" I'm going to assume it's a bad coil and replace it. "
And you will be wasting your time & money. Diagnose the problem before you start replacing parts. You don't have a parts problem, you have a troubleshooting problem.
There are two schools of thought when it comes to getting a non-running tractor to start. One way is to just start replacing every part you can get to until it starts or you run out of money. The other way is to take a step-by-step approach to solving the problem, working from most likely to least likely. The trick to fixing these tractors (or trouble shooting any piece of equipment) is to be systematic about it. You need to isolate the problem step by step and work from most likely to least likely. You have solved half the problem by determining it is a spark problem. That doesnít mean you need to replace all of the electrical parts on the tractor! :)
Coil problems are difficult to diagnose. For starters, round coils are pretty robust & square coils arenít (because of the difference in insulation used), but neither one will hold up to a poorly done 12v conversion that allows too much current to the coil or leaving the key on (see tip # 38). Too much current creates heat which melts the insulation. Insufficient resistance in a 12v conversion will do the same thing. Rarely do coils just ďgo bad.Ē
There are a few ways to see if a coil is bad, but itís not possible to determine if a coil is good w/o some expensive testing equipment. If you detect a dead short or high resistance in the coil w/ an ohm meter, itís bad. If itís cracked, itís bad. If a sidemount coil w/ battery voltage to the primary will not jump a ľĒ gap from the secondary wire to the block, itís bad. But, here is the hard part: even if you do not detect a short, even if it will produce a spark, even if itís not cracked, that doesnít mean the coil will work when itís hot & under a load. So, itís a process of elimination. If the tractor starts & runs fine for 30 minutes or an hour then cuts off & refuses to re-start, and you checked for spark at the plugs & it had no spark at all, AND you have the correct voltage at the coil thatís a good sign that you have a bad coil. Let it cool off, restart it & if you have a good spark, odds are itís a bad coil. But, even then, you might end up w/ a spare coil on the shelf!
Bottom line.......coils do go bad, but I'll venture a guess that 75% of new N coils sold today are sold to folks who do not understand how to diagnose a poor spark problem or how a coil works. So, for those who donít know any better, in a no spark situation the first suspect is usually the coilÖÖand, more often than not, it isnít the problem.
Get your meter out. The ignition circuit goes from the battery to the terminal block then to the switch then to the coil then by the primary (little) wire on the coil to the points. Trace the circuit.
Do you have voltage across the points when they are open? Verify the gap on the points at .025. Then, run a new clean dollar bill through them. New points sometimes have a residue & old points can corrode, or get grease from the distributor cam on them. Or, you could have used a dirty feeler gauge. (I always spray mine off w/ contact cleaner.) Make sure you have voltage across the points, as in past the insulator on the side of the distributor. That is a very common failure point on sidemounts & often hard to find because it is usually an intermittent short. (If you find the short there, the Master Parts catalog lists everything you need on page 154. You can make the strip and you could also make the insulators as well. But, somethings are just easier & in the long run cheaper to buy. Get the strip, 12209, screw 350032-S, 12233 bushing & 12234 insulator & just replace it all.)If you just replaced the rotor & lost spark, put the old one back in. Check continuity on the secondary coil wire. Make sure it is firmly seated in both the cap & the coil. In fact, replace it temporarily w/ a plug wire.
Post back w/ results; I'll be interested in what the problem was.