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Les Kerf 
Orange Level
Joined: 08 May 2020
Location: Idaho
Points: 1202
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 Posted: 02 Jul 2025 at 3:47pm |
 dfwallis wrote:
...The delco rotor has a very long curved section trailing the front edge. From the spark wear, it looks like it has been firing very close to the leading edge where I would expect it to be. I have considered sacrificing one of the caps to instrument it... |
It's a long shot for sure, but there is a slim possibility that there was a manufacturing defect affecting the rotor phasing that has plagued this since time immemorial.
dfwallis wrote:
... The caliper shows it now spaces 5/16 inch consistently all around but I don't know what it should be. The dust cap does behave as a spacer... |
Is it practicable to remove the dust cap as a test to minimize the rotor/terminal gap? Do you know how large that gap actually is?
dfwallis wrote:
Yes, it is difficult to tell if the signal dropouts are just because it lost sync or if the signal really went away. There is no trigger level adjustment. |
This is the major flaw I see with this gadget
dfwallis wrote:
However, the timing light appears to miss signals in the same pattern as the GTC505 (when it shows one miss, the timing light shows one miss, two, timing light shows two misses (roughly)). The RPM view with pulse count view shows this correlation best. The long duration voltage recording charts are difficult to interpret without a timescale and knowing where one sequence begins and ends. The instantaneous voltage chart shows the signal missing periodically, but again, it's hard to tell if it's just a sync issue. |
The high level of noise from the non-resistor wires/plugs seems to give this gadget fits
dfwallis wrote:
I'm hoping that the new scope will allow me to trigger on the points signal and more unambiguously show the spark pulse in relation to it. |
I am most curious to see what develops, please keep us posted 
dfwallis wrote:
I'm trying to work up the strength to recheck the valves next month. But if it's 100+f again in the barn, it'll be mighty difficult to make myself do it...( |
It's been mid-90's here in northern Idaho and less humidity than your area, and this old hillbilly pretty much shuts down above 80 degrees nowadays 
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Straanger
Bronze Level
Joined: 10 Dec 2019
Location: Michigan
Points: 56
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Posted: 03 Jul 2025 at 1:48pm |
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I’m more of a shade tree mechanic, so I’d be looking for a big shade tree with a cool breeze, and take a tool box with the tractor out there. I’m to lazy for that kind of heat.
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dfwallis
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Joined: 09 Mar 2023
Location: DFW
Points: 751
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Posted: 03 Jul 2025 at 3:15pm |
Straanger wrote:
I’m more of a shade tree mechanic, so I’d be looking for a big shade tree with a cool breeze, and take a tool box with the tractor out there. I’m to lazy for that kind of heat. |
I live in Texas. I'm mostly used to heat, but not usually that much humidity. Was more Houstonesque than Dallas. It's regularly > 105 in my TX workshop. I just keep a fan on me, drink water, go inside once in a while.
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1952 CA13092
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dfwallis
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Joined: 09 Mar 2023
Location: DFW
Points: 751
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Posted: 08 Jul 2025 at 6:31pm |
Brother spoke with the guy that rebuilt the head. He bought a complete valve train kit for it. He checked all valves, seats, and guides and replaced anything that was not to spec. It was rebuilt several times before, so not everything was 72 years old. Does not mean that something wasn't missed, but if there's anything wrong in the valve train, it would most likely be that 0.012 was not maintained. The misfiring symptoms were immediate after the rebuild. It doesn't seem to have changed since then (and of course occurred before the rebuild). He also milled the head, cleaned and polished cam and installed new cam bearings, cleaned the block, ground crankshaft to spec (took several attempts to get the end play acceptable).
I went ahead and bought a new knockoff zenith carburetor. That should help me quickly eliminate that as an issue. It didn't come with the adapter for the breather so I'll have to make one if I leave it installed :( That will leave me time to decide whether to follow up with a repair or replacement TSX 486 or attempt to locate a 670, 701, or 815. They all seem to be scarce as hens teeth.
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1952 CA13092
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Les Kerf
Orange Level
Joined: 08 May 2020
Location: Idaho
Points: 1202
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Posted: 08 Jul 2025 at 8:28pm |
dfwallis wrote:
... if there's anything wrong in the valve train, it would most likely be that 0.012 was not maintained. The misfiring symptoms were immediate after the rebuild. It doesn't seem to have changed since then (and of course occurred before the rebuild). He also milled the head, cleaned and polished cam and installed new cam bearings, cleaned the block, ground crankshaft to spec (took several attempts to get the end play acceptable)... |
We have all been doing a heap of guessing and spinning our wheels  There is something fundamentally wrong here, so it's time to get back to the basics.
If that were my engine, I would pull the valve cover, re-torque the head, and re-set the valves.
Then, I would verify TDC by actually measuring it (not by merely assuming the marks to be correct).
Next, with the valve cover still off, bring #1 to TDC on compression. Now observe the valves on #4, which will be at the tail end of the exhaust stroke and the beginning of the intake stroke. Both valves should be open the same amount at this point or at least almighty close to the same amount. If not, then rock the engine back and forth past TDC until they are open the same amount, and then decide if it is off far enough to matter, and why (or why not).
Now check #1 the same way with #4 at TDC compression. If both #1 and #4 check good you can probably skip #2 and #3, but I would still put my dial indicator on the rocker arms and check the lift on all eight cam lobes.
I have actually found incorrectly assembled engines that refused to behave using the above technique; it is also possible to have incorrectly manufactured components.
This may seem like a lot of fiddling, but this is a long-standing problem with with this engine that dates back prior to the overhaul; something is clearly amiss, and none of the usual remedies has cured it.
Have fun 
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dfwallis
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Joined: 09 Mar 2023
Location: DFW
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Posted: 08 Jul 2025 at 8:35pm |
Les Kerf wrote:
dfwallis wrote:
... if there's anything wrong in the valve train, it would most likely be that 0.012 was not maintained. The misfiring symptoms were immediate after the rebuild. It doesn't seem to have changed since then (and of course occurred before the rebuild). He also milled the head, cleaned and polished cam and installed new cam bearings, cleaned the block, ground crankshaft to spec (took several attempts to get the end play acceptable)... |
We have all been doing a heap of guessing and spinning our wheels There is something fundamentally wrong here, so it's time to get back to the basics.
If that were my engine, I would pull the valve cover, re-torque the head, and re-set the valves.
Then, I would verify TDC by actually measuring it (not by merely assuming the marks to be correct).
Next, with the valve cover still off, bring #1 to TDC on compression. Now observe the valves on #4, which will be at the tail end of the exhaust stroke and the beginning of the intake stroke. Both valves should be open the same amount at this point or at least almighty close to the same amount. If not, then rock the engine back and forth past TDC until they are open the same amount, and then decide if it is off far enough to matter, and why (or why not).
Now check #1 the same way with #4 at TDC compression. If both #1 and #4 check good you can probably skip #2 and #3, but I would still put my dial indicator on the rocker arms and check the lift on all eight cam lobes.
I have actually found incorrectly assembled engines that refused to behave using the above technique; it is also possible to have incorrectly manufactured components.
This may seem like a lot of fiddling, but this is a long-standing problem with with this engine that dates back prior to the overhaul; something is clearly amiss, and none of the usual remedies has cured it.
Have fun
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On the list. We did verify TDC matched valve position/state (I/E), piston at top, and rotor pointing in the correct direction. Doesn't hurt to look again.
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1952 CA13092
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Les Kerf
Orange Level
Joined: 08 May 2020
Location: Idaho
Points: 1202
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Posted: 08 Jul 2025 at 9:53pm |
dfwallis wrote:
...On the list. We did verify TDC matched valve position/state (I/E), piston at top, and rotor pointing in the correct direction. Doesn't hurt to look again. |
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dfwallis
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Joined: 09 Mar 2023
Location: DFW
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Posted: 09 Jul 2025 at 3:56pm |
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The parts manual seems to show two NPT ports on the manifold. It shows 70912725 1/2 NPT at the left/front flat of the intake manifold and it shows 70901651 1/8 NPT at the flat just above the carburetor. I purchased a vacuum gauge. If I decide to modify the manifold, do I need both ports for a complete test?
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1952 CA13092
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Les Kerf
Orange Level
Joined: 08 May 2020
Location: Idaho
Points: 1202
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Posted: 09 Jul 2025 at 4:22pm |
dfwallis wrote:
The parts manual seems to show two NPT ports on the manifold. It shows 70912725 1/2 NPT at the left/front flat of the intake manifold and it shows 70901651 1/8 NPT at the flat just above the carburetor. I purchased a vacuum gauge. If I decide to modify the manifold, do I need both ports for a complete test? |
No. A 1/8" NPT port at the flat above the carburetor is perfect. Bigger just means more drilling than necessary (unless you plan to run a milking machine like we do on occasion).
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dfwallis
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Joined: 09 Mar 2023
Location: DFW
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Posted: 3 hours 16 minutes ago at 11:26am |
The GTC-505 vendor has agreed there could be a trigger level issue and has provided a firmware update that adjusts the algorithm for distributor systems. If the voltage is held high too long (e.g. coil hysteresis), the trigger may not reset properly ready for the next cycle.
They are also working on an update to facilitate trigger level adjustment and will provide that update when ready. It isn't clear whether that will allow manual adjustment or whether it's just an update to the automatic internal adjustment process.
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1952 CA13092
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