CA erratic spark

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.

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

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 536 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 :)

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).

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.