VCCA Home Forums TECHNICAL - GENERAL Electrical - 6 volt Dwell Meter calibration?

Dick and I were trying to check the dwell on his 1931 engine. Wanted to see how the points gap affected the dwell. But we got some screwball readings. (And it wasn't related to our beverage of choice.)

Then we checked the dwell on a different 1931 engine (mine). The icing on the cake was when we checked the dwell using TWO different meters. Oops!!!!!

Got different readings.. approximately 40 degrees on one and 33 degrees on a second one... and both readings against an .018 gap. So, by the book the dwell angle should have read about 36 degrees.

So we've concluded that our dwell meters aren't calibrated properly. Sooooo, the question of this posting is "How can, or should, someone calibrate their dwell meter?" This seems to be a key item in fine-tuning our engines.

If you can chuck the dist. up in a calibrated rotary table, you can use the rotary to measure the dwell with an ohm meter.

Of course it would be easier to take the car to a shop with a trusted meter, if anyone has one anymore.

There are a few that still have the old machine that runs the dist into a bunch of sparkplugs. Does anyone remember the name of it?

If the meters that you are were using were the old analog type (needle) they sould not be trusted....if they are over 10 years old.Would be better to find a digital meter as they never loose their accuracy.Were they both set for 6 cylinders?

Years ago we had a Sun Machine in the shop that you could set a distributor in and spin.Could check dwell and advance (mechanical and vacuum).

I never bother to set my old 6 cyl engines with a dwell meter.A feeler gauge works just as well and they will run OK from .022"-.017".I set them at .021 with new points and let them wear in to .018".

I have found that some of the cheapo dwell meters do not read accurately. On my Sun distributor machine I have the on-board dwell meter and I also have two accessory dwell meters attached to the machine as well, and all of them are analog. When testing a six cylinder distributor with a point gap of .018" the dwell reading is 36 degrees and all three dwell meters are within 1 degree of each other. A good dwell meter will have a calibration knob where you can set the analog needle either on zero or on a "set" line.

Either you are using a cheap dwell meter or possibly the cylinder mode is not set to six cylinders. The point gap and dwell have a direct relationship to each other, therefore at a gap of .018" the dwell should be reading right around 36 degrees.

Since we have two meters, one reading 4 degrees high, and the other 3 degrees low, then I can use them and always split the difference? Right?

Don't need no stinkin' new meter!!!

Ha!.... thanks for the insight.

-Bill

Sounds like your trash can will contain two dwell meters very soon!

Dwell meters are ok if they work! just set your six cylinder points to .018 gap on a high point then you can check your dwell meter out!
Chevrolet didn't seem to dwell too much on dwell meters. even those with external poibt gap adjustments didn't call for a dwell meter. I seen a fellow employee a Senior Field Engineer(he wanted to meter everything from filiment voltage to the break of dawn) use a Simpson multi-meter to check dwell. sometimes the engine would even run!

Most old distributors have worn cams, and the wear is acceptable up to two degrees. With a worn cam the point gap will be different on each lobe of the cam, therefore, I prefer to go by the dwell instead of the point gap. With a worn cam the point gap can be adjusted accordingly first, and then the dwell can be set for the final adjustment. Again, a quality and accurate dwell meter should be used.

So JYD if the cam is worn uneven will the dwell fluctuate between cylinders or is the meter padded well enough in a quality meter to show a constant dwell value?

I have an old analog large face engine test meter with rpm, dwell, volts and a couple of other functions that works as well as the new digital rpm and dwell meter I bought a couple of years ago. I usually hook the digital up as a second opinon whenever I do a tune up. I also use a vacuum and pressure meter and a compression gauge on tuneups, which usually take a full day on the three old cars 28, 53, and 68 that I keep running. (all have points distributors). I have a Perxtronix for the 68 but the points distributor works so well I haven't changed over, may never.

On a good dwell meter the needle with not fluctuate between lobes. The meter will give the overall dwell reading. For example: If a distributor cam has a wear of 2 degrees for the six lobes, and the dwell is set at 36 degrees, when the point gap is checked on each individual lobe it will be found that one or two of the lobes will actually be at .018" and the other lobes are not.

On the reverse side of the coin, if the point gap is set on one lobe of a worn cam at .018" and then the dwell is checked, the dwell will be off some from the 36 degree figure because of the worn cam lobes. That is why I prefer to set the points on distributors with used cams by the dwell instead of by the point gap.

In testing I have also installed a new old stock cam and set it by the dwell of 36 degrees. Then, when each lobe of the cam was checked for point gap, all six lobes were found to have a gap of .018" since there was no wear on any of the lobes on the new old stock cam.

The first time I did a tuneup on the 28, which i didn't know squat about it's condition I used a jewelers loupe to inspect the distributor, and to my surprise it looked practially brand new, along with what I saw by looking into the sparkplug hole with a bore scope. It was a surprise since the old gal sings her own song going down the highway at 50 mph.Some of the noises was eliminated when I oiled the front bushing on the waterpump and dumped some engine oil on the felt pad under the valve cover.

In another forum someone posted an old hot rod tip that said they were getting improved performance by setting the point gap to 5 degrees which would obviously increase the dwell and limit point float. I suggested that that might work in a very dry climate, but that I wouldn't want to do so here in Oregon as moisture could make it appear that the points never opened, making starting impossible. My question is, why are 18 thou and 36 degrees so critical? The points fire when the points open and the coil recovers while they are closed and regardless of the dwell, the engine is timed to the opening of the points.

If the points are set too wide the spark plugs don't get enough juice. Set them too close and the points will burn.

The dwell angle is the number of degrees of rotation of the cam/distributor during which the points are closed. During each rotation of the cam/distributor, the points must open and close once for each cylinder. The points must stay closed long enough to allow the coil primary current to reach an acceptable value, and open long enough to discharge and produce a spark.

Having the point gap and the dwell dead on is actually not that critical on the older vehicles....especially if you adjust by dwell instead of by point gap, but the settings should be within reason! It's just that the point gap and dwell have a direct relationship to each other and the relationship from point gap to dwell needs to be correct.

However, some engines do run better at a specific dwell. For example, the 327 in my 1968 pickup and the 350 in my 1969 Impala seem to run better at exactly 30 degrees dwell even though the specs. call for 28 to 32 degrees. Again, having the dwell set exactly at 30 degrees in these two examples is not at all critical, but it does appear to make a difference on performance.

Thank you JYD,

I guess the points burn from arcing across an insufficient gap and/or insufficient acceleration of the points as they are breaking, it that right?

points can burn at any gap if the condenser is bad (open) if the condenser is shorted there won't be any spark.

So, my opener in this thread was to try to figure out a way to "set" my dwell meter. It's obvious that it's 4 to 8 degrees off... I thought that someone might have a neat trick as to how to connect it to something so that I could zero-out the needle.

If it is a jeweled meter, the screw in the indicator hand balance spring may be adjustable for mechanical zero. There is a "set" position on my meter knob to set the full scale reading but it is a real old meter. I think it is to compensate for the flashlight battery output.

There were dwell meters sold that ranged in price from $10 to $200...some battery powered-some not.Some adjustable and some not.What brand is it?If its a "professional" brand it may be adjustable,such as Snap-on,Fox Valley, Sun, etc.If its just a little toy amature one probably not adjustable.If it is adjustable it would be necessary to know how to correctly calibrate it......lots of IFS.

Independant Front Suspension??? :confused: :confused: :arrow:

I have about 6 good dwell meters and all of them but one have a calibration knob for the dwell. One (a Sears model) does not. And all of the dwell meters also have a mechanical screw set so that the indicator needle can be set to zero. None of the above meters are powered by their own batteries.

Billy Boy, if your meter is a cheap dude then it probably can't be calibrated and all of the cheap dwell meters that I had ended up in the trash can because they were not even close to being accurate.

:( :( :(

But if it can be recalibrated you would need a known accurate meter to compare it with.How else would you know its correct with out using what ever the factory used to set it when new.?

Absolutely! My Sun 404 distributor machine has an on-board dwell meter with a calibration knob to adjust the needle to the dwell set line. However, that same dwell meter can also be recalibrated via some simple tests listed in the Sun Service Manual. Whenever I "out test" a distributor I not only use the on-board dwell meter but I have two other quality dwell meters attached to the machine as well so that I can get an overall dwell reading from the three meters. And, I also can tell if one is out of calibration with the others if I get a weird reading from one of the meters. At any rate, all three dwell meters are within one degree of each other when setting the dwell on the distributor machine.

We tested four dwell meters this weekend. Three of them had a mechanical screw which allows you to "zero" the needle -- but only at rest. It is purely a mechancial adjustment against the lower end of the scale. This didn't really give much comfort by way of "calibration".

The fourth meter had the screw for this "zero" setting, but it also had an "active" switch which apparently invoked a live circuit and set the scale to it's maximum value. Of the four meters, this was the one that we ended up trusting the most after all of our testing.

That is good, but the feeler gauge setting as per the manual, with the points fully open have seniority to a Dwell meter on Gm cars! But it don't hurt to show off all that professional shop equipment to your friends and cronies, just make sure the beverage of choice is good and cold!

Actually, if the distributor cam lobes are worn on the older cars the dwell meter gives you a more accurate setting than a feeler gauge. And, on later cars with point systems the dwell meter is recommended over the feeler gauge.