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Centrifugal Advance Issue

73LemonTwist

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I have a stock 340, with the original distributor in it. I have been working on the new Demon carb to improve off idle throttle response.

Today I noticed that when I hit the throttle, the timing light shows that the advance first drops 5-7 degrees, then picks up as the engine accelerates and centrifugal advance increases. The distributor still has the stock springs and weights.

The vacuum advance is disconnected (but on the '73 it has the OSAC valve to delay vacuum advance anyways).

My base advance is set about 12 degrees BTDC.

Any ideas what would cause the initial drop in the advance?
 
You may want to remove and disassemble distributor for lube and inspection.
Mike
 
Sounds like the mechanical advance cam is gummed up. Take the felt out of the top of the shaft (if there) under the rotor and see if the cam plate moves freely if not you need to take the cam plate out clean and lube it. Just watch that clip on the shaft it will go across the room if it pops out. Lube it with engine oil only just a few drops not grease!
 
Should probably look at the advance weights themselves, too. Lead, or otherwise, they work off pivots, and springs, and have seen enough of 'em rusted up. Yes, the weight's pivots need a high temp grease, not oil, to work for any length of time.
I 'service' mine once a year.
 
I have to disagree on the grease it just gums them up from the heat , trust me I do a few hundred a year only light motor oil, the weights have a oilight bushing in them. And yes the do rust in place and the springs do get rusted and break. When you oil the shaft it will get onto the weights and pivots. Biggest thing I find is no one even knows to oil them during a tune up and then the shaft for the cam plate rusts solid. Hey any ideas I am all ears.
 
I have to disagree on the grease it just gums them up from the heat
That's why I said high temp grease. Either one, matter of preference. Going by past experience, every one I've gone into had grease on the advance weights.
 
Really. Seriously every time I find one with grease they are stuck. That one reason I cant pay much for core. Trying to download the service manual and see what it says, really Curious now.
 
Man I cant copy the page used a 66 manual all it says is oil the lubricating wick and three drops of oil on the felt pad. Guess we are both wrong.
 
Fig. 12-Testing Contoct Arm Spring Tension (3) Install cam spacer, chamfered end down on dis- tributor shaft. (4) Slide cam and yoke on distributor shaft, engage weight lugs with slots in the yoke. Install cam retain- ing spring clip. Be sure it is properly seated in the groove of distributor shaft. (5) Lubricate and install flat thrust washer. Posi- tion washer on distributor shaft and slide shaft into distributor body. Position lower thrust washer and drive the collar on the lower end of shaft. Install re- tainer pin. (6) Install oiler wick and oiler. (7) Install contact plate assembly. Align condenser lead, contact point spring, primary lead and install attaching screw. (8) Install felt wick in the top of distributor cam. (9) Attach vacuum advance unit arm to contact plate and install the retainer (Prestolite only). Install the vacuum unit attaching screws and washers. (10) Test contact arm spring tension, and adjust contact gap. (11) Lubricate felt pad in the top of distributor cam
 
Interesting I have a old mopar distributor book I want to scan, kind of my bible on the older ones. You know what the grease I do find is probably non synthetic also.
 
s-l1600.jpg
 
What do you use on the lobes? I have been using synthetic assembly lube.
 
Real dark brown, waxy grease (name I don't know) is what I'm used to using. For lack of knowing, I've always called it 'mag grease', since it's the same we used, rebuilding aircraft magnetos. Also used a 'mag oil', that I still have some, in place of engine oil, since it stays in place longer.
 
Interstin have a minute to talk on the phone?
 
What do you use on the lobes?
Always used that so-called mag oil, couple drops on the felt, very thin wipe on the cam. That oil is like rear axle lube, but high temp.
If there's an aircraft supply house around, might find it there.
Stuff you find in automotive parts houses...nothing like it, in them.
 
Not that I am aware of but think I know what it is I am a ex USAF avionics guy C-130 and H-60. Cant think of the name of it.
 
thanks for the advice everyone... will pull it out and see what I can see...
 
Interesting that the advance drops, unless it is because engine RPM is falling off. Any increase in rpm should result in an increase in advance, unless the distributor has wear or something loose causing erratic timing.

Ray, early distributors (Autolite and later Prestolite) have grease pockets in the weights (laminated type). The Autolite/Prestolite service manual talks about using a grease in those pockets. No filler type that has a melting point of 280deg F. Same goes for the cam, unless the points have a wiping wick which gets oil. As far as the pivots for the cam stop I do believe in using a thicker grease on the post for the cam stop slot. Anything to help reduce wear on the slot. This is all below the breaker plate so nothing really gets up into the points. Also, I have taken apart untouched and NOS distributors from autolite. They have a thick residual grease at the base of the distributor.

Remember your dealing with 40-50year old grease. Restored distributor with proper maintenance won't have an issue. I do agree with newer Chrysler weights with bushing not to use grease, though I use a little silicon lubricant with a drop or two of synthetic oil.

Other thing I do, especially when I repaint a distributor body is bake it at 200 in oven to dry paint and heat up bearings. I then remove the distributor and plug bottom and fill shaft with synthetic 20W oil. The absorbent bearing microscopically release oil when hot, and absorb when the cool. Manual calls for soaking them, I just do it starting hot, and let the cool in oil for a day.

105419.jpg 105301.jpg
 
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