Dwell Variance with Vacuum Advance

[Lugnut_66]

When testing dwell on my 318 poly, the dwell does not change from the 30* spec when revved to 1500rpm with the vacuum advance disconnected.

The dwell does drop, from 30* to 20*, when revved to 1500 with the vacuum advance hooked up.

The factory manual simply states that the dwell should not vary more than two degrees up to 1500rpm with the vacuum advance disconnected. There is no specific mention of the dwell varying with vacuum hooked up.

Does it indicate a problem if the dwell does change, when revved, with vacuum hooked up?? Or is it normal??

 

[Funship32]

It has been a long time since I've worked on a points dizzy but your 1st test w/o vacuum seems to indicate that the shaft bushing and such is in good shape. That is a good thing. As for with the vacuum hooked up, I can't say I know 100% but I would expect dwell to change as you are putting the dizzy under vacuum and that will change the timing/dwell. Personally I don't think you have much to worry about but considering the age of the car, I would test to make sure your dizzys mechanical advance is working; given the age it isn't unheard of to find the weights seized up due to lack of maintenance.

 

[vin_vl29u]

My understanding: Dwell should stay the same, it's the number of degrees the points are closed. If everything is working right, it should stay within the 2 degrees no matter the rpm. If the dwell goes down, the points are staying open longer. And I'd guess something like point bounce (weak spring) or something is worn like a bushing.

 

[Cranky]

I generally set dwell at idle and don't worry too much from there. Been doing it that way for way too many years. You can check your 'dist-stu-butor' for wear with a dial indicator if you want to go that far. If the shaft is moving, replace it. Is the car behaving bad? Set the points wide and the plugs close and the tune will 'drive in' and will be very nice in a few miles. Use Blue Streak parts only.....

 

[A383Wing]

the dwell dropping is normal...

 

[Cornpatch MO]

My understanding: Dwell should stay the same, it's the number of degrees the points are closed. If everything is working right, it should stay within the 2 degrees no matter the rpm. If the dwell goes down, the points are staying open longer. And I'd guess something like point bounce (weak spring) or something is worn like a bushing.

Ditto.........................MO