In your profile it says you have a 1973 R75/5, I suppose this is the bike in question?
Do you have a booster or any other modification to the ignition?
I would start with ignition. You have points and they can be loose. Check the point gap and then check timing.
I do not suspect the carbs at all. A little rich on the mixture screw does not blow up engines. It makes the plugs black and that is usually all it does. Many engines today are being run rich because of the crap we have for gas.
But while we are talking about carbs? The ones that your bike was built with are 64/32/9 & 10. If these have not been modified the mixture starting point for them is 0.75 (3/4) turn out.