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!