Spark mystery

640, 12V conversion, running well x 2 years. A bit ago I posted that it had died after a few hours mowing and there was burning/melting of the rotor.

Today mowed a bit, shut it off to move somehting, no restart. Fat spark at the coil wire, nada at the plug or plug wire.

Replaced rotor and cap with new form this site. No change. Fussed a bit, at one point caught and ran for a moment, then died.

Swapped with rotor and cap off another tractor. Nothing.

Still have fat spark from the coil wire but nothing from the distributor downstream. 3 different rotors and caps.

Any ideas?

thanks
 
Not quit sure but try a different coil .. new points & condenser
also there is a primary and secondary side of the starting circuit, the primary sends a full 12 volts the starting circuit, for starting, then when you release the key switch the secondary side cuts the voltage back to 8 volts to run.
This process cools the circuit components, coil and points

Not sure but look at the bearing in the distributor. I have had one go out and wipe out the cap and button
 
Not sure why I'd look for problems at the
coil when I have a good spark at the coil
wire the whole time.

The distributor shaft does not wobble at
all.
 
The dust cap or the distributor cap? The dust cap has nothing to do with spark. Most of the farmall tractors. The dust cap is the first thing that get tossed.

if the rotor is getting a hole burnt in it allowing spark to ground to the dist shaft. You need new plug wires
 
(quoted from post at 00:17:24 05/17/15) The dust cap or the distributor cap? The dust cap has nothing to do with spark. Most of the farmall tractors. The dust cap is the first thing that get tossed.

can't remember which models, and I'm too lazy to go outside and look,
but some Fords actually use the dust cap for distributor cap
alignment and upwards spacing.
Not one of Fords better ideas, but it is what it is....
 
yeah, I replaced the dust shield. In my original post I commented how I changed cap and rotor x 2 no change. Great fat blue spark at the coil wire and if I use the coil wire as if it were a spark plug wire - ie connect it to a spark plug- I get a spark at the plug.

Yes, the dust shield is responsible for distributor cap alignment, but the old one did the aligning just fine too. My only guess- and this is a guess - is that the new dust shield has a rubber oring at the bottom which the old won didn't have. Probably wore away in the last 60 years. This little or ring makes the dust sheild and thus the dizzy cap sit a tad higher- liek 1 mm - and changes the clearance between the rotor and cap.

Darn if I knwo why that did it, but it dod it.

opinions welocme.
 

could be lots of things.... bad spark wire from coil to dist.

moisture in cap shorting fire out. rotor shorting to center shaft as others said shorting fire out. but it sounds more like....

bad points with dirty contacts creating weak current path. bad coil or primary wire to coil. bad coil ground in clamp to block ie because a weak coil will fire better on high resistance than low resistance,, meaning it will jump a larger gap but be weak on a smaller gap.

broken down plug wires.. less likely, too small of a plug gap, wrong plugs for compression rating of engine,,, the higher the compression, the harder it is to jump the gap... back to bad coil circuit also. bad coil that weakens when warm or hot. bad wire from ignition to coil especially if resister in circuit, bad ignition switch also common. or wires to and from ignition switch. low battery voltage.
 
You said.....
I use the coil wire as if it were a spark plug wire; I get a spark at the plug
This little or ring makes the dust shield and thus the dizzy cap sit a tad higher- like 1 mm - and changes the clearance between the rotor and cap

You change the clearance between the cap in rotor.
This gap is critical and should be the same as your plug gap.
What you did by moving the cap away from the rotor is created a hotter spark.
I suspect what brought this problem on in the first place is your plug wires are going bad and require a strong spark since changing them out with the coil wire provided a spark.
This is why it worked with the old dust shield in the past. The spark plug wires were in better shape.
You most likely fast forwarded the demise of these spark plug wires by pulling on them to replace the cap.
 
Yes but over time they get cracks in them that hold water and that will short it out. The cap can do the same thing
 
Your welcome. A simple cracked distributor cap can and will cause all sorts of odd problems. I learn that one way back when I was a teenager and had a GMC truck that was prone to cracked caps
 

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