Motorcycle Repair: my fuel mileage is horrible., xv1700, road star 1700

Question
i have a 2005 Yamaha Roadstar 1700. my problem now is bad fuel mileage. On a whole tank of gas on the highway I get 90 miles and forget the city right now a half a tank is 30 miles..(tank size almost 5 gallons)
History:
I changed the stock filter to k&n, re-jetted the carb with the dynojet kit and pipes to cobra slip ons. i know i'm supposed to loose some mileage but this to much.
History Pt 2
I started loosing power only when i tried to go past 70mph if felt like the revs were higher but I was going nowhere. I thought it needed to be jetted a gain , they did it maybe an improvement but no go(mileage was still good). Then another guy told me its the clutch changed that and still no go (mileage was still good).  I went to another guy and he said the manifold is crack and your engine is sucking in air, changed the manifold and the power back but now the bad gas mileage. On forums they are saying wrong jets, the needle and so on, can you please at least guide me in the right direction.  
p.s. also what happens now if i'm at a high speed and i back of the throttle its almost feels like im braking without braking or hesitating...lot of info I think.
thanks Tony

Answer
Hello Tony,

First of all, you're NOT supposed to loose mileage by tuning your bike...
All jetting kits are supposed to improve fuel management, meaning performance AND consumption.
Stage 2 and higher though decrease mileage, but with significant gain in performance.
Now, in your case, I believe there is a mess-up/mix-up in stages.
What I mean is, that your jetting is stage two, but your airbox is stock.
Main Jet larger than 175 is unacceptable without a modified airbox.

I suggest to proceed troubleshooting in the following order:
a)Check and adjust engine oil level! 200ml beyond maximum is enough to loose more than 15-20%  power and equally increase consumption.
b)Check both wheels for free spinning.
c)Check and adjust if/as necessary fuel level by means of float height.
d)Check that vacuum slide moves freely up and comes down smoothly without slamming.
Make sure that it's membrane isn't caught or damaged by any means.    
e)Write down your current jetting (main jet, pilot jet, jet needle clip position, fuel screw position) airbox configuration (stock, modified or aftermarket) and come back to let me know.

Don't worry we'll crack this...

Best Regards,
Lazaros.