1978 yz250 no spark

First off....Clean all the coil connections and mounts to be sure all the rust and dirt are not creating the problem.
Then unscrew the plug cap from the wire, cut 1/4" off the wire and screw the cap back on to the wire.
Then do an OHMs test on the coil.
You are looking for a good Blue spark at the plug.
If you still have a problem, then pull the flywheel and clean it and the stator.

The service manual gives the coil OHM reading spec's. Each bike has different spec's for the coil. With that said you want a good bit of resistance on the primary side of the coil so it does not break down and creates the charge needed to fire the plug.
The secondary side of the coil should have as low a reading as possible so that the energy from the coil is delivered to the plug so it can fire.
Dirty or rusted mounts and the coil wire connection to the spark plug boot effect these measurements and can cause the bike to not have fire and or to stop running after it warms up.
The dirty stator/flywheel reduces the electrical juice to the coil thus reducing the coils ability to create enough energy to fire the plug. The stator also contains a trigger coil so it will send the voltage to the coil at the correct time.

You need to locate your manual or a replacement.

Paw Paw

Used to have a bunch of that 78-79 YZ250 stuff. Couple of engines, cylinders, ignitions, most any related engine part. Had a new 250 in 79, and in later years , used those engines in my short track bike. Only had one ignition failure, the pulse coil on one of the stators took a dump. They are the sealed plastic type on those bikes. Bought a new pulse coil, and all was well.

Most of the time there is an "open" in one of the windings on the stator coils that causes them to stop working. A broken wire in the winding. If they start and run for a bit ,then quit firing, usually engine heat has caused the "open", or break in the winding to expand, and lose what bit of contact it may have been making. If you let it cool off ,and it starts back up, runs a bit more, then quits again, you more than likely have an "open" some where.

Coils are fairly inexpensive, you may try another one and get lucky. There is a lot of interchangeability with Yamaha ignitions for that bike.78-to 89 YZ 250 have the same taper on the crank / rotor / flywheel. The only thing is you will have to make a spacer for the ignition cover if you use a YZ250 system past '85, as I believe they went to an outer flywheel in '86.

YZ80 stuff also uses the same taper from at least '85 and up, but a bit more work is involved in using this on the 78-79 YZ250.You need the cover spacer, as well as an eccentric ring for the outer edge of the stator plate, in order to center the YZ80 stator plate over the crankshaft end, along with grinding a new opening in the cases for the wire lead exit, as the YZ80 system spits the wires toward the front engine mount, instead of nearer the countershaft area on the stock 250 system.

I used this complete 87 YZ 80 system on my 78-79 YZ 250 engine in the blue and red bike pictured here. Other than the spacers and the weight I had added to the flywheel, it was a free system as I had a couple of YZ80 parts bikes to pick from. Once the spacers were fabbed, it fell right on, very simple install. Timing was at 1.9mm BTDC.

Think it gained some peak revs, as the 80 system had a bit higher rev limit. Ran good. Pulled it off the bike when I sold it, and sold the system to a guy , who is still using it to this day.