So nothing changed when you swapped the other coil in? Did you swap the wire's around on that coil? The 12v should only go to the one side of the coil, if you reverse them i reckon the spark might be tiny at the plug A back fire through the carby's sound's like the Ignition coil's are plugged in to the harness the wrong way around and it's firing on the wrong stroke? Maybe try swapping them and see if it fire's ? Both the coil's run off the same 12v coming from the TCI, one of the 12v wire's to the coil's is tapped off the 12v feed from the first coil where it come's from the TCI, guessing the join would be near the airbox area If only one of the coil's is getting 12v then there must be a break in the wire or corrosion on the joint under the insulation, you've checked the terminal's are ok in the plug / socket where the coil wiring meet's the main harness ? Also look for sign's of corrosion where the coil wire's are crimped into the terminal's on both end's of the coil wiring, and same at the other end of the main harness where it plug's into the TCI. You can remove the terminal's from the plug's / socket's with a small flat head jeweller's screwdriver if you push the retainer tab down a little (push the wire/terminal into the plug / socket at the same time as you push the locking tab) If there's any corrosion you should be able to trim the end of the wire back a bit to where there's no corrosion, strip the insulation back a bit and crimp a new terminal on then refit it into the plug or socket. If they are all good then push the retaining tab back out a bit so it'll lock back into the plug / socket, then refit the terminal into the plug or socket it came from (do 1 at a time so it goes back in the right position) If you find that the coil is getting 12v when the ignition is on ( killswitch turned on) Then it may be a problem with the earth wire from the TCI, or possibly the TCI itself as @Linkin mentioned. You could test the switching of the TCI if you ran a test lead off the positive side of the battery and put the tip of the test light on the Orange wire or Grey wire from the ignition coil depending on which coil you are testing. You should get a flash from the test light when the coil you're testing fire's (may only be faint as it'll happen very quick) Make sure the coil lead's are all connected to the spark plug's, and that the plug's are all fitted to the cylinder head when testing otherwise it'll put stress on the electric's inside the TCI. Let us know the testing result's and we can advise you from there.