*should* be (in theory) a little better for high rpm ignition performance.
But I can't imagine you'd be able to tell on an IH at only a ~3,800 rpm red
line.

If you really want to make life easier on yourself, replace the points with
a $65.00 Pertronix Ignitor.  You'll get better ignition performance,
excellent consistency and you'll never have to worry about gap or dwell
again... ever!

Take care,

John L.
-------------------------------------------------------------------------- 
jlandry AT halcyon DOT com      | 
Conservative Libertarian        |  "The road to  tyranny, we must never
Life Member of the NRA          |   forget, begins with the destruction
WA Arms Collectors              |   of the truth."
Commercial Helicopter - Inst.   |                     William J. Clinton
http://www.halcyon.com/jlandry/ |    10-15-95, speech at the Univ. of CT