up size your pilet to a 42 and upsize your main jet to 125 or 130. but first take off the gas cap and remove the metal piece. it is a tip over safty piece that plugs the vent flow so gas doesent spill. take out the metal piece under the cap. all of it. leave it out and that will help alot. I had the same problem with the one I bought used / new. but can ride all day long at 55mph. anymore ? please ask

 

So, you think my airbox and exhaust mod has caused the problem?
It seems to idle fine, just bogging when I try to accelerate.

I'll remove the metal too thanks.

 

ya when you open up eather intake or exit you need to rejet.

 

phew, sorted.

Ok, I paid someone who knows what they're doing to strip the carby again.

1. The auto choke - the clamp that screws to it goes into the first slot - the one that sees it in past the o-ring the least. (I'd put it in too far).
2. Clean all the jets PROPERLY - I now had a set of wire gauge drill bits and cleaned it properly with carby clean and compressed air. While I was at it, I went one drill size over the original hole on the main jet.

Tip: Don't use compressed air on it without taking out the vacuum flange in the top of the unit. The mechanic who worked on mine did and had to replace the vacuum flange as he'd stretched the heck out of the old one and it wouldn't go back in.

The thing has some POWER now!!! I haven't had a chance for a proper run or to set the carby mixture much (how do you get to that screw easily anyway?!?!?) but off the mark it just takes a little tug on the handlebars to point the front end skyward :-)