could some one give me an idea whats wrong.my brother and i both have xt600e standerd spec his has about 5500 miles 07 and mine has 8500miles 03 on it.he is heavyer and is still accelerating faster.my bike is giving less power but same top speed and a gentel rocking motion like on a boat when going about 30mph

Make sure your brake pads are clearing your discs... even if they are touching lightly they get bigger when they heat up and start to grip.

And clean your airfilter...

Vando

In my experience, a gentle rocking (or rolling) of the bike at low speed is due to the spokes on one (or both) or the wheels being loose. You need to check the spoke tensions asap because it is indicative of movement which will, over time, cause the spoke locating holes in the rims to wear. If this happens (or has happened) you may find that you can get oversized spoke tensioners (used to be available, but these days ... ?). If not you will need a new rim and wheel rebuild.

Anyhow, check the tensions soon, because eventually the wheel may simply collapse which ain't nice

Hope this helps

I found a lot of loose spokes on the rear of my XT600E when I serviced it a couple of weeks ago. They were loose enough to allow the wheel rim to move about half an inch under hand pressure - scary! Tightening the spokes has transformed the bike's handling - it is now tight and predictable and feels like a much newer bike altogether. The symptoms were not the same as the OP's however. In a straight line it was stable up to 80 or so. The problems came on roundabouts and tight corners, where the bike felt to be lurching about. I put it down to well-worn Trail Wings until I saw the state of the spokes. It feels like it's on race rubber now!

the wheels are ok the oil and air filter is fine.its had new spark plug and ht lead.its just not as responsive at the throtle. when i got the bike it was nt geting full speed because some dick had fited an alarm under the seat in the way of the air intake. i have removed the alarm.could this have caused any problem for the engine

I have the same "slow" feeling even though it does now rev cleanly out until the rev limit light comes on!

So far I've checked

valve clearence..ok

air filter..dirty with oil and now replaced

oil and oil filter/old..now changed

Chain tightness WRONG..TOO TIGHT! + (Turns out its also the wrong gearing..not the 15/45 it should be, will sort when funds allow)

carb and jets ..dirty and now cleaned out

Throttle slack / adjusted

fuel tank filter/ now cleaned

Plug/ re newed just incase.

Also re charged battery (now on an optimate) and soaked all connections in wd-40.

also look for any air leaks around the head / carb area

Cheers

Carwyn

Maybe you could do a compression test?

A few months ago I started noticing some power loss on my 90' XT600E until I was riding it at about 80km/h and it suddenly lost all power and the engine died..

problem? the cam chain broke and bent both intake valves.. :\

bike has only 30 to 40k km...

Vando