Dwell Meter Reading

[leadrsa729]

Hello -

When checking the dwell Angle on a 72' 911T, the needle on the dwell meter jumps all over the place. It never goes above the recommended 38 degrees though. I am checking the dwell angle by hooking up the positive clamp to a screwdriver and touching the screw driver to the metal contact that the CDI wire is connected to on the distributor - nothing is hooked up to the CDI unit itself. Then I've got it grounded by attaching the other clamp to the case. Can anyone tell me what the cause of this is? I've set the gap on the points to what the spec book says. The points are also brand new and are for the 3 digit number on my Bosch distributor (003).

The engine is a 2.4 case with S cams and S pistons from the 2.2 for the 9.8:1 CR.

I am having the hardest time setting the timing for this car.

[asphaltgambler]

First you need to have e good solid connection from your meter to the primary wire with the other to a good ground

[rwest]

Are you trying to check the dwell while the car is running? dwell is with the engine off and checked by making sure that the rub block on the points is on the dist lobe. I had a bit of trouble figuring it out myself last year because it had been so long since I had done it last!

[RoninLB]

in another thread pelicanheads did a write up on how MSD ignitions screw up reading a timing light with the dial-back feature... good info

point setting clearance is a non issue with a CD ignition just as long as it's opening somewhat. A matchbook cover thickness is good enough. New points need a few hundred miles to break in unless you grease them and then it may take 1,000 miles.

set points, then set timing

[911pcars]

Originally Posted by rwest

Are you trying to check the dwell while the car is running? dwell is with the engine off and checked by making sure that the rub block on the points is on the dist lobe. I had a bit of trouble figuring it out myself last year because it had been so long since I had done it last!

With the engine OFF, how do you mechanically measure the number of degrees the contact points remain closed? If you do, you're good. Most mortals require a dwell meter on a running engine that electronically measures the angle.

And as Ron points out, only a simple point gap adjustment is needed on an engine with CDI.

Sherwood

[Peter Zimmermann]

I beg to differ with a number of the above posts. Accurate point dwell (forget the gap) is absolutely necessary for a top running car. Dwell is the amount of degrees that the center shaft of the distributor rotates between the time that the points close, to the time when the points open on the next lobe. Dwell for Bosch distributors is normally set at 38 degrees +/- 3. It's best to set them between 35-38 degrees. Use a meter designed to measure dwell, and when you touch your long, skinny screwdriver to the wire on the outside of the distributor housing, take extreme care to not let it go to ground. If it does it's highly likely that you will zap your CD box. Your bouncing might be points that are set too close, put the rubbing block on the high part of a lobe, and take another look at them. Also, if the points are new, did you clean the contacts with lacquer thinner or MEK (using a Q-Tip)? That's always a really good idea to do.

[Walter_Middie]

I always found it much easier to remove the distributor, clamp it in a vise, and spin the distributor with a drill. The drill was attached to the dist drive gear with a piece of an old bike inner tube. Very easy, very accurate, and much easier to get my hands into the points for adjustment. Then simply re-install the distributor, set the timing, and off you go.

[rwest]

Sherwood,

Now you have me thinking! I think I went from flat to lobe by turning the crank by hand and reading that on my digital oscilloscope. I know when I tried it running the numbers just were moving all over. would a analog meter work better for a running engine? Sorry to all if my advice wasn't correct, thanks for the catch!

Rutager

[911pcars]

Originally Posted by rwest

Sherwood,

Now you have me thinking! I think I went from flat to lobe by turning the crank by hand and reading that on my digital oscilloscope. I know when I tried it running the numbers just were moving all over. would a analog meter work better for a running engine? Sorry to all if my advice wasn't correct, thanks for the catch!

Rutager

Using an oscilloscope is a new one. If that works, go for it.

Sherwood

[rwest]

Hi,

It's an automotive scope with a dwell setting! so I was using the dwell function not the scope. but I think it might show the opening and closing of the points(square wave?) though I'm not sure if it would give a good degree reading, but it might be something to try, but not until spring, unfortunately my susp went bad a few months ago so the car is tucked into the corner of the garage for winter.

Rutager