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ACinSC 
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 Posted: 15 Jun 2025 at 6:18am |
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Thanks for the explanation Steve B !!
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Steve in NJ
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Posted: 16 Jun 2025 at 10:40am |
No problem! I know a lot of us here are like me. Wondering what took place with stuff that doesn't work right and what the correction was. Sometimes we don't always hear the outcome of an issue. Makes ya wanna' know in case that particular issue happens to you. I've been doing this type of work for over 50 years, and every once in a while you come across something goofy and you say to yourself "what the h*ll?? Of all the Distributors I've done over the years, I've only changed a few cam assys. A couple were in Automobiles due to higher rpm ranges. Les is right. You don't need a 40K volt Coil. The standard Coils work fine unless you want to put one in. Not neccessary... BTW, I think the main reason why the one lobe wore out was lack of lubricant. The cam was dry as a bone, and the wick was there for the ride. Wasn't even touching the cam.
Steve@B&B
Edited by Steve in NJ - 16 Jun 2025 at 10:43am
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39'RC, 43'WC, 48'B, 49'G, 50'WF, 65 Big 10, 67'B-110, 75'716H, 2-620's, & a Motorhead wife
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dfwallis
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Posted: 22 Jun 2025 at 1:18pm |
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Well, there doesn't seem to be any change in behavior. The gtc505 waveforms are much more consistent and with improved characteristics, but the misfire is still there exactly the same. Timing light on cylinder 1 is very clearly missing it's going about 3 pops then misses. It could be that the voltage is a little below the timing light minimum, but I see misses on th gtc 505 also, just not as glaring as the timing light...timing light on 2 and 4 is much more consistent. Changed plug wire, new cap, bnb installed new points and condenser. I'll try a different rotor next, then see if I can scrounge up another coil. The timing mark was also bouncing around quite a lot, completely out of the viewing window.
Edited by dfwallis - 22 Jun 2025 at 2:06pm
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1952 CA13092
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dfwallis
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Posted: 22 Jun 2025 at 2:04pm |
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Changed plug wires, coil to 72 yo original, rotor to possibly original, no difference...timing light agrees with gtc505...cylinder 3 is reading 3000+ rpm at 920rpm on other cylinders...timing light cylinder 3 is flashing 3 times faster than other cylinders. Sneak path somewhere? Cylinder 1 still shows clear dropouts, but when flashing is at correct rate per 2 and 4.
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1952 CA13092
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dfwallis
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Posted: 22 Jun 2025 at 2:20pm |
OK, solved one problem, the coil ht wire is inducing voltage on the plug wires/jacket and confusing the instruments...I'll completely isolate the wires and try again 
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1952 CA13092
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steve(ill)
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Posted: 22 Jun 2025 at 3:55pm |
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thats an odd one all right... since STEVE rebuilt the distributor you got a 99.999% chance that it is NOT the problem.. That dont leave much to look at but coil , plugs, wires and the motor itself !
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Like them all, but love the "B"s.
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WF owner
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Posted: 22 Jun 2025 at 7:12pm |
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Have you tried running it in the dark? I had a WD45 that looked almost like fireworks in the dark.
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dfwallis
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Posted: 22 Jun 2025 at 8:30pm |
 WF owner wrote:
Have you tried running it in the dark? I had a WD45 that looked almost like fireworks in the dark. |
Worth a try...I ran out of gas, I'll resume tomorrow...does seem like it's arcing somewhere it shouldn't...I found the original dust cap wasn't sitting quite flat, but I think the rotor rides far above and wouldn't be rubbing on it. But I cleaned it up and will see if any effect.
Edited by dfwallis - 22 Jun 2025 at 9:39pm
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1952 CA13092
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Wispitfiremike
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Posted: 22 Jun 2025 at 8:48pm |
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Is it possible you have a bad ground somewhere? Or a wire grounding from vibration? If tests ok but when on running engine its different sure seems possible.
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Les Kerf
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Posted: 22 Jun 2025 at 8:59pm |
WF owner wrote:
Have you tried running it in the dark? I had a WD45 that looked almost like fireworks in the dark. |
Great idea! 
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HudCo
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Posted: 22 Jun 2025 at 10:37pm |
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i allways check more than one lobe with the feeler gauge , that will catch a wore lobe
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dfwallis
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Posted: 23 Jun 2025 at 11:59am |
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Interestingly, the timing light dropouts have all but stopped. Maybe the distributor cap simply was not sitting level enough. I improved the crunchy dust cap gasket. I'll look at that a bit more. It's still missing some. And every plug wire that runs close to the coil ht wire flashes the timing light much faster...I tried adding a rubber hose on the coil wire but didn't seem to dampen it much. Harder to diagnose with unreliable test equipment behavior:(
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1952 CA13092
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DonBC
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Posted: 23 Jun 2025 at 12:49pm |
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My experience with an engine that starts and runs for a few minutes and then starts to miss is a bad coil. When this happens, feel the coil and if it is very hot, it is a bad coil.
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Jack of all trades, master of none
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dfwallis
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Posted: 23 Jun 2025 at 2:38pm |
DonBC wrote:
My experience with an engine that starts and runs for a few minutes and then starts to miss is a bad coil. When this happens, feel the coil and if it is very hot, it is a bad coil. |
It's immediate, no delay. Swapped coil with no difference in behavior. I've now swapped everything but dust cap and plug wires. I did find an old make your own plug set and made a new ht coil wire. Did not produce a noticeable difference in behavior. I did try a few plug wire switches from the H, but may not have been the right cylinders. I did ohm all the wires and checked for intermittent connections but they all tested ok...not a good test for hvhf tho. Taking a break to work some other cleanup items. The plug wires are brand new, i meant i didn't get another brand new set.
Edited by dfwallis - 23 Jun 2025 at 5:51pm
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1952 CA13092
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dfwallis
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Posted: 23 Jun 2025 at 5:46pm |
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Just mowed another 2 ac field. It doesn't noticeably miss a lick when under load and at operating temp it only misses every now and then when not under load, several good seconds, roughly 1 miss. It may be a little improved from yesterday.
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1952 CA13092
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dfwallis
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Posted: 24 Jun 2025 at 7:21am |
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Inspected for arcing in the dark. Did not see anything.
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1952 CA13092
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steve(ill)
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Posted: 24 Jun 2025 at 7:58am |
it only misses every now and then when not under load
DFW... you may skip a beat when you get to be 75 years old also... ... I have seen that on more than one tractor... Have wondered if its electrical or gas flow thru carb.. Not really "common", but many have a similar effect...
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Like them all, but love the "B"s.
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Les Kerf
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Posted: 24 Jun 2025 at 1:22pm |
dfwallis wrote:
... only misses every now and then when not under load, several good seconds, roughly 1 miss... |
Just for grins, try opening up the spark plug gap a whole bunch, like maybe double the normal amount. At some point this will likely cause a miss while under load, but it just may smooth out the no-load situation.
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dfwallis
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Posted: 24 Jun 2025 at 8:30pm |
Les Kerf wrote:
dfwallis wrote:
... only misses every now and then when not under load, several good seconds, roughly 1 miss... |
Just for grins, try opening up the spark plug gap a whole bunch, like maybe double the normal amount. At some point this will likely cause a miss while under load, but it just may smooth out the no-load situation.
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I might try it. The gap recommended is from a time when octane was in the mid 70s and without alcohol...
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1952 CA13092
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Les Kerf
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Posted: Yesterday at 8:18am |
dfwallis wrote:
...
I might try it. The gap recommended is from a time when octane was in the mid 70s and without alcohol... |
These engines have a rather primitive combustion chamber; the spark plug location is way off to the side, the siamese intake ports will deliver different swirl patterns in all four cylinders, the squish patterns are haphazard, etc.
Under load, when the air-fuel charge is approaching maximum density, there will be ample mixture present inside the spark plug gap where (obviously) the flame kernel is initiated and then spreads smoothly (we hope) across the combustion chamber.
Under no-load conditions (even at high idle) the air-fuel density is significantly less which results in much less mixture being available at that spark plug hanging out way over there in the far corner of the chamber; if this mixture inside the gap happens to be too lean at the instant of sparking then a misfire is the result.
A wider spark plug gap increases the probability of having adequate fuel being present at the instant of sparking; it also increases the voltage required to initiate the spark. At low compression pressures this is not a problem, but it can be a problem at higher pressures.
At low idle speeds the compression pressures are quite low due to the throttle butterfly being nearly shut; the idle circuit in the carburetor throws in excess fuel in order to allow enough fuel for reliable ignition (idling results in very poor fuel economy).
Under load at wider throttle opening there is ample mass airflow to provide a strong vacuum signal to pull in adequate fuel through the main fuel circuit. Since you are seeing smooth running under load it tells us that you have sufficient fuel and good spark energy for the conditions.
Misfiring at no-load high idle may be a sign of inadequate fuel; the butterfly isn't shut but is open just far enough that the idle circuit is probably not delivering much (if any) fuel, yet isn't open enough to allow the mass airflow needed to pull enough fuel for steady ignition.
If a wider plug gap helps the no-load condition it may be a clue as to what is happening.
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steve(ill)
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Posted: Yesterday at 8:54am |
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that is a great explaination Les ...... I have SEEN it happen, but never thought of WHY... Engine design / combustion chamber size / air flow velocity is probably the answer.. THANKS !
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Like them all, but love the "B"s.
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dfwallis
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Posted: Yesterday at 9:47am |
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Changed to .041 and then to .055. If there was a difference in behavior it was too subtle for me to detect at either setting...although at .055, it did have very slightly less stable slow idle.
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1952 CA13092
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Les Kerf
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Posted: Yesterday at 2:35pm |
dfwallis wrote:
Changed to .041 and then to .055. If there was a difference in behavior it was too subtle for me to detect at either setting...although at .055, it did have very slightly less stable slow idle. |
I think we have beat the ignition system plumb to death  At this point I would contrive some way of hooking up a vacuum gauge to see what it says under your operating conditions.
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dfwallis
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Posted: Yesterday at 3:41pm |
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May have to wait until the next trip. I've cleaned up several items including the left side final drive oil pan and the sealant needs 72 hours to cure. I did also find a tiny leak at the carburetor main jet, but the fix is a kludge due to broken jet driver slots...don't have that small of an easy out. If the kludge helps (or even if not), I may either send the carburetor out for rebuild or break down and get the Zenith. Running out of things to change...
Edited by dfwallis - Yesterday at 3:42pm
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1952 CA13092
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Les Kerf
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Posted: 5 hours 38 minutes ago at 10:43am |
steve(ill) wrote:
...... I have SEEN it happen, but never thought of WHY... Engine design / combustion chamber size / air flow velocity is probably the answer.. THANKS ! |
If ever there were an engine design just begging for an MSD-CDI ignition it is our beloved AC's 
The triple-strike capacitor discharge can hide a multitude of sins inside that lousy combustion chamber; it is also by far the most expensive solution 
An MSD setup firing some iridium tipped spark plugs would likely last much longer than I will be able to drive the tractor.
Edit: This assumes proper carburetor function
Edited by Les Kerf - 5 hours 36 minutes ago at 10:45am
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ACinSC
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Posted: 5 hours 30 minutes ago at 10:51am |
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Les,is this the same as a pertronix ignition? Thanks
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Les Kerf
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Posted: 2 hours 59 minutes ago at 1:22pm |
ACinSC wrote:
Les,is this the same as a pertronix ignition? Thanks |
No. The Pertronix is simply a solid-state triggering device that does nothing but replace the original breaker points and condenser. They are available in both 6 Volt and 12 Volt versions, and can use the original coil, wires, and plugs (although Pertronix recommends using resistor wires/plugs).
The advantage of Pertronix is that you do not need to do periodic maintenance on the points, and performance does not deteriorate gradually like happens with points. If it fails, it tends to fail suddenly and all at once, and the points can be quickly and easily replaced if need be. Pertronix units have a pretty good reputation for reliability; I currently have one running in my Model C.
Since it uses the same coil as the original breaker points, the ignition energy is no greater, and no less than, that which is delivered by the breaker point triggering.
The MSD is a whole 'nuther animal.
It uses Capacitor Discharge Ignition (CDI) whereby a special module charges a capacitor up to maybe 400 Volts, and then discharges the capacitor through a special coil. This results in a spark that has a VERY fast rise time and is well-known for its ability to fire a fouled spark plug.
The MSD system produces three separate spark events in rapid succession, the theory being that if the first one doesn't light the fire then maybe the next two will. Better late than never. Due to the speeds involved, most MSD's only produce one spark at high RPM, say, above 4000 RPM. But then our tractors never come close that speed.
There are CDI ignitions that are not MSD and only provide one spark per event. CDI is the main reason why your modern chain saws start so very much easier than the old magneto ignition McCulloch saws I grew up with.
The MSD unit still requires something in the distributor to trigger it; this can actually be done with the original breaker points (which will now last practically forever) or I believe even a Pertronix can be used.
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dfwallis
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Posted: 2 hours 11 minutes ago at 2:10pm |
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I have taken a video of dwell time %. Essentially every large spike in the dwell time is an engine miss. I'll upload next week. My nephew left me a little more work than planned. Also had to fix the brush hog pto shaft for my brother. Running out of time for this trip.
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1952 CA13092
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Les Kerf
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Posted: 57 minutes ago at 3:24pm |
dfwallis wrote:
I have taken a video of dwell time %. Essentially every large spike in the dwell time is an engine miss. I'll upload next week. My nephew left me a little more work than planned. Also had to fix the brush hog pto shaft for my brother. Running out of time for this trip. |
I am curious to see the video. Meanwhile, can you describe the aberration? Is this 'large spike in the dwell time' being measured on the primary (low voltage) side?
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ACinSC
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Posted: 54 minutes ago at 3:27pm |
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Thanks for the explanation Les.
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