Hot coil on my 300U

I'm having problems with my 300U coil getting so hot you can't hold your hand on it after running for about an hour. At that point the tractor runs really bad and not much power. When everything cools down it runs fine again. Its a 12 volt system with a generator and a 6 volt coil with a resistor. Been like that since the 90's with no problem. Problem started last year when I was brush hogging and I decided the coil was bad so I replaced it with a 12 volt coil that required no resistor. Same problem. I now have a NAPA 6 volt coil with a resistor again. Generator is different than last year as last fall my original one had a short and I replaced it. Seems like something is making the coil hot other than a bad coil. I have a lot play in the distributor shaft, both in and out and rotational play. I compared it to my H which has virtually no play any direction. I get how that could make it skip a little which it does sometimes but the real problem starts when the coil gets hot and it looses power. I haven't changed plugs in a long time and I have carbon core wires to the plugs which have been there a long time also. Any ideas what to be looking for?

I just did a voltage measurement and have 12 volts going to the 6 volt coil. Will that make the coil hot? I don't understand how it can be getting 12 volts after passing through a resistor that measured 2 ohms when I checked it. Testing was done with the tractor off so its voltage from the battery only.

Answer: a resistor does not react to reduce voltage unless it is under load in the circuit. Your meter is reading an open circuit, so it will read 12.xx volts on both sides of the coil small terminals. The coil should read about 1.2 to 1.5 ohms when disconnected on one side. When the points are closed, the voltage on the connected coil should be ~6 volts on the resistor connected terminal, and zero volts or very close, on the distributor side. If the coil resistance is less than an ohm, it coil is going bad. But, and it is a big but, The distributor shaft issue can be the problem. If the point gap is kinda random, it could be putting the coil under stress by having it "on" too much of the time. Dwell is an angular measurement of the degrees of distributor rotation that the points are closed. Dwell (with a meter is probably good when at 38 to 40 degrees. Which X4 equals 160 degrees of distributor rotation, or less than 1/2 of 360 degrees. This means the coil is turned on (making heat) for less than 50% of the time when running. With noticeable slop in the dist. shaft, that could put the coil on time at 70 or more % of the time. It will overheat. Jim

Jim - always appreciate your replies to a problem. The voltage measurement I'm referring to is after the external resistor. There is only one hot wire going to the coil and it is from the resistor. The resistor measures 2 ohms but I'm not seeing a reduction in voltage. I'm confused.

WHen the points are closed, the distributor side of the coil is grounded through the points to the distributor housing, then to the engine. This grounding (points closed ignition on) caused the coil to be "on" when on there will be no voltage on the distributor side of coil, and ~ 6 volts on the resistor side. If the points are open there will be 12+ on both sides. Jim

Sorry to be a pain on this this. Sounds like what you are saying is for an internally resisted coil. I have an external resistor on mine going to a 6 volt coil. I have the same set up on my M and it reads 6 volts at the distributor. My 300U acts like there is no resistor but there is and reading 2 ohms when I measure it.

There is no such thing as an internally resisted coil. (though it is implied by the wording external resistor required, or no external resistor resistor required) they are just made with internal windings appropriate for the voltage. Just like a 6 volt light bulb runs on 6 volts. and 12 olt bulbs run on 12 volts. If you run a 6v bulb on 12v, it needs a resistor to keep from melting down. The engine normally stops with the points closed. so if they are, the voltage will be ~6 on the resistor side, and nothing on the distributor side. Jim

Jim explained it, perhaps a picture will help visualize it

Add or change your ballast resistor, using the 12v coil.