Point Gap and Dwell - 1968 Ford Mustang

mustang

#1

Doing some searches on the site around the topic of Point Gap and Dwell I found something http://community.cartalk.com/discussion/1053609/dwell-angle/p1. I also navigated to the link they mention to bring back to life that lost memory.

Any way, I notice in my car that when I adjust - increase- the dwell that the point gap decreases and was wondering if that is normal. It seems to me that I am then out of spec based on what the settings are to be for both gap and dwell.


#2

The dwell is the time that the points are closed. The less gap, the more dwell. When the dwell is right on spec, the point gap should be also. Generally if you have a dwell meter, you don’t recheck the point gap once you have the dwell right. You do need to reset the timing after changing the point gap.


#3

If your point gap is way off when the dwell is correct, are you sure you have the dwell meter set for the correct number of cylinders?


#4

With 4 and 6 cylinder engines, it doesn’t matter that much, dwell angle is not critical. But with 8 cylinder engines, it becomes very difficult to get enough dwell time especially at high RPM’s…With a brand new distributor and point-set, you can achieve 30 degrees of dwell and a gap of .017" or whatever the spec is. But as things wear, especially the point cam, the point gap gets smaller and smaller in order to achieve that ideal 30 degrees of dwell…That’s why duel-point distributors were so popular as a performance option. They allowed the dwell angle to be increased while maintaining a reasonable point gap…

Another problem, if the top bushing in the distributor wears, it allows the shaft (and point cam) to move around which causes the dwell and timing to change with each shaft revolution. This condition makes doing a tune-up impossible…

A BIG improvement would be to install the electronic ignition Ford used 1975-1981 on these engines…Get rid of the points and condenser which is Model “A” technology…


#5

As pointed out earlier, the dwell is the time the points are closed. It is during the close time that the coil is charged up. The longer the points are closed, the greater the charge on the coil, up to a point. As your RPM increases, the charge on the coil decreases because it has less time to charge, that is why the spark decreases with RPM.

You can only decrease your point gap so far or you risk arcing across the points during the coil discharge.

Electronic ignitions increase your dwell time to the maximum. As soon as the coil is discharged, the electronic ignition starts the charge the coil for the next cylinder in the firing order. It does not have to wait for the points to close.


#6

We are looking at the electronic ignition technology but as this is a project car with my boys - want to teach them a thing or two before making life easy on them.

I will double check the items pointed out here - wear on the distributor shaft, timing and making sure the meter is set correctly.


#7

Being off a few thousandths is not anything I would worry about. It’s also possible the lobes on the distributor shaft are worn. These lobes should get a drop of distributor cam lube about every 5k miles but that is seldom ever, or ever has been, done.

Good condenser, properly adjusted points, etc. and there should not be a problem with a set of points going 30k miles before replacement is necessary.


#8

“Good condenser, properly adjusted points, etc. and there should not be a problem with a set of points going 30k miles before replacement is necessary.”

You might be right, but in those days, the recommendation was to change the points, condenser, rotor and spark plugs every 12k miles.


#9

Caddyman has the best idea…

Get a Electronic ignition system. You can even buy a new one for that car.


#10

Ah, sweet memories. My '72 Vega didn’t have a little door in the distributor cap, and to adjust the dwell rather than just the gap required checking the dwell angle, removing the cap and physically tweaking the points mounting bracket, reinstalling the cap, and checking the dwell again. It probably didn’t really make much difference on that 4-banger, but I wanted the dwell to be right.


#11

I had always found using a dwell meter better than setting points with a feeler gauge. 30k is a high estimate as I recall checking mine every 3 k. Points can get a buildup, and make a feeler gauge unreliable. When you adjust dwell, you are adjusting the point gap. Set it to specs.


#12

I’d use a remote starter switch, key on, cap and rotor off, and set the dwell while turning over ther car. One-shot deal.


#13

Yes, a dwell meter or scope can be used to set the dwell with the cap off, coil wire grounded, plugs out and using a remote switch to operate the starter. But if the point gap is carefully adjusted and the distributor is in good condition checking the dwell should confirm that it is within spec.


#14

Back in the days…The BEST tune-up shops never worked on the distributor in the car…They were pulled out, cleaned, inspected, lubricated, new points and condenser installed, the points being carefully adjusted so the surfaces came together in perfect alignment. Then the distributor was placed in a Sun Machine that could spin it up to operating RPM and display the firing sequence on an illuminated degree wheel…Any problems were plain to see…The advance curve could also be observed and adjusted if necessary…along with the dwell. The distributor was then installed back in the engine, the timing set, and the ignition hooked up to another Sun Machine, a specialized oscilloscope that could display both the primary and secondary ignition circuits while they were in operation on the car…Many of these Sun Machines also had an exhaust gas analyzer so the carburetor could be adjusted very accurately…Few shops could afford this top of the line tune-up equipment but the ones who had it stayed very busy…


#15

@texaes - i like that ideal

@Caddyman - I worked with one of those Sun Machines in school - we had two - and they were great, loved to have one in my garage at home as I am looking at getting a second 40+ year old.


#16

There won’t be a problem with points if set correctly and a drop of dist. cam lube is applied ever so often.

You can also make points last longer by using a different condenser. Most cars come with an all-purpose condenser designed to work ok, but not great, in all driving conditions. It’s kind of like tires.
If the bulk of your driving is city or vice-versa the condenser farad rating can be changed and this will slow down or eliminate any material transfer that may occur on one side of the points or the other.


#17

I had a '67 Mustang (bought it new) with a 289 V8. Back in those days you set the points with a feeler guage to get it close and then used a dwell meter to get the points set. The dwell meter is more accurate, so the dwell meter setting trumps the feeler guage. After setting the dwell you set the timing with the vacuum line removed from the distributor and plugged. Then you reattach the vacuum line and reset your idle speed.

You need to change the points about every 10K to 15K miles, so you might consider a conversion to an electronic ignition. I think you can fit a Pertronix unit on your distributor for about $100.


#18

After a weekend of setting the dwell, adjusting the timing, resetting the idle speed along with the idle fuel mixture we were able to get the dwell to 36, spec is 38. The point gap went from .027 to .015. I feel that if I push it to get the dwell to 38, the point gap will be really small.

So how small can a point gap go and still have the system work right?

Based on the results I would have to say that the lobes on the distributor is very well worn and needs to be replace. So doing a search to compare the cost of a new/rebuilt one vs going with electronic ignition. I know that electronic would be much easier to deal with but I would have to say me and the boys had a great time of making the adjustments and hearing the improvements in the engine. Along with having them get a better understanding on how the engine and all its parts work. Much better then playing X-Box any day of the week.

Next project - replace emergency / parking break.


#19

With that kind of info, I’d definitly replace the distributor with a points type, you and your boys will enjoy working on that much more. And Mustang distributors are easy to find, I assume you’ve got all the Mustang and Ford classic vendors’ web sites bookmarked by now!


#20

You need the gap and the corresponding dwell to be as close as possible. Start getting the gap too close (just like too wide) and the point transference problem will surface.

If you get a point distributor I would advise getting a tube of distributor cam lube at the same time. Pop the cap, apply a few drops every 5k miles, and you will find that the points will pretty much stay gapped correctly, distributor cam lobe will be near non-existent, and you will not have to replace points that often.

Also consider the type of driving and alter the condenser a bit from stock. Mostly city driving go to a higher rated condenser; mostly city go to a lower rated one.