Coil shorting out on '73 GT250

[Johnny5]

I am having an awful time tracking down a short on my new '73 GT250. The right side coil is shorting out and burning up. When I bought the bike the people told me that all it needed was a coil. After replacing the coil the bike would run, but the coil then began to overheat and shorted itself out. Even while the bike was not running, thus ruining the coil. I am new to these bikes and I do not have a manual yet and I was just wondering if anyone else has ever run into this problem before. Any and all info would be great!!! Thanks!!

[pearljam724]

I would start by inspecting or replacing the ignition and kill switch. Also check the coils ground to the frame. If you remove the switch and disassemble it. There is a good chance you will find a ground or coil wire pulled from it's solder. It's common on older style ignitions that the ignition or kill switch wiring can be compromised from side to side motion of the handlebars. Caused by moving a bike around in a tight radius. If the ground has been pulled from it's solder it will cause the bike to continuously run after it's been started. The only way to shut it off is through the kill switch. Your coils positive wires run to the ignition switch. Both it and the kill switch act like a fuse to supply and shut off power to your coils. You can easily test a set of coils with an ohm meter. Find out what ohms your specific coils are rated for. You insert one prob into the end of the coils positive wiring and the ground prob into a spark plug lead. This will tell you if that specific coil is admitting the proper amount of spark to your spark plugs or not. If the reading on the meter is within less of an ohm as to what the coils are rated for. The coils are good. It's a good idea to be using a coil or set of coils that are equally rated for what the manufacturer suggested using on your specific bike. If the bike came with 4.5 ohm coils. That would be the way to go on purchasing another. Any coil will work as long as it has the same ohm rating as the original coil or coils. Make sure the coils are grounded properly via a clean / tight connection at the frame and ignition switch. You have to open the switch up to determine that. Unless, you just buy another switch to go around it. Make sure the ignition switch is wired properly and not grounded to an adjacent connection inside the headlight bucket or the positive wires from the coils are not being grounded to the headlight bucket itself.

[jabcb]

Some likely problems that could cause this:
incorrectly installed right points (have short so it doesn't matter if points are open or closed)
incorrectly adjusted right points (points never open)
bad condenser (shorted out)
wiring short between the right coil & right points

Was the bike running on just the left cylinder?

Did you check to see if you get a spark after replacing the right coil?

Do you have a multimeter? Do you know how to use it?

[Johnny5]

I would start by inspecting or replacing the ignition and kill switch. Also check the coils ground to the frame. If you remove the switch and disassemble it. There is a good chance you will find a ground or coil wire pulled from it's solder. It's common on older style ignitions that the ignition or kill switch wiring can be compromised from side to side motion of the handlebars. Caused by moving a bike around in a tight radius. If the ground has been pulled from it's solder it will cause the bike to continuously run after it's been started. The only way to shut it off is through the kill switch. Your coils positive wires run to the ignition switch. Both it and the kill switch act like a fuse to supply and shut off power to your coils. You can easily test a set of coils with an ohm meter. Find out what ohms your specific coils are rated for. You insert one prob into the end of the coils positive wiring and the ground prob into a spark plug lead. This will tell you if that specific coil is admitting the proper amount of spark to your spark plugs or not. If the reading on the meter is within less of an ohm as to what the coils are rated for. The coils are good. It's a good idea to be using a coil or set of coils that are equally rated for what the manufacturer suggested using on your specific bike. If the bike came with 4.5 ohm coils. That would be the way to go on purchasing another. Any coil will work as long as it has the same ohm rating as the original coil or coils. Make sure the coils are grounded properly via a clean / tight connection at the frame and ignition switch. You have to open the switch up to determine that. Unless, you just buy another switch to go around it. Make sure the ignition switch is wired properly and not grounded to an adjacent connection inside the headlight bucket or the positive wires from the coils are not being grounded to the headlight bucket itself.

Thanks for the reply! The replacement coil I got was just a new one off ebay. It looked exactly the same and mounted the same, but I didn't check the resistance. This is all amazing advice, I suspected the ignition switch first. I am going to go through the system and figure this thing out. Thanks again for the advice.

[Johnny5]

Some likely problems that could cause this:
incorrectly installed right points (have short so it doesn't matter if points are open or closed)
incorrectly adjusted right points (points never open)
bad condenser (shorted out)
wiring short between the right coil & right points

Was the bike running on just the left cylinder?

Did you check to see if you get a spark after replacing the right coil?

Do you have a multimeter? Do you know how to use it?

Thanks for the reply! That was my other thought, the points grounding out. The bike was running on both cylinders after replacing the coil. The left coil was getting warm while the switch was on, but the right one was the one that cracked, smoked, and burnt up. I do have a multimeter, but my buddy was working on the bike while I was at work and he only uses a test light. I TOLD him he needed a meter to do the job correctly. I believe that between the points and the ignition switch I should find the short. Thanks again!! I'll keep you updated if I find the problem.