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A question of dwell

Printed From: TheAMCForum.com
Category: The Garage
Forum Name: AMC 6 Cylinder Engine Repair and Modifications
Forum Description: AMC-made I-6 engine mechanical, ignition and fuel from basic repair to high-perf modifications
URL: http://theamcforum.com/forum/forum_posts.asp?TID=99908
Printed Date: Aug/19/2019 at 1:36pm


Topic: A question of dwell
Posted By: blumontag
Subject: A question of dwell
Date 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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72 Gremlin X



Replies:
Posted By: tomj
Date 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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1961 roadster american, 195.6 OHV, T5
1968 american, 199ci, T96
sr-ix.com



Posted By: blumontag
Date 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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72 Gremlin X


Posted By: 304-dude
Date 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.


-------------
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker


Posted By: billd
Date 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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Posted By: 304-dude
Date 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.








-------------
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker


Posted By: billd
Date 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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Posted By: Trader
Date 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. 


Posted By: billd
Date 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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Posted By: tomj
Date 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.




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1961 roadster american, 195.6 OHV, T5
1968 american, 199ci, T96
sr-ix.com



Posted By: blumontag
Date Posted: May/10/2019 at 10:23am
Thanks for all the input.
I'm borrowing a friends dwell meter to see how it reads.
The points I bought at NAPA are Echlin (not cheap) and are spec for the 232 and 258.
I will check that they are aligned properly.
I also have a new set of cheaper Standard Motor Products points I could try on there.


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72 Gremlin X


Posted By: 304-dude
Date Posted: May/10/2019 at 11:37am
Originally posted by blumontag blumontag wrote:

Thanks for all the input.
I'm borrowing a friends dwell meter to see how it reads.
The points I bought at NAPA are Echlin (not cheap) and are spec for the 232 and 258.
I will check that they are aligned properly.
I also have a new set of cheaper Standard Motor Products points I could try on there.


Its a great idea, and recommended... condensers are very much rated like capacitors, not only by farads, but also by tollarances to its value. I figure out of 10 new packaged condensers, many will give off different readings on their values when measured by a capacitance meter.

Given the values from the previous reply on engine types having specific condensers, a larger value will make the points act like they are delayed on timing, and the switching current will act closed slightly longer, when the points are actually opening. A lower value will make the the points more repsonsive and will cause premature burn up of the points at high RPMs. Thus the reason for dual points on very high performace engines, that run top end 90% of the time.


Not many check capacitance on new condensers any more, but back in the day i am sure many racers have done so, and probably kept it a garage secret, with many other odd tweaks and checks before assembly.


-------------
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker


Posted By: tomj
Date Posted: May/10/2019 at 10:02pm
Originally posted by 304-dude 304-dude wrote:

Its a great idea, and recommended... condensers are very much rated like capacitors, not only by farads, but also by tollarances to its value. I figure out of 10 new packaged condensers, many will give off different readings on their values when measured by a capacitance meter.
Not many check capacitance on new condensers any more, but back in the day i am sure many racers have done so, and probably kept it a garage secret, with many other odd tweaks and checks before assembly.


good ideas all.

the capacitor needs to "match" the coil. it's a series-resonant circuit, but ideally, in the instant after the points OPEN the magnetic energy built up in the coil during dwell folds back on itself ("collapses"), and transforms the energy stored over 5 to 15 milliseconds, into a huge brief (microseconds) voltage spike -- if all is well, that energy dissipates into the spark plug gap. if it doesn't, the coil "rings" (the series-resonant thing).

there's some particular capacitance value that provides a low-impedance path for the coil primary, but doesn't induce too much ringing after the plug dissipates the coil energy. there's probably a world of theory and practice tweaking the cap value for some characteristic or other. i bet there's a bunch of geezer racers taking that knowledge with them.

any idea what the criteria is?



-------------
1961 roadster american, 195.6 OHV, T5
1968 american, 199ci, T96
sr-ix.com



Posted By: 304-dude
Date Posted: May/11/2019 at 6:03am
Originally posted by tomj tomj wrote:

Originally posted by 304-dude 304-dude wrote:

Its a great idea, and recommended... condensers are very much rated like capacitors, not only by farads, but also by tollarances to its value. I figure out of 10 new packaged condensers, many will give off different readings on their values when measured by a capacitance meter.
Not many check capacitance on new condensers any more, but back in the day i am sure many racers have done so, and probably kept it a garage secret, with many other odd tweaks and checks before assembly.


good ideas all.

the capacitor needs to "match" the coil. it's a series-resonant circuit, but ideally, in the instant after the points OPEN the magnetic energy built up in the coil during dwell folds back on itself ("collapses"), and transforms the energy stored over 5 to 15 milliseconds, into a huge brief (microseconds) voltage spike -- if all is well, that energy dissipates into the spark plug gap. if it doesn't, the coil "rings" (the series-resonant thing).

there's some particular capacitance value that provides a low-impedance path for the coil primary, but doesn't induce too much ringing after the plug dissipates the coil energy. there's probably a world of theory and practice tweaking the cap value for some characteristic or other. i bet there's a bunch of geezer racers taking that knowledge with them.

any idea what the criteria is?



Gosh, i gave up on the matching coil and condenser idea long ago when I made HEI my choice.

NOTE: To buy 20 oem condensers to match to your coil of choice would be like finding a needle in a haystack. So, will explain the procedure in broadening the variances by being selective enough to make a cost effective and highly effective option for matching.

Reason it is not so critical in grabbing two dozen of one type to find a few that will match, is that even with a standardized tollarance set in condenser values, you may only see 1 to 2uf variance with a budget handheld meter. Also some internal characteristics in the way they are manufactured, can allow for properties that effect high rpm use. As when testing, the data gathered will make sense as to how frequency effects dwell over points adjustment. Usually the optimum setting for daily driving will not be the optimum for performance. Since rpm ranges will increase beyond what is set for oem operation.

Part two of the equation is resistance... some coils require a resistor and most using points will need one inline with tye coil at tye battery side. In a recent AMCENTHUSIAST'S post on finding a resistor for his dual points and coil setup, he had to hand select a proper resistor, to make street driving better. Note, street driving RPM was not the goal at first when tuning the ignition.

Given there are a dozen after market coils and hundereds of OEM coils to pick from.

My option to make a static control, is to pre-setup before testing live.

You will need a 12v bench test with a small automotive or flashlight light bulb as a replacement for coil, and electric motor drive set at 1000 to 1500 rpm. This will assist in obtaining static dwell around factory spec adjustment of around .016" gap. Verify with billd's measuring guide, that was posted a wee earlier, as a reply. Dial in with the best value on the meter. You may find that you may not get optimum dwell value, even if you have moved points gap up and down the adjustment scale. It is critical the points adjustment are left are static (untouched), by not using kit with integral condenser, as each change requires removal of the points.

As for your small army of condensers... for the fun of it, you can go with a few oem replacements for your engine and mark them at the bottom flat, as you want to keep the body clean from anything that may limit electrical contact. To add with the fun, obtain some big three factory labeled variants from similar engines of known high performance builds.

Install distributor with your choice of OEM condenser made for your engine, back on your engine. You will want to properly time the engine, before testing. Once everything is set. You will need to collect dwell measurment data from idle, 1500, 3000, 5000, and 7000 for your performance engine. Dwell will vary as RPM increases. You may also note that low rpm dwell also changed from being on the bench from being on the engine.

Do the same test for each of the condensers, and validate which one performs best at keeping dwell within the RPM range intended.

If any further adjustment is required, measure the resistance wire or resistor to the coil. Make note, and if you need to bring more dwell into the high rpm operation a higher resistance will need to be changed out. If you get too much dwell in high rpm range, a smaller resistance is needed.

Here is the link to AMCenthusiast's coil and resistance findings, to add to this information and topic. You will need to select privious pages to get the full details, as the link opens to his reply about his coil and resistance choice.
http://www.theamcforum.com/forum/xrv8-gremlin_topic76151_page47.html" rel="nofollow - http://www.theamcforum.com/forum/xrv8-gremlin_topic76151_page47.html

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71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker


Posted By: amcenthusiast
Date Posted: May/11/2019 at 11:41am
Your dwell should be just about 'perfect' if you set the gap to specifications.

My Chilton's Repair Manual says your 6cyl point gap should be .016" and your dwell reading should be 33 (degrees)

My '71 AMC Technical Service Manual tells about installing a new set of points using a dwell meter method only... as follows:

" ...Turn car ignition 'on' and crank engine with remote control starter switch while observing the dwell meter reading. Adjust dwell angle by inserting a screwdriver blade in the adjusting slot of the breaker plate and moving the plate until the specified setting is indicated on the the dwell meter (33 degrees) Tighten retaining screw and recheck dwell angle. Install distributor rotor and cap, verify dwell angle with the engine running."

***NOTE: this is obviously done with the distributor cap and rotor taken off (so the car can't start) -so you can turn the engine over and adjust the points with the distributor spinning, while watching the dwell meter... 

----------------------------

If point gap or dwell meter reading is not close to specifications, this very likely indicates some type of mechanical error... could be wrong set of points and/or like points aren't sitting flat on the breaker plate, something bent ...only guessing now ...but there are always a thousand ways something can go wrong & this is called 'Murphy's Law'!

IMO, it is very-very doubtful the metal lobes on the distributor shaft mechanism are worn; the plastic rubbing block on the breaker point is made to wear down first.

----------------------------

Just keep trying.

It's not hard 'to master'; installing a new set of points.

We all have our 'good days' and 'bad days'.

Some days you have to try ten times harder, just to do 'something simple' on any other day.




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Link to XRV8 Race Parts website: www.amcramblermarlin.1colony.com


Posted By: blumontag
Date Posted: May/11/2019 at 3:43pm
Will check new points for straightness or other install error on my part tomorrow. Raining right now. We took the Gremlin out for a local car show earlier. "You don't see these things anymore, do ya?!!"

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72 Gremlin X


Posted By: farna
Date Posted: May/13/2019 at 6:25am
If it's a driver get a Pertronix and be done with all this. I'd only run points in a 100% restored car. Yes, points work just fine, and if you really like tuning every 3-6 months or just like them because they are a little anachronistic and you just like things like that, nothing wrong with it at all. All I can say is that if electronic ignitions were relatively cheaply available back then car makers would have used them. Higher performance and emissions (mostly emissions!) is why the bigger makers developed their own electronic ignitions in the 70s.

I always suggest updating a driver... really no point in not doing it (unintended pun there....). Better, more consistent spark = better efficiency and power, though you may not notice much of each if you keep the car well tuned.  The main thing you will notice is you don't have to get under the hood and tune as much as with points, and only have to replace the cap and rotor on occasion, unless something goes wrong. And of course spark plugs... though they will last longer due to the better spark.

A Pertronix can fail, nothing lasts forever or is failproof. Keep a set of points, a screwdriver, and a matchbook in the glove box in case the Pertronix does fail... though it's not likely. The only drawback is there is one extra wire that needs to be run to power the Pertronix unit. Use a black wire and it will only be noticeable on the closest inspection.


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Frank Swygert


Posted By: blumontag
Date Posted: May/13/2019 at 8:12am
This is a photo of the Pertronix 1162A, which I have read is recommended for the 258 in our Gremlin.
I printed out the install instructions. Second photo is my current coil setup. Would I just hook up the 2 Pertronix wires to the coil (leaving the wires currently attached...the brown and green), or is there some other wire involved? Sorry, but I don't have a strong grip on electronic stuff in cars! 







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72 Gremlin X


Posted By: tomj
Date Posted: May/13/2019 at 10:50pm
a good power transistor suitable for reliable ignition coil service, the transistor alone cost a modest dinner in 1970 dollars. that's why the good stuff (MSD, etc) was expensive and the cheap stuff was unreliable (Prest-No-Lite). today you can't even buy a transistor that crappy. for five bucks i'm buying an 80-ampere "smart" high side drivers that has built-in overload detection, overheat detection, load monitoring, you name it and acts like an "ideal" part. that was 2016. now i'm looking at one that has four drivers in one package, MORE monitoring and processing guts, and less money. and smaller. that's two years of change.

the revolution in electronics is hard to grasp. every field that electronics has touched has advanced more in the last 30 years than it did in the last 300 years, without exaggeration (eg. navigation, communications, ...)

here's a 1962 transistor price list. 2N174 and 2N176 were used in car radios (final audio). awful things, but so much better than tubes. https://worldpowersystems.com/ARCHIVE/solid-state-datasheets/Motorola/misc/images/price-1.jpg" rel="nofollow - https://worldpowersystems.com/ARCHIVE/solid-state-datasheets/Motorola/misc/images/price-1.jpg

a phone today has 2 billion transistors.



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1961 roadster american, 195.6 OHV, T5
1968 american, 199ci, T96
sr-ix.com



Posted By: blumontag
Date Posted: May/14/2019 at 8:30am
Borrowed a dwell/tach meter from a friend. It read 33, while mine read 28 (at about .015 gap). I reset gap to .016 and now have 31 dwell.  Bottom line , I believe my ancient dwell meter (a Craigslist buy) is off. Thanks for all the input...and I'm looking hard at the Pertronix as an option.

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72 Gremlin X


Posted By: farna
Date Posted: May/15/2019 at 6:37am
Did you not get the instructions with the Pertronix? The red power wire should go to a switched power wire. You can attach the red wire to the positive side of the coil, black to negative. If you have a ballast resistor (most sixes don't) attach  the red wire to the end of the resistor that comes from the wiring harness, not the coil end.

70s cars usually have a resistance wire instead of a ballast resistor. In that case you need to find a wire that supplies 12V with the switch in the crank and the run positions, or find the resistance wire in the harness and splice in front of the wire.

http://support.pertronix.com/file.php?key=6SwQxQSwbFwXJsbp7O1jtCXnJ_DI1rdI&expires=1558051200&signature=3c9321290c0442ce9d49c41cce77ba7933e64812" rel="nofollow - http://support.pertronix.com/file.php?key=6SwQxQSwbFwXJsbp7O1jtCXnJ_DI1rdI&expires=1558051200&signature=3c9321290c0442ce9d49c41cce77ba7933e64812


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Frank Swygert


Posted By: blumontag
Date Posted: May/15/2019 at 7:44am
I didn't buy the Pertronix. See my previous post.

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72 Gremlin X



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