Backfiring on 1975 Johnson 115hp with later twin power pack ignition system fitted

[Gilera]

Ok so on the weekend I spent a bit of time on the motor. Time is not my friend so I didn't really get to try one thing at a time, but here's what I did:
Tried without the kill wires - didn't make any difference. Stripped carbs back down and cleaned them. Checked link and sync. Replaced the NGK plugs I had bought with new Champion QL77JC4. Replaced fuel line from tank to motor with a premium 3/8" one ($70!). Still wasn't running right but maybe better.

I had made mounts for the coils out of aluminium. All of the bolts on the side of the coils with the earth strap were going into the top bracket where the original coils went except for one (but the aluminium plate was in contact with the top bracket). I figured the aluminium plate would give good earth but I made an earth lead and connected it to the block and I THINK this made a difference or at least at that point it seemed to be running more smoothly.



I then noticed via a nice shock that the spark plug wire from that same coil has a crack in the insulation so replacing it is my next job. I was getting good spark across 7/16" when I tested but it can't be helping the situation.

[Gilera]

Here is a video before. It was weird as sometimes would take a while before it would start to run rough and die. This was what made me consider a possible fuel supply problem.

https://drive.google.com/file/d/1mdF...w?usp=drivesdk

Here is video now

https://drive.google.com/file/d/1mtq...w?usp=drivesdk