9N intermittent spark

My 12V front mount 9N had no spark. The points are new, and gap checked. The coil is 2.5 ohms. I checked out the distributor and coil for operation and continuity, following the advice in the archives. All tested fine. Coil pigtail spring makes good contact.
However, the only way I could see to check the coil-tab connection to the contact on the top of cap was with a continuity tester while pressing the cap and coil in together in my hands (with gasket in place) while having one lead touching the tab and the other lead on the center of cap. I can see the coil-tab deflect slightly when putting the coil on the cap, but continuity seemed intermittent. Maybe my hands weren’t so steady. To be sure I had continuity, I took the copper strip from old set of points and wrapped it around coil-tab to act as shim/bias spring. This helped get regular continuity with the hand held test.
After re-installing the distributor and coil, I checked spark (on No. 4)-- now it did have spark (even if a bit on the yellow side). It then started right up, and ran for about 5 seconds, then cut out. I re-checked for spark (on No. 4 again), and it had no spark again.
I’m gonna need it to run for more than 5 seconds at a time to get the hay cut. Any suggestions where I might be going wrong?

also - I just reviewed Bruce(VA)'s comments on a recent post about common causes of failure on front mounts and can say that items 1-4 and 7-10 are OK. I talked about item 5 above(the tab on the bottom of the coil not making contact w/ the brass button on the cap). I will check item 6 (Incorrect positioning of the spring clip on the plate causing the pigtail to ground) when I pull the distrib on Friday.

I should add: the distrib plate is new and I replaced lower bushing when I did the points.

You can gently/slightly bend that tab out to make better contact.
I would clean the tab and the mating button too.
With contact cleaner if you have it.

as a test, have you hotwired the coil to rule out a bad connection in the wire harness / switch?

I am not sure what you mean by "hotwired"... I did check that top of coil had 12V with key on, and went from there.
I didn't advance to check voltage when points closed though.

really? you've never heard the term 'hot wire' before?

hot wireing is a slang reference.. mostly going back probbaly to car theft. it involved of using a wire from the battery 'hot' terminal' directly to the ignition, thus powering the ignition. ( in a car setup.. one would then find the wire to the starter solenoid coil and arc it against the hot wire to make the starter spin.. on simple old ignition setups and on columns with no locks.. all you need is power to the coil and a spinning starter. )


anyway.

hot wireing the coil is a good test as while your meter with 50k ohms of input impedance see's 12v on the coil... it may bee too poor a connection thru a switch or wire connection to pass enough current to run the ignition.

thus you run a 'hot wire' from the battery to the coil, thus bypassing any other wire connections and igniton switch in the line... like my previous message specifically stated...

(quoted from post at 10:51:38 06/06/13) really? you've never heard the term 'hot wire' before?

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Ha! Of course I've heard the term hot wire before-- thats where I get all my discount flights and hotels.

Seriously though, I totally missed your idea. I was so focused on voltage and continuity, I forgot all about the amps. (And the ignition switch wire to the terminal block looked like 3 lines spliced in series, so there may be some high ohms in there somewhere. I hope it works out that easy. Either way, I gotta fix that.

We are in for yet another 24 hours of heavy rain here so i won't be going out to the field to check until later tomorrow or Sunday. I'll let you know.

yep.. cruddy wires and lotsa splices can still show 12v to a meter with high input imp. but when the coil wants 3a.. it may not get it.

Well, I tried the hotwire to coil. No Joy.
Re-tested all the connections, etc. Turns out: by checking continuity from tab on coil through cap to tip of rotor, even though it looked "clean and bright", the rotor tip was bad. Put in new rotor, and she fired right up.

Also, part of the fun was I started testing continuity with a brand new spare distrib cap for comparison. This new cap that i was sure must be good was defective out of the box. Wish I could remember where I got that cap.

Thank you for the help.

(quoted from post at 12:34:00 06/09/13) Well, I tried the hotwire to coil. No Joy.
Re-tested all the connections, etc. Turns out: by checking continuity from tab on coil through cap to tip of rotor, even though it looked "clean and bright", the rotor tip was bad. Put in new rotor, and she fired right up.

Also, part of the fun was I started testing continuity with a brand new spare distrib cap for comparison. This new cap that i was sure must be good was defective out of the box. Wish I could remember where I got that cap.

Thank you for the help.

Click to expand...

hat is a "bad rotor tip"?

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No continuity when one meter lead is touching the tip of the rotor.
When I removed the coil to check continuity, I held one probe on the contact on top of the distributor cap and the other through the hole to the tip of the rotor (at 12 oclock position), and got an open circuit. With a new rotor, I had continuity. Old rotor back in, no continuity. Then, with new rotor, I had spark and ignition.
My guess is it was corrosion or something, even though it looked clean. When I get a chance someday I will clean it or file it to see if it makes a difference.