Tach dwell meter on dual ignition system. How to interpret?

[m-mman]

The car is a 1929 Cadillac. Single distributor, single coil but dual points. One set fires 4 cylinders the other points fire the other 4 cylinders. 
 

Connect a tach dwell meter.  Connect it “backwards” because it’s positive ground.  6/12 volt? Not sure that it matters with this tester. 
 

Dwell reads 15(?). The distributor has been professionally set up so I know that the points are correct. Is the dwell doubled? 
Hummmmm…. I Know the points are correct, reading is academic and not critical. 

BUT

what about the tachometer?  Wide open it reads 2000 RPM.  Hard to estimate if that’s correct.  Do you double the tach reading also? 

[Grimy]

4-lobe cam in distributor?

[EmTee]

6V system?  If so, is the meter calibrated for that?

[Bloo]

To even read the dwell, it would have to read 0 with the points open and the maximum possible degrees with the points closed. On the meter in the picture, maximum is all the way to the left. Normal on most cars is a little more than half of what is theoretically available.

 

I think there is almost zero chance the meter in the picture will even function on a 6V positive ground car. I would like to see the manual for the meter. There is one other likely problem:

 

2 hours ago, m-mman said:

Single distributor, single coil but dual points. One set fires 4 cylinders the other points fire the other 4 cylinders. 

I am guessing as @Grimy did that it has to be a 4 lobe cam. If that's true, the points just about have to be wired in series. This makes the dwell meter reading meaningless. A correct dwell reading would occur with a correct setting, and also with 2 wrong settings in opposite directions.

 

Then there is the timing issue on dual setups like that. Once the points are set, one set has to be moved so the timing is the same on all cylinders. It depends on the distributor. Some have a way to change the timing on one set of points. The goal is to have the timing set normally on #1 or whatever cylinder it is supposed to time on. Then check the next cylinder in the firing order. You have to change that adjustment until they match. If there is no separate timing adjustment for one set of points, then you have to set the dwell (or gap) slightly incorrectly on one set to get the timing to match.

 

The only easy way to do this is on a distributor machine. If you are a complete masochist, and have a lot of free time and the patience of Job, there might be another way. Post back if interested.

 

Unlikely for the RPM to be correct unless the meter explicitly supports 6V positive ground. RPM is a tougher technical problem for the meter than dwell. Dwell is easy.

 

Edited December 23, 2023 by Bloo (see edit history)

[Rusty_OToole]

You could bypass the whole issue with a feeler gauge and a 6v test light. I have set up ignition this way then double checked with dwell gauge and timing light and found no difference.

[Matt Harwood]

I never tested the dwell on my '29 Cadillac, so I'm not positive that it works the same, but on my Lincoln, the dual points read as two separate 6-cylinder engines and I set the dwell for each set individually using the 6-cylinder scale. With a single coil on your Cadillac, I'm not sure it would be the same, but potentially it might be acting as two 4-cylinders and the 4-cylinder setting might give you a correct reading?

 

 

Edited December 24, 2023 by Matt Harwood (see edit history)

[maok]

IMO, there would be an 'overlap' of dwell between each series of cylinders, which would show as a low dwell angle on a dual points, single coil setup like this. The meter is reading how the coil is responding to the individual points opening and closing in series.

Edited December 24, 2023 by maok (see edit history)

[Grimy]

My 1930 Pierce 8 has two coils, two condensers, and a 4-lobe cam (Delco 668-E distributor), and my 1934 and 1936 Pierces use 662-Js with one coil but two sets of points, one condenser, also 4-lobe cams.  Some 20 years ago I found on ePay a Bear (made by Echlin) mechanical synchronizer set with 12 different degree discs and a test light.  It has been awhile now, but I synch points with the dist head in the vise and use a 6V lantern battery to power the test light.  I have the instructions on a pdf which I can't post here if anyone is interested (if so, pm me your email address).  The chart showing which disc to use for which engine ends with 1934 applications.

[hchris]

Do as Matt H has said, set the scale to 4 cylinder. Why, because the meter is reading 4 impulses each revolution.

[m-mman]

Thanks everyone.
 Yes it is a four lobe cam.  Ed set up the distributor on his machine so I know that it is correct for gap, dwell(?) advance, etc. 

 

there are marks on the flywheel and the distributor rotates (can adjust the timing from the dashboard) I used a timing light to adjust the timing for both sets of points/cylinders so they are correct. 
 

The car still runs poorly but that’s because of the original Johnson carb that both Jon and Ed condemn highly. 
(both have no ready suggestions for a replacement). 
 

But my concern was evaluating the RPMs. At some moments it accelerates great and the next moment it “hits the wall” and revs only slightly above idle.  Then it may/may not run well for the next shift.  
I’m confident that it’s all carb, but I was going to try to track how much it revs and if it hits the wall at a consistent top RPM. 
 

Other Cadillac folks have custom built a downdraft manifold and use a modern carburetor.  

I don’t know, I like original, but this thing is almost as frustrating as Matt’s Lincoln. 

I work on it, get frustrated, put it away for months then drag it out and tinker with it some more. I drug it out now to see if I can get it usable for our “Holiday Motor Excursion” tour, but maybe it still ain’t ready enough. 

[Lee H]

7 hours ago, m-mman said:

But my concern was evaluating the RPMs. At some moments it accelerates great and the next moment it “hits the wall” and revs only slightly above idle.

I think it would be easier to judge whether RPM is reading correctly at idle. So while I don’t know what your actual idle speed is, I’m going to assume it is in the 300-400 RPM range. Set your meter on the 1000 RPM scale, and then see if you get something close to that on either the 4 or 8 cylinder setting. Then you can use the same setting on the high RPM side
 

BTW, while a few here have speculated that your points are wired in series, according to my literature they are actually parallel. And I’m still trying to wrap my brain around how that would affect (or not!) the reading of an electric dwell meter.

 

OK, got my brain adjusted. The whole purpose of this (fiddly) system is to enable longer dwell times than possible with an eight lobe single breaker system. Theoretical s/b 34 degrees.

Edited December 24, 2023 by Lee H (see edit history)

[Bloo]

If the points are truly in parallel, and they are 45 degrees apart as you would expect, and there is one coil, and four lobes on the cam, the ignition would never fire because one set of points would always be closed.

 

If the points are at the same angle, everything else as above, only 4 cylinders would fire.

 

If the points are only separated by a very small angle, in the interest of increasing dwell, as is seen commonly in postwar setups, and everything else is as above, only 4 cylinders would fire.

 

If the distributor were driven at crankshaft speed (double speed), and everything else is as above, then all 8 cylinders would technically fire, but in that case the cap and rotor would have to be elsewhere, driven by a different shaft (at normal speed) or the rotor would not line up to the right cylinders at the right time.

 

EDIT: Hmmm... maybe if the cam lobes were extremely wide, and the first set of points is still open when the second one opens, maybe it could work with the points in parallel. That is a bit of a brain teaser. The signal at the coil for purposes of RPM would be the same as a modern 8 cylinder in that case. Dwell too, although the dwell reading wouldn't be very useful. Wrong point settings in opposite directions could produce a correct reading. Offhand I would expect a dwell number higher than a typical V8.

 

Edited December 24, 2023 by Bloo (see edit history)

[Bloo]

18 hours ago, m-mman said:

Dwell reads 15(?).

A dwell meter is just a duty cycle meter. To read correctly, a dwell meter needs to read 0 with the ignition on and the points open, always off, a 0% duty cycle. It needs to read 45 degrees (for 8 cylinder), or on 100% of the time, with the ignition on and the points closed, a 100% duty cycle. The meter cannot be pegged to the right or the left. It must be 0 and 45 (for 8 cylinder). I suspect the red rectangle is SnapOn telling you anything in that rectangle is close enough at the 45 degree end. If you cant make it behave correctly at both ends, the meter cannot give an accurate reading on that particular car.

 

If the meter achieved both ends of the scale correctly, but backwards, then 15 degrees was really 30 degrees, a fairly normal dwell reading for a modern V8.

 

Dwell meter designs have been all over the place in design over the last 80 or 90 years. Ones that connect with 2 wires and have a zero adjust knob offer the most chance of success on cars that are 6 volt, or positive ground, or both. That is not a rule, just a probability.

 

[Johan Boltendal]

Just a remark, if Ed or anyone else has set the distributor on a Sun tester or a like,please don't touch the distributor with a dwell meter or any other tool, no matter how good yopu are at this you will never achieve the exact setttings of the machine, believe me.

 

As far as the Johsons are concernd, they have a bad reputation but if in a correct factory alike condition and function, they work well, mine does for 40 year now and I have done a few Johsons as well.

Most overlooked is the flapper valve spring, the correct tension , lenght and fit will make a huge difference. Best way to find this out is by testing on a 4 gas meter. The reading should be about even, at idle and half throttle.

Depending on the condition of your engine. the reading should be in the range of 5 - 7 % Co .

 

Tension to big in the spring, or to short a spring, will give reading in the 10-12 % range or even above, too weak, or too long a spring will

lead to reading in the 2-3% range or lower. Too strong a spring give too rich mixture, due to lack of air, too weak a spring a lean mixture, due to  too much air.

 

New springs should not be accepted having the tension intended by the factory, find this out by 4 gas testing is my advice. You will be surprised how well the engine runs with the right spring and setting

The new spring wire thickness comes in 4 different tension versions. The screw setting the mixture, was not intended to ork with the wrong spring tension. That is why the WS Manual ask for special care, handling this spring, in detail. Hope this will help Johan

[Lee H]

2 hours ago, Bloo said:

If the points are truly in parallel, and they are 45 degrees apart as you would expect, and there is one coil, and four lobes on the cam, the ignition would never fire because one set of points would always be closed.

 

If the points are at the same angle, everything else as above, only 4 cylinders would fire.

 

If the points are only separated by a very small angle, in the interest of increasing dwell, as is seen commonly in postwar setups, and everything else is as above, only 4 cylinders would fire.

 

If the distributor were driven at crankshaft speed (double speed), and everything else is as above, then all 8 cylinders would technically fire, but in that case the cap and rotor would have to be elsewhere, driven by a different shaft (at normal speed) or the rotor would not line up to the right cylinders at the right time.

 

EDIT: Hmmm... maybe if the cam lobes were extremely wide, and the first set of points is still open when the second one opens, maybe it could work with the points in parallel. That is a bit of a brain teaser. The signal at the coil for purposes of RPM would be the same as a modern 8 cylinder in that case. Dwell too, although the dwell reading wouldn't be very useful. Wrong point settings in opposite directions could produce a correct reading. Offhand I would expect a dwell number higher than a typical V8.

 

Bloo, it took me a bit to wrap my brain around it, but the points are not exactly 45 degrees, but staggered so that there is a short period (briefer than a single point system) where both points are open, allowing the coil to fire. I found this theory of operation very helpful to my understanding:

[Bloo]

2 hours ago, Lee H said:

Bloo, it took me a bit to wrap my brain around it, but the points are not exactly 45 degrees, but staggered so that there is a short period (briefer than a single point system) where both points are open, allowing the coil to fire. I found this theory of operation very helpful to my understanding:

Lee H, thanks for that! Still though, I think the points have to be staggered 45 degrees for an even fire V8. The slight offset seen on more modern dual point designs couldn't work because there are only 4 lobes. The staggering magic must be in the cam.

 

Consider this, maybe the first set of points opens to fire, but then doesn't close because of a really long lobe. After all the next opening ramp of a lobe for the first set of points is 90 degrees away. 45 degrees later, a lobe opens the second set of points and fires, because the first set of points is still open. Shortly after the second set fires, the first set of points closes, as it's cam lobe is more than 45 degrees long, but less than 90 degrees. The second set of points is still open, as it's cam lobe is also more than 45 degrees long but less than 90 degrees. Another 45 degrees later, the first set opens and fires again. Shortly after that, the second set closes, and so on. You would be effectively using one set of points to make and the other to break, just like a more modern dual point setup with 8 lobes and the points slightly offset from 45 degrees, except that the make points and the break points would alternate.

 

My head hurts.

 

Edited December 24, 2023 by Bloo (see edit history)

[m-mman]

2 hours ago, Johan Boltendal said:

Just a remark, if Ed or anyone else has set the distributor on a Sun tester or a like,please don't touch the distributor with a dwell meter or any other tool

Absolutely not!  I’m not interested in messing with the inside of the distributor after a knowledgeable person has set it up. 
 

My original question was about evaluating the engine with a tachometer. 
 

2 hours ago, Johan Boltendal said:

As far as the Johsons are concernd, they have a bad reputation but if in a correct factory alike condition and function, they work well

Yeah that’s pretty much any old car part.  The hurdle is getting it into that factory correct condition.  ☹️

Technical information on Johnsons is not “abundant” even about the theory of operation.  
Ed says that he has spent up to 80 hours fixing a Johnson , including machining.  That is beyond my resources.  
And when Jon doesn’t express confidence in being able to make one function I know that I am not going to tinker my way into a solution.  

[Lee H]

20 minutes ago, Bloo said:

Lee H, hanks for that! Still though, I think the points have to be staggered 45 degrees for an even fire V8. The slight offset seen on more modern dual point designs couldn't work because there are only 4 lobes. The staggering magic must be in the cam.

 

Consider this, maybe the first set of points opens to fire, but then doesn't close because of a really long lobe. After all the next opening ramp of a lobe for the first set of points is 90 degrees away. 45 degrees later, a lobe opens the second set of points and fires, because the first set of points is still open. Shortly after the second set fires, the first set of points closes, as it's cam lobe is more than 45 degrees long, but less than 90 degrees. The second set of points is still open, as it's cam lobe is also more than 45 degrees long but less than 90 degrees. Another 45 degrees later, the first set opens and fires again. Shortly after that, the second set closes, and so on. You would be effectively using one set of points to make and the other to break, just like a more modern dual point setup with 8 lobes and the points slightly offset from 45 degrees, except that the make points and the break points would alternate.

 

My head hurts.

 

With apologies to the OP for the hijack, this was an opportunity to discuss an ignition system I’ve managed to overlook all my life. This trick to increasing dwell was apparently used by Mallory for some of their hot rod distributors as well. This HAMB link is to a very informative how-to when aligning one of these on a distributor machine, around post #20.

Equally spaced lobes, but it works.

 

https://www.jalopyjournal.com/forum/threads/mallory-four-lobe-service.1085538/

 

[TerryB]

I have the instructions for the Snap-on MT418 dwell tach meter.  I think it’s just a slightly newer version of the 416 meter. Looks like the Snap-on meter would require 12 neg ground power to operate. Older Snap-on dwell meters had built in battery power to handle 12v versus 6v and positive ground versus negative ground.  Note they recommended using dwell readings on dual point systems taken during cranking, not running. Instructions attached.

 

[Bloo]

11 minutes ago, m-mman said:

My original question was about evaluating the engine with a tachometer.

I've not had much luck with that on 6 volt cars. I'm restoring a period 6V tachometer, but it isn't working yet. There are optical tachometers used to set RPM on diesel engines, and the best advice I can give is to try to borrow or rent one. Don't buy the harbor freight one. I did and it is truly worse than useless.

 

Another possibility is an Innova 3568 timing light. It works fine on 6V (unlike most similar lights), just connect polarity correctly at the battery. It uses the inductive clip for RPM, so no coil connection, and no questions about polarity or battery voltage for the RPM sensing part. I've not had much underhood time with mine yet, but RPM *seems* to be accurate. Of course I don't have anything else to compare it to.

 

[m-mman]

Photo to add to the this informative discussion.  

[m-mman]

More images. 

connected as if negative ground. 
 

wires reversed (positive ground) dwell setting. 15(?) double for a four cylinder(?) is 30 degrees for an 8 cylinder??

[m-mman]

Tachometer settings 

Slow idle 500-600 RPM shown. 

[m-mman]

If the carburetor provided a good consistent mixture I’m sure that it would be a wonderful driving experience.  

[Bloo]

13 minutes ago, m-mman said:

wires reversed (positive ground) dwell setting. 15(?) double for a four cylinder(?) is 30 degrees for an 8 cylinder??

I remain unconvinced that meter can do positive ground at all. Most 3 wire meters cannot.

 

The manual TerryB posted shows the 2 different "dwell" positions on the switch, but does not say why they exist or which one to use when. Are they the same?

 

The reading in your picture is 16 degrees on an 8 cylinder, and 32 degrees on a 4 cylinder, assuming the dwell meter is doing what it should.

 

As in my earlier post, if the meter can show 0 degrees with the points open and 45(90 4cyl) degrees with the points closed, and not pegged at either end of the meter, you can believe the 16(32 4cyl) degree reading.

 

If it does the exact opposite, and is not pegged at either end of the meter, you can read the scale backwards. That would be 34(68 4cyl) degrees in the picture.

 

If it does not go to one end of the scale with the points open, and the other end with the points closed, without pegging, then the dwell readings are completely meaningless.

 

 

[TerryB]

The two dwell settings are for operating rpm selected.  If using 1000 rpm tach range use left dwell, use right dwell for tach setting of 5000 rpm.

[m-mman]

Just to add to the discussion. 
 

Connected to positive ground. 
Point(s) closed 

 

 

 

Point(s) open 

[Bloo]

Well.. looks like it's working.

[Lee H]

Just conjecture, but I would assume that given the age of the meter, there were still enough 6v systems in the field that Snap-on probably wouldn’t sell a meter that wouldn’t work on that voltage. Also, I have to assume that the voltage used at the meter level is stepped down (<6v?) with a zener, or other voltage regulation. If the full voltage were applied, it seems it would be subject to voltage variations from the charging system and/or the ballast resistance at the coil. 

[m-mman]

3 hours ago, Bloo said:

I remain unconvinced that meter can do positive ground at all. Most 3 wire meters cannot

Oh, just discovered in rereading. 
The meter I am using is 2 wire.  One to coil and one to ground.  

[EmTee]

19 hours ago, Bloo said:

If it does not go to one end of the scale with the points open, and the other end with the points closed, without pegging, then the dwell readings are completely meaningless.

Well, maybe not completely meaningless.  I was able to 'calibrate' my 2-wire, 12V Dixco dwell meter for use on my 6V Buick by correcting the readout to account for the error in the full-scale meter reading with the points open.  I had been struggling to reconcile the 34° dwell reading with the 0.015" gap specified in the shop manual.  At 0.015" the dwell is supposed to be 30°.  Based on a different but similar forum discussion I discovered that my meter did not deflect full-scale on the 6V Buick.  Accounting for that error, it turned-out that the 34° reading actually equates to 30° when adjusting for the difference in full scale deflection.  So, everything finally makes sense and I'm able to use my dwell meter to keep tabs on the point gap.

[tom_in_nh]

On 12/24/2023 at 2:00 PM, m-mman said:

Photo to add to the this informative discussion.  

The contact point on the right looks like a mess.

I suspect it is not a genuine Delco...the rubbing block is different than the one on the left side.

I would investigate further. 

This might be a partial clue as to your rough running issue.

Good luck.

[Oldtech]

Don't know what the point gap is sposed to be but they look too wide. 

[tom_in_nh]

m-mman :

Did you ever solve your rough running issue?

Does your distributor still look like the photo above?

If so, I would be happy to look it over for you, as I am very curious as to why it looks the way it does.

Regards,

Tom

 

[m-mman]

No. It’s much more than “rough running”. This is a reproduction distributor (original pot metal distributors literally shatter)

So it is functioning properly. 
 

The car falls off when accelerating and sometimes acts like the throttle plate is stuck partially closed.  it isn’t. 
 

I know that the real problem is the darn Johnson carburetor. Barely functional when new impossible 9 decades later. 
 

I was actually looking for a tachometer reading to compare when the acceleration stopped.  
 

I have basically given up and will have to engineer and fabricate a downdraft manifold.  A few others have done that and they report good results. 

[tom_in_nh]

If that distributor photo from above is the current state of your reproduction distributor - it needs attention. 

In the photo it clearly shows the point set located on the right side to have an issue.

This could explain the loss of power - your Cadillac running on 4 cylinders instead of all 8.

If this is an older photo before repairs, of another distributor, etc. - disregard the above comments.

Just trying to help.

Tom