Spark /lead / coil / problem

Hi
Helping to set up bike. It has new carbs a new Boyer MK3. I have rewired the pick up & Boyer wires, with an extra earth. We now have the bike running after doing the float heights, tick over is a bit high but it pulls really well. We have a misfire & popping & banging that occurs after approx 10 miles of riding. Nearside is always affected. We are putting on new spark plug caps & leads tomorrow & swapping them over to the other cylinder to see if
a) the new leads/caps sort the problem b) to see if the problem swaps to the other cylinder ie prove its the coil.
I had swapped the coil but I only had old spares on the shelf so can not be sure if they are alright. We have new plugs, I have stripped the new carbs,checked & rebuilt them. The coils are original Lucus 6 volt ones. The plugs nearside plug has a dry black sooty coating that you can wipe clean. The bike will restart & run fine again for another 10 miles or so
Anything I am over looking? We are slowly working through everything & it feels really close to being right but the bike keeps moving away from us.
Thanks Chris
ps does anyone know the ohms for a Kawazaki w650 coil? The owner had one as his first bike back after 40 years & has a brand new 2 lead coil.

It sounds like you have a carb running rich and fouling the plug to the point that the spark shorts to ground on the carbon rather than jumping the gap. First check that the needle clip position is correct and the needle is in place under the spring. Check the idle mixture screw for best running, usually 1 to 2 turns out; and that the main jet block did not ubthread itself from the body.
A weak spark or plug gap closed up to a few thou by dropping the plug may also problems, but check the carb first.