Checking Dwell With Engine Running
 

My engine has points and CDI.

I set the dwell by placing the red lead of the meter on the side connector of the distributor for the thin wire, which I have disconnected. I place the black lead on the nut of the fan loop. I pull the fuel pump relay. Then I have someone crank the engine, which won't start of course, and set the points. I have always followed this procedure to prevent any damage to the CDI system.

I would like to check the dwell periodically without outside help. If I connect the red meter lead to the side wire or to the side connector without removing the wire then I can check the meter while the engine is running, since I can start it myself.

I don't think this would cause any damage to the ignition system but I just want to be sure before I try it. Can anyone verify that this method will not cause any harm?

I added a spare lead with a removable insulated cap to the distributor points wire I can clip my dwell meter to. You want to check the dwell with the motor running so you can check for dwell variation at different engine speeds, i.e. little to no variation idle to red line.

That's about what I was thinking of doing. Just wanted to confirm that having the meter connected while running wouldn't cause a CDI issue. Thanks.

I used to actually set the points while the engine was running on older cars of mine, but none had a CDI box.

Just want to make sure that if I try to set the point gap with the engine running, which will introduce screwdrivers into the distributor points area, that no damage will be caused.

How can you set the points gap (ie dwell) with the engine running? I only know adjusting the point gap with the distributor cap removed. It’s not a GM V8!

Going way back in time, it was common to set the dwell using a remote starter switch, dwell meter and a screwdriver. Crank it over, look at the meter, and set the dwell with the engine cranking, locking it down when the dwell was at spec. Kettering style ignition systems would wear the points considerable faster than CDI systems because of the higher current flow across the points.
CDI systems in practical terms, only use the points to trigger the spark, not to build voltage for the secondary side.

Buy a petronics or other breakerless ignition. Best mod you will ever make. Never have to mess with points again. Fits in distributor cap.

Chris

73 E

dude, RIF. This is for checking dwell and dwell variation. If you can set the points gap perfectly with out checking the dwell angle, more power to you.

I keep points because they are original and old school cool. They also require an almost lost skill set these days, much like carbs.

Jackstands,

Of course you are correct. The engine will not start with the distributor cap off. As you mention, GM had a nice system for that.

Looks like the only thing that can be done is checking the dwell, not adjusting it. At least this can be done safely with the cap on and the engine running via another lead attached to the thin wire. I may set up a remote4 starter switch or just go back to utilizing a helper.

dwell is a pain in the ass. Only way I knew was trial and error. Set, assemble, test, repeat .....Car would run great for a few hundred miles then be ever so slightly off. Petronics....always has that perfect dwell feel.....last plug.

Chris
73 E

Thanks Chris.

If you happen to come across another related post I had recently, you'll see that I actually have quite a few points that I want to use up. Then I may make the switch to breakerless, like a Pertronix.

Anyone still making dwell meters?
I have CDI and always check the dwell at idle. If it is off, take distributor cap off and adjust. Ideally you can do all this on a distributor test bench, but I don't think they make those anymore either.
I still use an old Bosch mini-tester, I guess i'll go pertronix when it dies.

I believe dwell is a function of point gap. You can check dwell with it running. You need to set the point gap as you stated. The lower dwell angle implies the points will hold their position longer.

Before I had Pertronix, I’d Remove the distributor, clamp it in a bench vice, and spin it using an electric drill. The connection between the drill and the distributor was using a piece of old bicycle tube and a hose clamp. Worked great, and I found removing and replacing the distributor was pretty easy. Setting dwell was easy too, with good access to the points as the distributor was spinning.

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Prior to electronics appearing in ignition systems, it was more important to maintain a specified breaker point gap, a compromise between an adequate gap to collapse the magnetic field in the coil and an adequate point closure time (dwell) for that magnetic field to build (coil saturation) for potentially higher voltage.

With electronic circuits, a specific point gap and closure time (dwell) is less of an issue. All it takes is a clean make and break signal to trigger the high voltage spikes.

However, breaker points are a mechanical component that relies on a fiber/resin rubbing block maintaining contact with the rotating distributor cam lobe. As the rubbing block wears, the points open slightly later which has a direct affect on ignition timing (and power). Thus the advantages of pointless ignition modules.

Sherwood

A clarification FWIW. I was referring to checking/verifying the dwell angle while the engine was running, not to set the gap while running. As Jack Stands pointed out, this is not possible on my car.