Loss of power & backfiring

Hi all, my CT 70 has a loss of power and and backfiring. It starts ok and runs great but after about 10 mins of riding it losses power. If I go down a gear it runs ok then if I change up the problem appears, it will backfire through the carby and start missing and have no power. This one's got me buggered.
Any suggestions will be greatly appreciated.
Cheers

Just guessing but wonder if you may have the type of fuel filler cap fitted with the butterfly valve as on an ST70 that seals the vent and creates a partial vacuum in the tank. I had similar symptoms with a new ST70 I bought in 1976 and didn't realise the vent was shut!. It ran for a few hundred yards then then the engine stopped, but started again after a few minutes before spluttering to a halt.
Even if you haven't got a filler cap like this it could still be a bunged-up filler cap vent - maybe with one of your indigenous spiders!- creating a vacuum in the tank.
Good luck
Brian

I'd start by changing the points and condenser and go from there,
Let us know who you get on,,,,