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A question of dwell

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blumontag View Drop Down
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    Posted: May/07/2019 at 9:49pm
Hi-
 Just did points, condenser, cap and rotor on our 72 Gremlin with mildly cammed 258 and automatic transmission. Did spark plugs last year. Points Gap set at .016 per manual. My dwell reads 24, manual says should be 31. I closed points further to about .014 and now get dwell reading of about 27. Should I keep closing points until I actually achieve 31...or could my dwell meter be off? Or am I missing someting?  Engine runs fine. Thanks!
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Post Options Post Options   Thanks (2) Thanks(2)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: May/07/2019 at 10:02pm
dwell over gap. "dwell" (the ratio of points closed, to points open) determines how much energy is in the coil. gap settings are seat of the pants value to get things running.

in ye olden dayes, before dwell meters and when engines didn't turn fast, and no one really cared about state of tune, gap was deemed "good enough". however you really want maximum spark energy; set by dwell.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote blumontag Quote  Post ReplyReply Direct Link To This Post Posted: May/09/2019 at 12:36pm
Any reason to be alarmed that the points gap will be much smaller than spec?  The distributor lobes don't appear to be overly worn.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: May/09/2019 at 1:08pm
Originally posted by blumontag blumontag wrote:

Any reason to be alarmed that the points gap will be much smaller than spec?  The distributor lobes don't appear to be overly worn.


Some have reported new points and condensors have plagued tune ups.

A failing or poorly made condensor will also effect getting a proper dwell. But mostly the points need to be properly set to obtain dwell.

If .016 is far from obtainting dwell, then its possible your lobes have worn. The main issue that causes premature lobe wear, is lack of lube, and constantly reusing old points after the lube has dried or worn off. Points are cheap, especially back in the day, and shouldn't be used no more than two plug replacements. Older style plugs wont hold up as well as the new designs made today. Though I have had seen old engines run on the same plugs for ages.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: May/09/2019 at 3:27pm
So explain how a failing condenser affects dwell.  It prevents the collapsing secondary field from re-introducing a higher voltage in the primary which is capable of jumping the gap. 
Speaking of gaps - points that don't open far enough won't last as long.
Dwell is the number of degrees of distributor rotation that the points are closed. It's not related to condenser action.

Frankly, I'd not close the gap any more than .014

>>If .016 is far from obtainting dwell, then its possible your lobes have worn.  <<

Uh, no. Worn lobes INCREASE dwell and RETARD timing. That's because worn lobes cause points to open later, not open as far, and close sooner - it's like worn CAMSHAFT LOBES - you LOSE lift, you do not gain it. So you lose point opening or "lift" and this increases dwell.
So if his distributor cam lobes are worn he'd have trouble getting the dwell down - he'd have trouble reducing it and he'd have to open the points more because the tops would be off the lobes, causing the points to not open far enough.

Since dwell increases with wear and time, I always have set it on the low side of spec knowing it will increase as the point rubbing block wears and the distributor cam lobes wear. Given that a V8 has a spec typically hovering around 28-32 degrees, I typically set about 29-30.
After several thousand miles it will get up to 31+ degrees. 
I do similar for the 6's - I set dwell - if I even use a meter, often I don't - to a bit over the minimum knowing it will increase with wear on the rubbing block.

Now one thing bothers me - your dwell should be higher than typical if your distributor cam is worn at all - not lower - so I actually question the accuracy of your METER. Did you zero it? Check it against another. I've seen 'em off in the past. 
I'd not close the points any more....... I'd not go lower than .014

Here's a spec for a Chevrolet 6 cyl engine - and there's no difference in the distributor settings - 
 Point gap:  New .019, used .016
Don't go lower than .014 - and frankly if it was any of my cars, I'd go by gap over dwell  - and there's a story behind that, too.......


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Post Options Post Options   Thanks (1) Thanks(1)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: May/09/2019 at 5:00pm
Billd, to answer the reasoning on condeser effecting dwell, I did not mean physical dwell, but electrical properites of dwell... depending on the type of equipment used, the theory of RC time constant, being R is the resistance in the curcuit, and C being capacitance, a failing condensor or poorly made one, can effect how dwells effect on coil saturation in producing optimal spark. Usually a condenser fails hard and spark is not good enough to create a contious cycle of ignition.

If the capacitance is not stable with the condenser, it can move dwell below or above the range set by physical measurements, by delaying (by storing charge), or in shortening (internal resistance becomes lower) the electrical action of the points closure for the coil to saturate. Which may effect some meauring devices on reading dwell.






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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: May/09/2019 at 6:31pm
A dwell meter measures "duty cycle" in a way. It is taking a percentage of the time the points are close (zero resistance - full scale) to open (full resistance, infinity)
This means that the dwell meter is just a dedicated version of a VOM.

If you have a multi-meter, either analog or digital, you can measure dwell pretty accurately. (the cap has nothing to do with this - that's over-thinking it and missing the real reasons that beasty is in there (which is actually sort of a dual purpose I could point out if you were standing with me next to a Sun scope or even my Sears scope))
Points are mechanically designed to be closed 2/3s of the time, and 
open 1/3 of the time. On a 360 degree distributor rotation, each of 8 cylinders get 1/8 
or 45 degrees. 2/3 of 45 degrees is 30 degrees. On a 6 cylinder its 60 and 40 degrees. 

If you disconnect the coil & cap from the points, and connect an analog ohm meter, it 
will read full scale (ANY scale) when the points are closed and back to the beginning when 
they are open. So when spinning the analog ohm meter should read 2/3 of full scale for 
any even fire engine. 

Another way, put a voltmeter across the operating points. Note the voltage with points 
open, you should see 1/3 of this when the engine spins. There is some error caused by 
coil interaction and voltage change from starter loading. Digital volt meters do not work 
well trying to average voltage. But a digital meter with DUTY CYCLE capability should be 
able to show the 1/3 or 2/3 (33% or 67%) figures

OR

To use a Fluke 88V to measure dwell, first measure percent duty cycle (%DC) and convert to dwell using this formula:
Dwell = (360 divided by # of cylinders) x (%DC divided by 100%)​

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: May/09/2019 at 10:14pm
Just to add a little to this.
If you cannot get the gap and dwell close, you likely have the wrong points if the distributor looks good. Just return them and try a set for the 232. The manufacturers just don't know what they are selling as far as I can tell - one size fits all????
On the condenser, measure it. Most places now no nothing about points ignition systems and will sell you a condenser for a lawn mower!
For an AMC I6 you should be looking at 21uF to 24uF. If you get a 28uF (V8, I8 condenser in"most" cases) you will be burning points, and 18uF (lawn mower) will foul points. 32uF is likely for a 6V volt system. 


Edited by Trader - May/09/2019 at 10:16pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: May/09/2019 at 10:45pm
If you go to a real parts store they can still look them up by application and get the right ones.......
Besides, having been in the small engine repair business for a few years and setting up same in an ACE hardware store, being certified with Tecumseh/Power Products, Kohler, Jacobsen, B&S and others, I can tell you most lawn more capacitors won't fit....... physically.  I still have dozens of NOS parts from those days in my garage loft. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: May/09/2019 at 11:08pm
billd's description of events and effects is spot-on, his theory and practice are perfectly correct.

when the points are closed, (they "dwell" in the closed position) the capacitor is shorted by the points so the cap have zero effect on coil charge. when the points open, the cap absorbs the primary side inductive kick, current then flows through cap and coil for a few milliseconds, which makes the big magnetic field, etc.

how are you measuring points gap? if the points have been run for any time, the two faces are no longer flat, as metal (often, usually) transports from one electrode to the other, so there's a bump and a pit, and a flat feeler gauge can't (accurately) be used. you can knock the bump off, but better to use a wire feeler not a flat one.

also, the points contacts may not be perfectly parallel. they should be trued before installation. if you're over 30, with a magnifying glass!

last, the quality of points in the last 20 years is total crap. junk. i doubt if anyone makes good ones at all. i've never seen 'em. long gone are the annoying but reliable Keps nut, replaced with a poo-quality slide-in "friction" ... thing that uses only the tension on the tail end of the points spring.

<oldphartmode>WHEN I WAS A KID WE BROUGHT POINTS MADE LIKE THAT BACK AND ASKED FOR REAL ONES</oldphartmode>

seriously.


points ignition is simple, but very subtle.  in the old days the quality of engine state of tune was on average very poor, it's easy to forget what it was like then. parts quality today is crap. combined those and it's a bad deal. you pretty much have to learn enough electrical physics to make this stuff work in spite of itself. this is part of why i think it's easier and more reliable to use Pertonix or something.




Edited by tomj - May/09/2019 at 11:12pm
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