Suzuki 2 Strokes

my gt750 1976 starts first touch of the button but under acceleration it looses power like a misfire the middle plug is darker than the outer two changed points and condencers

Welcome
Many things can cause issues like this, when you changed points did you set timing?
Are the carbs clean and proper jets with float levels set at the right height?
Are the mufflers open?

What Karl said above. To add to the bit about the pipes being clear, the triples do tend to get carbon / crud build up in the centre header pipe right where it divides into two to feed each silencer.

Similarly, the outer pipes get a ring of carbon build-up where the header is welded to the body of the silencer, and over time it starts to act like a restrictor washer.

If you have a spare couple of hours, drop the pipes, get an old clutch cable, cut the nipple off one end and fray the strands out so it looked a bit like a small chimney-sweep's brush. Stick the 'brush' end into the pipe and fix the nipple at the other end in the chuck of your electric drill.

Then hold the cable outer, switch the drill on and move the cable in & out so the frayed ends scrape away at the inside of the pipe. A couple of minutes each pipe, then tip all the crap out. You'd be surprised how much carbon comes out. If you don't have an old cable, one of those springy 'drain clearing snakes' from DIY shops is a good substitute.