Setting Dwell

[72Plymouth]

One thing you may want to check anytime before you use the meter is to make sure it is zeroed out. With it off you can adjust the needle to zero it with the screw in the middle at the bottom.

Thanks. Yup I did check that and adjusted it before I used it. But still the meter was diff from my Autometer so I trusted the auto more. Are Autometer gauges ever off?

 

[camd64]

They can be off. I would test the meter on another car and see how it compares. I thought my old Sears one may have been off but when we tested the car using my buddies snap-on it was the same. 20 year old meter and still accurate.

 

[67Dart273]

One thing you may want to check anytime before you use the meter is to make sure it is zeroed out. With it off you can adjust the needle to zero it with the screw in the middle at the bottom.

Another thing to be careful of is static charge on plastic face meters. Often you can run your finger gently around the face and see the needle "magically" move. Gently wipe with damp soft soapy towel or find an "antistatic" cloth. Used to get them for "records", remember them?

 

[dgc333]

It most certainly is NOT adjustable!!! The air gap has no effect on dwell, all the dist. does is to trigger the box.

I agree! Dwell is the amount of time that current is flowing through the coil expressed in degrees. In a points system the points open on the leading edge of the cam and close on the trailing edge of the cam so increasing the point gap at the peak cause the points to open sooner and close latter (same idea as changing lash on a solid cam). This has a two prong effect opening sooner increases the timing and decreases the dwell.

In an electronic system the reluctor passing the pick-up creates a very short duration pulse that triggers the circuitry in the ecu, it's the delay built into the ecu circuitry that will determine how long the current flow is interupted and hence the dwell. Late model electronic ignitions actually vary the dwell with rpm. They increase the dwell at high rpms for a more reliable spark and reduce it a lower rpms to prevent the coil from over heating. That is why the new stuff doesn't need a ballast resistor.

Adjusting the air gap serves the purpose of ensuring that the trigger signal occurs reliably. To much gap and the signal may not be large enough to cause the ecu to trigger resutling in a mis-fire and to little you could get false early triggers.

FWIW, the only reason that a non-magnetic feeler gage is recomended for setting the air gap is that you can't get the proper feel with a magnetic one because it wants to stick to the magnet in the pick-up. If you are stuck without a brass feeler gage a match book cover is .007-.008" thick and works quite nicely.