Dwell Hell

jcwolff · 05-14-2009, 11:50 AM

I have a 1973.5 911T and I replaced my points the other day. I now want to check dwell. I made a jumper cable with spade connectors on either end so I could easily measure the dwell. I clamped one end of my multimeter on the jumper connection and the other end on the fan shroud. I first measured rpm and it worked great. Then I clicked the multi meter over the 6cyl dwell. The reading said 60 and then went through a bunch of numbers before settling on 16.2. I measured it a bunch of times clamping it different ways and it always ended up on 16.2. My spec book says it should be 38 +- 3. I have searched and searched and I have never seen anything where it says I am supposed to double the number that I measure. Am I missing something? I can't believe that number is accurate. I wouldn't think the car would even run. Thanks in advance. - J

304065 · 05-14-2009, 12:04 PM

Dwell doesn't make a big difference for CDI because it's the rising edge that triggers the capacitor to fire. Widen the points gap to spec and the dwell should be close enough.

Hugh R · 05-14-2009, 12:10 PM

John, with all due respect, but in most cars, don't you close the points to raise the dwell?

db_cooper · db_cooper's Garage

Had a 73.5 that I enjoyed for many years.

The dwell was tricky as one meter might work fine and another would read 1/2 scale or bounce around. Don't know why.

I used a Mac dwell meter with some built in impedance that served me well.

Would suggest that you gap the points (was it 15 thou?) and see if you can get some meters and try them out to see which ones work. Maybe some buddies at the local shop will help.

304065 · 05-14-2009, 12:27 PM

Quote:

Originally Posted by Hugh R

John, with all due respect, but in most cars, don't you close the points to raise the dwell?

Hugh, absolutely correct. Narrow the gap to raise the dwell. Thanks for the correction.

jcwolff · 05-14-2009, 12:47 PM

Thanks. Glad to see I am not crazy. A couple of friends have analog meters. I will try. Thanks again. - J

john walker's workshop · 05-14-2009, 12:52 PM

the battery in the meter must be fresh or the reading will not be accurate. i clip the + lead to a long skinny screwdriver and reach in to touch the terminal on the side of the distributor. points cars only.

jcwolff · 05-14-2009, 01:15 PM

I tried the long skinny screwdriver with no luck. I was not good at playing "Operation" growing up. I took a piece of 18ga wire 6" long, and crimped a male and female spade connector on either end. I connect the female end to the side of the distributor and the male end to the two trigger wires. I will try replacing the battery to see if it makes a difference. Thanks. - J

RoninLB · 05-14-2009, 01:51 PM

a business card was my tool of choice with this car

Paulporsche · 05-14-2009, 07:10 PM

Did I miss the obvious here? You say you clamped something which I'm assuming was the meter leads several different ways but got the same result. You did try resetting the points using a feeler gauge didn't you? Did you try adjusting the points while you were "on the gauge"?

jcwolff · 05-15-2009, 08:29 AM

I did change the gap a little bit and the meter would read a little different but nothing huge. I am going to try an analog meter this weekend. I think it's reading at half scale. - J

john walker's workshop · 05-15-2009, 08:39 AM

agreed, digital timing lights and the like jump all over the place. i have an old fox valley dwell/tach/ohm/volt meter i've used for years. i bought a snapon digital timing light once and returned it the next week. too wierd for me.

ljowdy · 05-15-2009, 09:10 AM

When I had my original 73.5 engine in my car, I decided that a Petronix was the way to go.. No more points and no more setting the points. If you do decide to go this direction, call Petronix (909) 599-5955 as their instructions were wrong when I installed my unit. Over the phone, they gave me the correct installation information and from there on out, it worked flawlessly.

cmcfaul · cmcfaul's Garage

Ditch the points and dwell. The best modification you will ever make is going to a breakerless ignition (Petronics or other).

There is no greater PITA than adjusting Dwell.

Chris
73 911 E

Hugh R · 05-15-2009, 01:55 PM

One trick is to pull the distributor cap off and the rotor and have someone crank the engine while you read the dwell meter and adjust the points as the shaft spins. GM cars used to have a window in the distributor so you could turn and allen key and adjust the dwell while the engine was running.

304065 · 05-17-2009, 04:02 AM

Quote:

Originally Posted by cmcfaul

Ditch the points and dwell. The best modification you will ever make is going to a breakerless ignition (Petronics or other).

There is no greater PITA than adjusting Dwell.

Chris
73 911 E

Actually, the only greater PITA I can think of would be the Pertronix or other electronic ignition failing at the side of the road due to voltage spikes and then not being able to diagnose the problem without a handheld oscilloscope. All Pertronix users are cautioned to carry a spare set of points in the glove box, at about $8 they are cheap insurance.

Pertronix for a CDI-equipped car is a solution to a problem that doesn't exist-- the current through the points is only 420mA so they last for tens of thousands of miles with no issues. The OPs car probably runs fine at 16, or even 50, degrees of dwell.

berettafan · 05-17-2009, 04:35 AM

geez i've never bothered to check dwell on my car. not sure what i'd do if i found it out of spec as the only adjustment is the points gap right?

ljowdy · 05-17-2009, 05:28 AM

By adjusting the point gap, you also adjust the dwell... somewhat of a trial and error If dwell angle is too high (points closed for too many degree of rotation), you will have to carefully reset your points to a slightly larger gap. If the dwell angle is too low, the points will need to be set slightly closer. Adjust the gap .002" or .003" at a time for best control.

Walter_Middie · Walter_Middie's Garage

I haven't had points now for a couple of years (Pertronix). But I had found a method of checking the dwell that was really easy. I'd just pull the distributor out, mount it in a vise on the work bench, and spin it with a drill. I'd connect the drill to the dist drive gear using a piece off an old bicycle inner tube and a small hose clamp. There's just not enough room to get your fingers in there with the distributor mounted in the car. My method was quick, accurate, and easy.

Paulporsche · 05-17-2009, 08:34 AM

There is a small nub on one side of the points so you can get it close w/ a feeler gauge, tighten the hold down screw slightly only, spin the engine either with or without starting it, and adjust the gap using a wide blade screwdriver between the nub and side of the points until you get the correct reading.

2 things:

1. It takes less time than my feeble attempt @ describint the procedure, and

2. Lately there have been threads on this stating that for a CDI ignition, the actual dwell isn't important, and does not affect the timing, as it does in non CDI systems.