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The stock distributor advances as RPMs increase by centrifugal weights and springs. The springs break down over time and have to be replaced, and sometimes the bushings that the weights swing on get worn. If you have any looseness in the system this will cause unsteady idle and related problems.
Later models had a vacuum advance. Sometimes the two systems were used in combination.
You should be able to get a rebuild kit for these parts. Or buy a rebuilt distributor, or go with an electronic distributor. It depends what you want to accomplish with the car.
The advance curve can be changed by altering the weights and springs. Along with cams and head modifications, prior to electronics, that was how the engine was tuned for more power.
Original distributors almost always need adjusting. Most of the time, the weights are the same. Spring tension, primary and secondary, is adjusted by bending the spring mounting post. Vacuum advance adjustment is typically achieved by altering the thickness of the spacers inside. Aftermarket advances have a screw inside the vacuum hose nipple. This is all best done on a distributor machine.
Getting the dist cam lobe to a peak to set the points is hard to do, it is easier to rotate the rotor a little to get it on a peak then hold it, to check the clearance between the point contacts, its actually easier to do it with the dwell meter, remove the rotor, connect the dwell meter, put a screwdriver in the adjustment slot, and use another screwdriver to loosen the two screws a little, enough so you can move the points with the screwdriver in the adj slot, then have someone crank the engine, and adjust them where they need to be, then hold the adj screwdriver, and tighten the other two screws down. Then put everything back together and start it up and recheck the dwell to make sure it is okay, then recheck the initial ignition timing. Good Luck.
I ignore the gap and just set the points by dwell angle - that's what really matters, anyway. The ignition system doesn't care what the gap is, just that the coil loading and discharging times are in the correct proportion. Gapping is just a way to get you close to that proportion; dwell angle is that proportion. Trying to get both gap and dwell to match factory spec at the same time with a used or rebuilt distributor is an exercise in futility. The tach is probably moving around because 15* does not allow the coil to build up a strong enough field for proper spark.
i forgot to tell you that this is a rebuilt dis. from carquest all new parts in side...
Bull. If they told you that, they lied. The only new part available for these distributors is the shaft bushing and drive gear. All of the signs of wear posters are giving you must be checked. As far as adjustments of the vacuum and centrifugal advance curves, I have never seen a rebuilt distributor that was even close, and of course if the advance curves are not set right it just kills performance.
"Rebuilt" often simply means cleaned and repainted. There were also about 15 different distributor curves for any year of 289. How can one variety of 'rebuilt' distributor satisfy all 15 possibilities? That's why I try to fix what I have instead of trading for an unknown unit that someone else threw away...I mean traded in as a 'usable core'.
Another reason to use dwell instead of gap is for used points. Once they have a few miles the point surfaces are no longer smooth and gap measurement with a flat feeler gauge is no longer accurate. Delco, who invented points and condenser, long ago recommended to NEVER file the points but leave them alone and simply measure dwell. By filing lumpy points flat again you are simply reducing the available area that can make contact which will cause the remaining point area to degrade more quickly.
Dwell is a measure of how long the points stay closed so the coil current can build. 30 degrees at 3000 rpm allows only 1/6 the coil charging time as 30 degrees at 500 RPM. So at idle you could have as little as 30/6 = 5 degrees dwell and the engine should run just fine. If its not stable at idle then you have something else wrong other than dwell. A dwell of 15 at idle should run just fine - at idle. Check the distributor cam: are all the lobes the same height or are some worn down more than others.
Here's a little insight to the dist curves, the weights used limit the total amount of mech advance, they should be stamped in dist degrees, their weight is also important, the two springs used on the weights, one has less tension and keeps tension on the weight to return the rotor to its resting position, the other spring has more tension and is generally a loose fit, so as the dist spins the lighter spring and weights starts the curve fairly quickly (more advance per rpms), then as the weights move position the heavier spring contacts the perch holding it and it then has tension on the weights to slow the advance curve (less advance per rpm, so it looks like climbing a steep hill, until you get near the top, then its not so steep. Good Luck.
hi i went ahead and replaced the points with the pertronix ...man what a diffrence....i also found that i had a bad coil ...up up and away....i love it.what a snap it fit real nice..thanks for all your help