its charging at a higher voltage than it should. Could this perhaps be the problem. If so how?

No, that would have no effect on your current problem. If it's charging at more than 15V I would check your regulator/rectifier as it may be about to cook your battery. Check level of water in your battery .

OK the only other issue I could suggest would be that your float levels are too low, and if you fitted bigger jets, this is possible.(but not likely)
I still believe this is a fuel/air issue and has nothing to do with the electrical system.
Clearly there's a problem with the fuel/air delivery, either too much or too little, but from here it's hard to be specific.
Do you have a big enough diameter fuel line ? If you have one that's too narrow you could get fuel starvation...................

 

That sounds like classic float level being too low.