Mike's Most Excellent Power Tips
Contributed By: Michael Baxter
Date: Sat, 6 Mar 1999 05:20:04 -0500
Richard Hager >firstname.lastname@example.org< writes:
>> You really have me thinking about the vacuum advance. The way you described it, that -does- sound just like my symptoms doesn't it? <<
>> Does the old trick of advancing until idle stumbles, then backing off a little, work well for these engines? That, I can do myself in the driveway. <<
Richard, I assume from the above you don't own a timing light? Borrow one from a neighbor or a friend. You shouldn't have to look far. Somebody you know will have one.
The most likely culprit is the centrifugal (or mechanical) advance and not the vac. advance. Although both should be checked. It is common for the vac. advanced to be hooked-up to manifold vac. rather than port vac. because of either a failed CTO vac. switch or by a DSPO. Tim Savage just described that his '77 401 is hooked-up to manifold vac. Tim, if no one with a '77 TSM helps you out, I have a '76 TSM (which should be close enough) and I'll be glad to work with you via e-mail.
One thing to keep in mind is the both advance mechs. are additive. Meaning the vac. advance adds timing advance above whatever the centrifugal advance happens to be at that moment.
To check the vac. advance, you can indeed just remove the cap and suck on the hose. Watch the mech. inside the distributor and you'll see it move if it works. If you're really lazy, just pull the vac. hose off at the CTO and suck with the engine running. The idle speed will increase if the vac. advance is working.
You should have no, or very little, leak down on the vac. advance diaphragm. Meaning when you suck on the hose and then hold the vac., the mech. will not return until you remove the vac. You should have full vac. advance by 10-13 HG if you happen to have a vac. pump & gauge.
Most Motorcraft vac. advances are adjustable via a allen screw which is accessible by removing the vac. hose and inserting the allen wrench through the nipple opening.
To test the centrifugal advance:
It should start to advance by 1200 RPM. The advance should be smooth at least until you can't see the line anymore.
If you happen to have a digital advance timing light, your looking for max. advance in the range of 14-19 degrees by 4000 RPM or so for the centrifugal advance. The vac. advance should be in the 22-27 degree range. These are 360 specs.
The advance curves are graphed in the General sections of at least some of the Service Manuals including the '79.
If you have to service the centrifugal advance mech, it's in the bottom of the dist. bowl. The reluctor is retained via a small roll pin which will be pressed into one of the two grooves in the dist. shaft. Don't loose the pin when you remove the reluctor (by just pulling-up on it sharply). This applies to the Duraspark II-III distributor.
-- Michael Baxter, MBaxter@Compuserve.com
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