lexmoto xtr s 125 problem at top speed

Right ok, bought the lexmoto xtr s 125 off a friend recently, took it out for the first time today and noticed a few things. 1. I'm not sure if it's normal or whatever but the bike feels as though the back tyre is loose there's movement but could this be down to the fact I had a 97Honda cg before and it's just different suspension? 2nd of all when I'm on a straight and it's at top speed it all of a sudden starts slowing down, starts jerking but then all of a sudden starts accelerating when I leave it roll or if I pull over for two seconds, at first thought it had run out if fuel.

Any help would be much appreciated

Pritchard it's Jensen.

The problem you described sounds much like a fueling problem, based on the fact that it only happens at top speed I'd say that your main jet is too big.

Open up your carb and check for any blockages in the jets and for any gunk and check the seals for leaks, if that fails swap your main jet for a size smaller (i.e. if it's a 120 then change it for a 115) and see if the problem goes away or improves, if it gets worse then go a size bigger. If that fails either get some carb cleaner and spray it inside your carb or get a carb cleaning fuel additive. If that fails get a new carb, they are pretty cheap.

Before you buy a new jet, might be worth giving both of them a clean anyways. Just remember to record what they were on when you removed them!