dfwallis wrote: Analyzing spark via a gtc505 analyzer.

I am not familiar with that analyzer, is it an actual oscilloscope?

dfwallis wrote: All 4 cylinders look horrible.

Please describe "horrible". Is this at idle?

dfwallis wrote: Cylinder 1 and 4 have the best signals but 2 and 3 are spiking randomly. All have a peak voltage of around 6kv, but 2 and 3 have extra random spikes to 9kv.

Do you mean that there are additional spikes other than the one, correct voltage spike?

When the points open, the electro-magnetic field in the coil collapses; the resulting 'flyback effect' induces a voltage in the secondary windings of the coil. This voltage increases until the spark is able to jump the gap. It is not unusual to have variations in the firing voltage as an engine runs; I would not be alarmed seeing 6-9kV ranges.

dfwallis wrote: ... I think to get higher spikes, the dwell time must randomly increase?? ...

No.

The required firing voltage was higher on that particular event. In order to fire at all, the dwell had to be at least 'adequate'.

The first two things I would want to know is what each cylinder has for compression, and next would be to hook up a vacuum gauge and see what that looks like.

Also, do you have any way to take analyzer readings under load? That would be most interesting.

dfwallis wrote: Stanr...Those are all new...completeish rebuild...from the waveforms, I think the coil is fine. I think more likely erratic points...the analyzer also gets confused with lots of extra pulses, declaring rpm 5000 instead of 1040 momentarily...cylinder 2 and 3 mostly.

You posted this while I was composing my previous answer.

Your points might be bouncing around, also poor grounding can give fits. The breaker plate needs to be well-grounded to the outside.

dfwallis wrote: ...   Does not seem to be any side to side distributor play, but there is up and down (in/out) play...

I've not scoped the signal but believe I'm having a similar issue on the redneck B & (timing "fluttering" all over the place due to slack in it's ignition timing chain setup, needs a new bit of chain. So could be caused by the lash in your distributor in your case

Also, if the condenser is new new & not NOS then perhaps switch back to the previous one/a known good one. Modern parts are/were trash last I checked.

Would certainly attend to the distributor on your tractor, especially as Jay has experience of this being a cause.

I just hope it's fixed.  It's hard for me to imagine a CAM wear pattern happening that would give those results, unless it was pretty severely lop-sided.  I polished up the CAM to try to improve it (pitting), but I did not check for roundness.  When I set the points, I just picked one lobe and did not check if it had the same gap on other lobes.  I'll try to be more thorough if there's a next time.  

dfwallis wrote: ....  It's hard for me to imagine a CAM wear pattern happening that would give those results...

For those who were curious about this particular Distributor, here's the poop. It's very rare I change a cam. When I first ran that Distributor out of the box, the dwell was all over the place. First thing you think of is main shaft bushing. Not this time. Main shaft spun nice n' true. Up n' down, side to side all in spec. Looked good in the 504 too. Point Contacts were worn funny with a matching rub block which led me to a possible cam problem. Inspected the cam and one lobe looked almost round toward the middle. Installed a new NOS cam, ran it on the 504 again (same points) and the dwell straightened out quite a bit, but the misfire still was there. Put a new set of my points and condenser and ran it again. Misfire gone, dwell perfect, and full spark to all plugs.  The ole' 504 said she was good ta' go! Should run pretty nice now. Let us know how she runs Gary.

Steve@B&B

Thanks, Steve!  Kinda strange there was points wear.  They were brand new and the engine probably hasn't run more than 4 hours since rebuild.  I guess maybe they really WERE crappy quality.

And that's the difference between having the proper test equipment and merely stabbing around guessing!

Yeah, I may get me one of those...only about $4k...but I may never need one again at my age...:(

dfwallis wrote: Should I get one of those super duper coils?  I see a 40kv, a 55kv, and a 60kv option.  Does that risk burning the points/plugs faster?

Complete waste of money. No, it won't hurt anything other than your bank account.

My last job before retiring was in the electronics engineering department at Encoder Products Company doing R&D work; we had lots of really cool test equipment that I sometimes wish I still had access to, but my pockets aren't deep enough and it is difficult to justify for a one-time use

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

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!

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.

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.

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.

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

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.

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?

dfwallis wrote: Per the user manual it is the coil ht. I'll see if it has a low voltage option...

Ahhh... generally when I think of 'dwell' it is in reference to the amount of time (actually rotational degrees) that the points are closed; in which case if there was a double dwell period it would mean that the points didn't open that time around.

I would be greatly curious to see whether that anomaly occurred at the exact moment when a blip in the vacuum signal happened.

ACinSC wrote: Thanks for the explanation Les.

You are most welcome!

dfwallis wrote: When idling very slow it sounds almost like the key was switched off momentarily...

I have actually had this happen with poor contacts inside the switch

dfwallis wrote: ...but the voltage to the coil or signal to/from the points never shows any hint of a dropout on a voltmeter. Wish I had an oscope to look closer. Oh boy, a new toy...hard to get my tek 536 on the plane.

My Fluke 88 meter will record Min/Max readings and would pick up a momentary disconnect; just watching the meter probably would not be noticeable though. I do have an oscilloscope too

Hi Guys!  Just getting on here this evening. Haven't been on in a while. Been pretty busy in the shop. Gary, if your still having problems with that Tractor, I can tell ya it ain't the Distributor that's fir' sure! That unit didn't miss a beat on the last run test on the 504. I ironed the couple issues out with that. You have another problem there. Maybe a sticky valve when running. Higher rpms the valve may not stick. Lower rpms it starts to hang up and cause a dead miss. Gotta' be something with the Tractor. Possibly a fuel issue. A lean condition off idle till you get into the enrichment circuit, then it runs better and the miss is gone because it has a fatter mix and runs good under load. It's gotta' be somethin' like that.

Steve@B&B

Seems doubtful that it would be valves.  It had a complete, professional shop valve job.  Compression seems reasonably good.  The operating symptoms date back to the 80s or 90s (per my brother).  It isn't something new.  In the 80s, it had an automotive style toggle switch for ignition.  Same symptoms.  The key switch is new, the wiring hardness is 100% new.  It is burning very rich at all RPMs, but not enough for visible black smoke except under load.  It was burning leaner when I had installed the incorrect venturi (smaller diameter) in the carb last month.  When I installed the new, correct venturi, it is again blowing black specks of carbon onto the hood.  So, about the only thing left in common that hasn't been changed is the carburetor itself.  It has been thoroughly cleaned and all jets verified open, but the main jet was not changed because the screw driver slot was severely damaged and I didn't have a super tiny easy out (plus it's at a weird angle and not sure I can get to it).  I wanted to keep a TSX 486 carb on it and not switch to a Zenith, but I didn't find a source that I trusted so far.  I'll perform a few more ignition checks in July (e.g. direct wiring battery to coil, direct ground wire distributor to chassis, perhaps oscope some signals).  The GTC 505 provides oscope features but only for the high voltage signals.  Seems like slightly short sighted design.  

Anyway, I've been wanting to get a small portable oscope for some time (they're pretty cheap now).  My Tektronix 516 weighs about 500lbs and is a pain in the butt to move around.  I should have done some of that low voltage signal testing this last trip but I wanted to get it out in the field and use it some (the fun stuff) :)  The main glitches with that was the PTO shaft guard kept pulling too hard and stretching the chain clips (brand new it was very hard to turn in spots because it was severely warped), and on one occasion, the hydraulics would not go down (perhaps the hold valve stuck).  The rest of the trip was spent raising the tree canopy around the yard to make it easier to mow.  ~1 acre, 39 acres to go...I lost 6 pounds :)

Edit:  My Tek Oscope is a 516, not a 536.  516 is 15Mhz, 536 is 11Mhz.

dfwallis wrote: Additional info Compression 1 - 4 130, 129, 132, 129 ...

Those are excellent compression numbers; this does not eliminate the possibility of sticky valve guides though, or too tight valve lash, thus the recommendation for vacuum gauge testing.

dfwallis wrote: The operating symptoms date back to the 80s or 90s ... it is again blowing black specks of carbon onto the hood...

Assuming the correct size of jet(s) then the fuel level in the float bowl must be too high; perhaps the float isn't floating?   A crack somewhere inside the carburetor leaking excess fuel in where it shouldn't? I certainly dunno why, but if it has been blowing black specks onto the hood since the 1980's it must be getting too much fuel from somewhere

We set valve clearance at .012.  Doesn't mean it stayed at .012.  We had plans to recheck it at some point.  Maybe when it cools down a bit in the fall.  Needs to be done, regardless.  I don't know that it was blowing carbon specks in the 80s.  He only said it was misfiring similarly.  It did not misfire the last I used it before then, but started at some point and continues to this day, regardless of nearly everything being overhauled  to spec to the best of our and the machine shops ability.  I checked float level this week.  It was as close to 9/32s as I could measure.  The float does not leak.  It is dry as a bone.  I've checked it a dozen times.  I inspected the carburetor pretty carefully.  I saw no obvious cracks or leaks beside the one I sealed that was leaking fuel from the bowl at a very slow rate (the screw (plug) at the bottom front of the bowl).  I've ordered some tiny easy outs to try to fix it correctly.  All jets were replaced except the one at the bottom rear of the bowl.  There was no screwdriver slot material left to latch on to.

But, just because I express doubts about a particular cause, it does not mean that I've ruled ANYTHING out.  Until it's resolved, everything is suspect.  

I've ordered a new portable oscope that should allow me to actually correlate the points signal with the resulting spark of the secondary, Ch1 vs Ch2 (10000x reduction probe).

Edit:  I found a pic of an original manifold and it appears to have a pipe plug at that same flat spot.  If I were to drill and tap it I'd want to take it off so that I could clamp it to the drill press to drill a nice straight hole.  Not ready to do that at this point.

dfwallis wrote: New manifold looks like it has a flat that could be drilled/tapped to provide a vacuum gauge attachment point.  Don't think I want to do that.  Not finding a good attachment option.

Bummer. A vacuum gauge is a valuable troubleshooting tool.

I put a new manifold on my Model C a couple years ago and it had a small pipe plug that we pulled out to run the old Surge milking machine.

I just re-read this whole thread to try to cipher out what we have all missed

dfwallis wrote: ... the engine probably hasn't run more than 4 hours since rebuild...  dfwallis - 14 Jun 2025 at 1:53pm

I found no mention in this thread of re-torquing the head, re-setting the valves, replacing spark plugs, or re-tightening the manifold bolts since the rebuild; you have added a few hours since that post

dfwallis wrote: ...The timing mark was also bouncing around quite a lot, completely out of the viewing window...  dfwallis - 22 Jun 2025 at 12:06pm

This one is somewhat puzzling. At what RPM was this noted? At low idle there should be no advance, and at high idle it should be at full advance. I can see it bouncing around at mid-range while it is still partway advanced though.

That wouldn't have Champion Spark Plugs in it would it? I've seen and heard of so many goofy issues with them things. Screw in a real set of plugs like NGK's or Autolites and all the Gremlins go away..... Black carbon specs on the hood?  Hmmmm....

Steve@B&B

These are under the playlist "misfire analysis".  They will go public around midnight 1 July (CST).  

It isn't a single cylinder misfiring.  It is all of them at different times.  I've searched for intermittent shorts, intermittent opens, gremlins...The new 6V coil has no external resistor.  I switched it out and it made no difference.  I've switched everything out related to the distributor, wires, plugs, points, rotor (3 different ones), cap (3 different ones), distributor partial rebuild (not the dust cap).  No manifold leaks (and brand new).

Theyre autolite.  I know there is a carburetor issue.  GTC 505 was showing electrical issues, that's where I started first.  I still haven't ruled electrical out.  GTC 505 and timing light still show electrical dropouts.  You can see them better on the long duration recording than on the single trigger waveform.  But they happen at all 4 cylinders, not just one.  But the analyzer is easily confused by proximity of other HV wires so I'm not 100% confident in my interpretation.  I plan to try to find some high impedance tubing to wrap the wires in to improve it a bit.  It says you need 2 inches of clearance, but that's hard to do coming right out of the distributor. 

I also found some fire resistant fiberglass braid tubing that should help me better isolate the spark plug wires and eliminate the interference between plug wires.  25KV breakdown voltage.  Hopefully it will allow a more accurate view of what's happening.  I received my new oscope/dmm/signal generator and ignition probe.  It's quite nice for the price.

Edit:  I only find stills of the long duration recording showing the apparent dropouts, no video.

I'm keeping it as original as I can stand.  I did add a regulator to the generator.  I had to drill one hole in the drawbar to attach my new bail.  I bought a new PTO guard off ebay and modified it (original straightened and stored away) to clear the rockshaft mounted top link bracket.  I used the second light switch detent to turn on the rear spotlight.  Everything else is bolt on additions using existing mounting points.  I'm not a fan of 12V conversions.  6V works great if you have good wiring/connections and the engine timed.

It draws 195 amps when cranking.  Is that about right?

Both cylinder 4.  These are both before the distributor rebuild, the signals are now much more consistent, but still show these "apparent" dropouts.  All cylinders look similar to each other if you get rid of the proximity interference:

RPM (pulse count):

Every one of those "wide" pulses seems to correlate with a misfire.  These appear on all 4 cylinders.  It also correlates with the meter having difficulty reading the correct RPM rate (cause or symptom of the instrumentation??).

Voltage (Again, before the dist rebuild, it's now much more consistently around 6KV):

I selected "hold" during a period when it couldn't read the RPM rate.

I'm also thinking there must be something amiss in that area as well. It is the only place that affects the high-tension delivery to all four cylinders.

I would start by measuring the length of the rotor to see if there is excessive gap to the terminals, and then I would sacrifice a distributor cap by drilling holes in it to see if the phasing is off (i.e. is the rotor actually pointing at the terminal when it fires).

I watched those videos several times and I am not quite ready to run out and buy one of those gadgets just yet

That's plenty high but if it gets the job done it's ok; does it seem to drag?

I tried 3 different distributor caps and 3 different rotors.  Two were new and one each of the old (delco branded) ones.  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.  

I also added a thin gasket under the dust cap to get the distributor cap up just a hair higher since the old rubber seal is crunchy and I had to grind it in spots to get the cap to sit perfectly flat.  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, but adding the thin gasket made no difference.

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.  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.

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'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.  I spent thousands to have the head professionally rebuilt (among other things).  I'll be extremely upset if that turns out to be a problem (other than gap setting, which we did)...:(

I'll give the carburetor one last hurrah with the screw extractors and replacing the last jet, but I have doubts that will help...unless someone had bored (gouged) the jet out or something.  I guess I could figure out the spec and check the size.  I might also try lowering the float even though it shows to be in spec at 9/32 inch.  I was going to try some things like that last trip, but it was 100+f and 65% and I was loosing mojo. :(

It spins quite fast when the engine is timed right.  If not timed right though, it does struggle a bit (engine firing too soon).

Definitely been a thorn in my side :(

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

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.

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.

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

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.

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).

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.