'bouncy' dwell measurement?

[waqas]

My dwell reading is 'bouncy'. The digital readout on the dwell meter rapidly varies between 39.2 and 39.8 degrees.

Is this normal or does this mean that the cam lobes on the distributor shaft are unevenly worn? In case of the latter, how can one tell when the cam lobes are too worn? (I've already greased the cam lobes)

From my understanding, having an uneven dwell measurement means that the intensity of the spark going to each cylinder is different. When is the variation is too great? (for the sake of protecting the cylinder head from damage, etc)

[thelews]

The specs say a variance of 3 degrees is acceptable and 5 degrees to change the points. Wear in the distributor and play in the shaft can cause variance.

[waqas]

This was after installation of new points. The car runs fine, but I wanted to find out if there's a spec. I suppose I'll need a distributor rebuild at some point soon.

Thanks

[thelews]

This was after installation of new points. The car runs fine, but I wanted to find out if there's a spec. I suppose I'll need a distributor rebuild at some point soon.

Thanks

It doesn't sound like your .6 variation is an issue.  What does it do when you rev the engine to 4K?

[Allenh]

This was the main reason why I converted to electronic when I did.  I could see the erratic dwell readings as well as jumpy timing at idle.  Shaft wear in my original distributor. 

The change eliminated any issues.  I have been reading the posts that tend to trash to Pertronix unit but the installation is virtually invisible and I haven't experienced any negative vibes on my car. 

[waqas]

It doesn't sound like your .6 variation is an issue.  What does it do when you rev the engine to 4K?

At 4k, the dwell measurement runs about 39.2-39.3. (less variation). But this may just be the effect of aliased measurements at that rotational speed. Is there any reason it would get better at that RPM?

I think I'll keep an eye on it to see if it gets worse. The timing numbers are pretty steady.

[al_lieffring]

Dwell variation usulaly is caused by wear in the breaker plate. the center hole where the two pieces fit together will wear to an elipse and cause lateral movement, changing the dwell as the vacuuim advance (or retard) moves the plate.
When I changed to the Pertronix ignition this eliminated the spring pressure from the points so now there is less effect on the breaker plate and the advance works more smoothly, and of course the dwell is generated electronically so it remains within specs.

Al

[Benz Dr.]

If the breaker plate is worn it will affect dwell angle. The worn plate will affect timing as well because the plate will still move when the plate moves. Normally there is no play at all and just a smooth movement. You can adjust some of the play out but if it's really worn it's not going to work right. Only about 1 in 10 units will be worn this badly.

There will be some sideways pressure from the points as they push against the cam - that's normal and the whole distributor is designed to compensate for that. If there's no wear in the unit it will function prefectly and can spin up to 6,500 RPM without the points floating. This is generally more than enough RPM for the average driver.
 I like to see them hold dwell up to 6,000 RPM but I won't throw one away if they only work well up to 5,500. Most drivers are only going to run up to 4,500 or less during normall outings and a small drop in dwell angle won't be noticed at high speed anyway.
 If the main shaft is worn and the breaker plate is worn there's little you can do with a mess like that. On good units I can easily drive several years on one set of points. It's all about wear....

Not all cars are 113's after all....

[merrill]

Waqas,
After reading this post  & noticing my dwell changing (which I have a post in this forum)
I wondered if a change in voltage to the coil would cause a change in the dwell.

I measured the dwell & voltage at the same time today.
I found that when the voltage was low say under 12 volts the dwell was at 37
As I increased the RPM's and the voltage went up say 13.5 volts the dwell was at 35.5 or 36

I let the car run for a while and found this to be fairly consistent.

I thought this was interesting.

Also note in the technical manual for this car it says it is acceptable for the dwell to drop up to 3 degrees at 4000 rpm.

I am sure others will have an explanation of why this occurs. 
It may be as simple as understanding that a dwell meter is a modified volt meter.
As voltage changes dwell will change..

The dwell changing that you noticed may be / probably is expected as voltage changes.
If dwell was super critical there would be some sort of voltage regulator on the input side of the
ballast resistor.....

[Benz Dr.]

I don't think the two are all that closely related. Dwell angle is simply the amount of degrees that the points stay closed as the shaft turns. Radial play in the shaft will cause it to deflect and move sideways away from the points. By closing the points ( increasing dwell angle ) slightly there's less defelection and generally the dwell will stay in spec over a wider range. It's the spring pressure that's produced by the points that's behind dwell variation. This is why I set the points to about 40 degrees on some units. I watch how it works and make small compensations so it will function properly.
Put your dwell meter away. It's only going to drive you nuts. Maybe me too.....

  Most distributors work better at speed than at idle. My tester shows me this and the way the engine runs confirms it as well. Some of that has to do with valve timing and how valve overlap affects idle quality. At higher RPMs the cam starts to work better and the engine will smooth out. Also, the more the cylinders, the smoother the idle. It's called volumetric efficency.
 On cars like 300SL's the idle can be quite rough but when the cam kicks in it's like a turbo charger. The sound is like no other car.....