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Les Kerf 
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Location: Idaho
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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
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Location: Michigan
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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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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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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
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Location: Idaho
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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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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
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Location: Idaho
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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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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
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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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Posted: 15 Jul 2025 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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Les Kerf
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Posted: 15 Jul 2025 at 4:59pm |
Hope it helps
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dfwallis
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Posted: 15 Jul 2025 at 5:50pm |
Les Kerf wrote:
Hope it helps
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Hope I can figure out how to install the update 
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1952 CA13092
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dfwallis
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Posted: 19 Jul 2025 at 6:46pm |
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New Zenith carburetor...no more misfires...not adjusted right yet, dies at high throttle...these Zeniths based on 12566 and 12522 are not acceptable substitutes for a tsx486...many mods needed. Throttle link rubs the block unless you saw off the throttle flap through bolt. Would be better if I took off some of the lock nut holding the lever arm. Had to borrow some parts from the tsx to make it fit correctly. Boy, that's a load off.
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1952 CA13092
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dfwallis
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Posted: 19 Jul 2025 at 7:48pm |
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Does not seem to be any carb setting that allows it to run at high rpm without very slight choke. Has both adj screws. Vacuum shows steady 28 cm hg vac
Edited by dfwallis - 19 Jul 2025 at 7:51pm
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1952 CA13092
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Les Kerf
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Posted: 19 Jul 2025 at 8:06pm |
dfwallis wrote:
... Vacuum shows steady 28 cm hg vac |
At what RPM? That's pretty low if it's at idle.
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dfwallis
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Posted: 19 Jul 2025 at 8:16pm |
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All rpms, changes very little
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1952 CA13092
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steve(ill)
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Posted: 19 Jul 2025 at 8:21pm |
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I bought an "aftermarket" carb for a B that did the same think... FInally decided is was missing a little FUEL at high RPM... i had about 10% choke, so i pulled the carb, pulled the main jet and made the main jet .003 inch bigger diameter and that FIXED the lack of fuel problem.
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Like them all, but love the "B"s.
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Les Kerf
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Posted: 19 Jul 2025 at 8:48pm |
dfwallis wrote:
All rpms, changes very little |
Hmmm... assuming your elevation is under 1000 feet above sea level, that engine should pull 40-50 cm hg (18-20 inches) at low idle, you are at about 11 in hg, not enough to run a milking machine (usually about 14-15 inches hg).
If the throttle butterfly is open too far then the idle circuit is ineffective and you are then trying to run it on the main circuit, which also appears to be wacky
Steve's suggestion makes good sense.
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dfwallis
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Posted: 19 Jul 2025 at 9:42pm |
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Yes, the meter itself shows a green good range as you suggest. I agree it is starving for fuel. I will consider a tweak to the jet.
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1952 CA13092
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dfwallis
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Posted: 20 Jul 2025 at 9:43am |
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I updated the gtc505 firmware...surprise surprise...the electrical misfires are gone. They've updated the cycle menu with a distributor specific selection and algorithm...if you have one, you'll probably want the 211c or later update. It could be costly otherwise:(
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1952 CA13092
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dfwallis
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Posted: 20 Jul 2025 at 10:28am |
And in another laughable (or angering) episode...my cheap Chinese vac gauge was not calibrated. I had to open it up and manually adjust it to zero. Now the vac pressure reads in the normal range at about 45 in hg. 
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1952 CA13092
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dfwallis
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Posted: 20 Jul 2025 at 10:40am |
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My brother will bring his high $ gauge out and recheck it
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1952 CA13092
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dfwallis
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Posted: 20 Jul 2025 at 11:42am |
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Question...the Zenith main jet is not the same size all the way through...the very tip is much smaller. Either it is stepped or they didn't bore all the way through...docs online seems to suggest Zenith does not use stepped or tapered jets ?? 12566
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1952 CA13092
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dfwallis
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Posted: 20 Jul 2025 at 11:47am |
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Much smaller than the tsx486 main jet also, all the way through...not that they're necessarily comparable
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1952 CA13092
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dfwallis
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Posted: 20 Jul 2025 at 12:12pm |
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Zenith: .0640 > .0600
TSX new main: .0775
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1952 CA13092
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steve(ill)
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Posted: 20 Jul 2025 at 1:03pm |
I bought a tsx428 and it was .041 inch... and i made it .043 .... ......... the 428 was used on a 120 CID FORD tractor..
remember on the jet size your interested in the AREA of the hole... The DIAMETER might change .002-3 for a 5% difference, but the AREA gets 10% bigger since you Square the RADIUS when you figure the AREA..... dont go too large, to fast.
Edited by steve(ill) - 20 Jul 2025 at 1:14pm
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Like them all, but love the "B"s.
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dfwallis
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Posted: 20 Jul 2025 at 1:21pm |
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By accident I found a drill bit .0630, about to try it
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1952 CA13092
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dfwallis
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Posted: 20 Jul 2025 at 2:12pm |
No difference...idles perfectly, any attempt to go beyond mid throttle dies without choke regardless of adjustment 
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1952 CA13092
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steve(ill)
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Posted: 20 Jul 2025 at 3:13pm |
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if the choke keeps it running... that means your running out of gas.. Could be the main jet or could be the venturi in the throat ?
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Like them all, but love the "B"s.
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dfwallis
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Posted: 20 Jul 2025 at 3:17pm |
it's brand new...shouldn't have that sort of problem
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1952 CA13092
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