Norton Commando 750cc 72 model

Not reving passed 5000rpm? What is the likely problem

Stock carbs? Stock ignition? Does it run properly at lower RPMs?

Open the throttle a bit more...

I would check that both plugs are the right colour; if not, go for the ignition wiring and plug caps and then the carb jetting. If OK then pull the air filter element and try it.

If you are running stock Amal Mk1 carbs make sure the needle didn't drop down on one of the carbs, plugging one needle jet.
You are not giving us much to work with here with your subject statement and diagnostic of nothing past 5000 rpm. Try giving us a picture. There would be a thousand word in there somewhere.
Cheers,
Thomas

Check your plug color, Does it give you an indication of rich or lean? My commando used to accellerate up to 82-3 mph, then hit a wall and bog down. I put the proper sized main jets in it and now it flys right up to 100mph. My issue was too large of a main jet. As a young man I put those larger mains jets in the bike thinking, "more gas = more go!" I found out that's so not true...