I didn't say 'working', I said 'working correctly'. Without an adjustable
timing light you would have to paint lots of marks around the pulley to
check the centrifugal advance is giving the correct advance at various rpms.
Merely to see that it is moving as you rev the engine is not enough, the
usual mode of failure is it reaching maximum advance way too soon which
causes pinking under load, poor performance and economy.
If you "only get a momentary glimpse of the vac unit working as you open the
throttle" then maybe yours is not working correctly and you need an
adjustable light to diagnose it properly :-) Manifold vacuum gives max
advance at idle and reduces gradually as the throttle is opened. Ported
vacuum gives zero advance at idle, increases the advance rapidly as the
throttle is opened part-way, then reduces again as it is opened further the
same as manifold vacuum.
Dialling in the amount of advance expected then setting up the correct input
conditions (rpms in the case of centrifugal, vacuum with a MityVac in the
case of vacuum) then checking to see how close the TDC mark is to the
pointer is precisely how they are intended to be used.
----- Original Message -----
From: "Charles & Peggy Robinson" <ccrobins@ktc.com>
To: "Telewest (PH)" <paul.hunt1@blueyonder.co.uk>
Cc: "Mike Lewellen" <Mike.Lewellen@insightbb.com>; <mgs@autox.team.net>
Sent: Monday, October 14, 2002 6:17 PM
Subject: Re: email sent to Bob Shaw
> Shux, you don't need an adjustable timing light to check out the
> centrifigul and vac advance. Any timing light will let you see that
> those things are working. Anyway, with the engine unloaded, you only
> get a momentary glimpse of the vac unit working as you open the
> throttle. I suppose you gould dial in the amount of initial advance +
> the total vac and see if the light strobes at TDC on the damper, but is
> it going to be worth anything unless you are measuring the manifold vac
> too?
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