Motorcycle Repair: xs 750, k n filters, style filters

Question
Hi
I have a xs 750 1979 , it has been made into a trike , I have had to fit K and N style filters as there is not enough room for the air box , The bike had stood for 15 years when I got it so I do not know if the problem has been there before .
The problem is it will not run over 4000 rpm under load , the timing has been set and 155 main jets fitted but still runs like a pig .
Thanks Steve

Answer
Hi Steve,

As you probably already know, when you ad K&N filters they will lean out your jetting and this is noticeable mostly on the top end. It looks like the stock main jet for your bike is a 120.
My guess is that it is running too rich on the the top end; the 155 main jets are too big.
If the bike is running rich it should feel like it is running "bubbly" or rough above 4000 RPM (if it were lean, it would feel flat).
If you are running the stock exhaust the you should be using somewhere around a 132 main jet. If it is an aftermarket exhaust or the baffles have been removed, then you should run around a 136 main jet.