I have recently acquired a 1999 TRX-250 fourtrax which has been neglected by the previous owner. I have been working through the basics of cleaning it up and servicing which includes changing oils and filters, adjusting carb, cleaning fuel tank and running some fuel system cleaner, new spark plug etc.

Generally it starts and runs ok I think. However when I have been riding it hard for an hour or so, like through soft sand or revving high to get over rocks for example, I’ve noticed it is getting quite hot and some smoke is coming from the bottom rear area of the engine. I think it also struggles a bit at this point as in seems to lose some responsiveness to the throttle particularly on lower throttling.

Could anyone give me some ideas as to what could be causing this smoke/behaviour? Or is there any further servicing i should be doing assuming it has been poorly looked after in the past?

Thanks a lot.
Luke