Some time ago I asked here about low dwell readings, on what was supposed to be a fairly recent rebuilt. It was suggested that the lobes on the distributor cam may be worn. Also recently found the bushing is somewhat loose. Unfortunately I missed a chance sometime back, to get a N O S distributor. My error..

Questions; 1) How close do the point settings have to be to get proper readings? I recently reset them twice. Best reading is about 21 deg at about 1500 & as low as 16 at idle. (the engine seems to have a constant miss at idle)

2) Does anyone know what the dimensions of a new distributor cam were supposed to be?. Unfortunately, in western Canada there is only one rebuilder as far as I'm aware. And that's where this "dud" came from. A friend suggested going to a Pertronics unit.

3) How successful would that be on a defective distributor?

4) I have a 55-56 distributor on hand. As a check, would the cam lobes dimensions be the same?

Any help appreciated. Thanks.

Paul

Can't help with dimensions at this moment. The points rubbing block does not make contact the entire height of the cam. Therefore, if it's worn so much that they can't be adjusted properly, it should be easy to spot.

When you adjust the points do you use a feeler gauge or the dwell reading? A feeler gauge on used points can provide false settings.


Lawrenceville, GA

Dwell is simply a measurement of the time (measured in degrees of rotation) that the points are closed. Remember the cylinder fires when the points open and the electrical field collapses. A worn cam or point rubbing block has the effect of increasing dwell time and retarding timing. I have seen points installed with weak springs or in some cases no spring just the grounding strap, this will cause erratic and low dwell time readings. Mike

1956 Fairlane Victoria (ORREO)

 

Overlooking Beautiful Rimrock AZ

To add further to what Mike said, dwell is simply a measurement of the time that the points remain closed. A low dwell reading indicates a large points gap and if the dwell reading remains steady when idling or at speed there is no variation due to worn distributor bushing, contact plate etc.. As Mike has said if you are using worn points it is almost impossible to get a correct reading.

From what you have written it would seem that the distributor is OK as if the distributor lobes were worn you would not be able to get as big a point gap (small dwell angle) that you have.

I am sure that every one of us has at some time set the point gap too big using feeler gauges, this is why dwell meters are so much better to use. There are so many pitfalls setting points by hand for the uninitiated.

I would suggest you close the points, disregarding the feeler gauge and see if you can get a reading closer to what is recommended, about 28 degrees from memory.

The point replacement systems will work in distributors with worn bushings and give very accurate dwell readings. This is because they work on magnetism in the case of Petronix (Hall effect) and breaking a laser signal in the Accell unit. Whichever you go for will work better than points.

Symptoms of point gap too wide would be smooth idle but miss getting progressively worse as revs build with no power on accelleration, this is because there is not enough time for the coil to build up a spark.

Hope this wasn't too long winded for you.

Regards

Rick - West Australia
Do Y Blocks Downunder run upside down? Gravity Sucks!!

One thing I didn't think to check was where the timing was after setting the points. Interestingly the engine burbles or misses at idle, but when driven moderately does not exhibit any noticeable missing. One issue was that the points seemed to hang up on the plate, no matter which end I tried to snug up first, I couldn't seem to hold a setting. I'll restart by installing fresh points & check the plate for a burr or whatever. Maybe the lobes aren't badly worn! Thanks everyone for the diagnostic information.

Paul

Hi Paul. I was having trouble with my rebuilt unit getting the timing to stabilize.When I checked the freeplay(vertical movement) measured between the colar and the lower body of the distributor it was well outside specs.Should be beteen .022 and .034 with closer to .022 the better.I had over .050 on mine.Bought a shim kit from Mr.Gasket and removed gear and colar to install.Got down to .022 and problem cleared up.The rebuilders dont sem to check that spec.If the shaft can move up and down excessively timing will change.