Tempramental Gilera Cougar 125cc 4-stroke (2003)?

I was riding Cougar last sunday on a duel carrige way, doing about 60 in 5th gear, I pulled in to the fast lane to overtake someone but as I accelerated the bike lost all power. I pulled into a lay-by and turned the engine off because I thought it was a spark problem (I had had to bump start the bike due to a flat battery). Having cheked all the basic things (battery, spark plug, fuel line, choke etc) I couldn't find a problem but nor could I start the bike up again! I called a friend of mine to come look at it and I think he may have attempted to adjust the air (or fuel?) screw on the carb. It still woulnd't start.
When I got it home i charged the battery amd tried again and it started up but the tickover speed was really high and the engine note was higher pitched. I tried riding it anyway but it kept randomly cutting out (although it would start up again fine).
Frustrated, i left it a day and came back to it. low and behold it wouldn't start (this was a rainy day among otherwise sunny days - don't know if that makes a difference). My friend (studying engineering) said maybe he had adjusted the air scerw(or is it a fuel screw/ I have no idea - it's accessible from the side of the bike though) too much and maybe i should try that.
I turned the screw all the way in and opened the choke fully and started the bike. Again, it started at a really high tickover and engine note. I then turned the screw out untill the tick over speed dropped to what I was used to and the engine note was nice and deep. However, when I accelerated, even in neutral, the bike cut out (this haddn't happened in neutral previously). i started it again and let the choke in and the bike cut out and since then the battery has gone flat because I keep trying and failing to start it.

Prior to all this I had been complaining to my friend that the bike had become rather tempramental, it didn't feel right, it felt like there was some air trapped somewhere or something because the exhaust kept popping and the accelleration was no longer smooth when changing gears... the vey day that i mentioned this to my friend was the day it conked out on the side of the road.

So, any ideas as to what tyhe origional problem might have been or what the botched repair might have done to get me to this state. More importantly, any ideas on how to fix this without going to a garage (I've checked and they're all booked up till mid may in my area' plus i'm flat broke in any case!!).

Any help would be appreciated as i'm new to the world of motorbiking (well, even driving in general!)./

Thanks.