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From: highflyer <>
Newsgroups: rec.aviation.homebuilt
Subject: Re: Engine/Mag Timing
Date: Tue, 15 Sep 1998 09:21:01 -0500

JIM wrote:

> Can someone explain to me in laymen's terms how to set engine timing
> (Continental) and magneto timing (Slicks)?   Thanks!

It is pretty simple.  There are two kinds of timeing that have to be
taken care of with a magneto.  There is the internal timing and the
external timing.  The internal timing is done on the bench with the
magneto out of the engine.  You have to ensure that the points open
at the correct spot in the magneto revolution in reference to the
E-gap.  There is usually a set of markings on the magneto to help
you get this properly lined up.  Then set the point gap and opening
correctly by adjusting the point position relative to the rotor.
Once the internal timeing is correct the magneto should put out a
good hot spark.  If the internal timeing if wrong, the magneto will
put out a weak spark or even no spark at all.

With the Slick magneto there is a hole in the case.  You put a timeing
pin into this hole to lock the magneto in position with the points
just opening at the E-gap.  This, of course, depends on the proper
internal timeing of the magneto.

Then you bring the number one cylinder to top dead center.  You can
do this by putting a special plug into the cylinder that stops the
piston from comeing all the way to the top.  Put your degree ring
on the spinner or prop and read the setting where the piston stops
with BOTH VALVES CLOSED.  Then rotate the engine back the other way
until it hits the stop again.  Read the setting there.  Top dead
center is halfway between those two readings.  Adjust the degree
ring so that the zero reference is halfway between those two positions.
Now you have your degree ring set.

Remove the blocking plug from the spark plug hole and rotate the
engine in a forward direction until you are coming UP toward the
zero reference mark on the compression stroke on number one cylinder.
Stop at exactly the correct advance setting prior to top dead center.
If you go past the point, continue forward two more revolutions until
you stop on the correct advance setting while turning the prop in the
correct direction.  Needless to say, you want to do all of this turning
of the prop with the spark plugs removed from the engine.  We would
NOT want it to START while we were playing with the prop! :-)

With the engine positioned at the point in its rotation where you
want the magneto to fire, and the locking pin in the Slick magneto,
insert the magneto into the proper location on the accessory case.
Do not force it, but let is slide into engagement with the gear that
drives it.  Snug the magneto down in position.  Now your timeing is
approximately correct, both internal and external.

Follow this procedure for both magnetos.

Now, with both magnetos installed, connect your magneto timeing box
to the p-leads of the two magnetos.  Rotate the prop again, once more
in the proper direction, two revolutions.  As you come up on the
second revolution, watch the degree ring carefully.  The lights on
the buzz box should switch at the proper advance point for both left
and right mags.  The buzz should let you know the status also.

When both mags are exactly right, as driven forward by the engine
gearing with all slack taken out in the direction of rotation, you
can disconnect your buzz box and reinstall the plugs and ignition
wireing.  Make sure the P-leads are properly connected to the
ignition switch and are grounded when the mag is switched off.

Some engines do NOT have adjustable magneto mountings.  These magnetos
have to have adjustments made to the timeing by repositioning a
splined coupling in the magneto drive shaft.  There are a different
number of splines on both sides of the coupling and by going forward
on one side and back on the other side of the coupling you can make
small and precise adjustments in timeing.  The shaft timeing is not
likely to change if you do not remove the magneto.  However, the
magneto timeing will still change as the rubbing block on the points
wears down with use.  That is the primary reason that you recheck the
magneto timeing at every annual inspection.

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