I will be finding this out shortly. Im a motorcycle mechanic from Sydney, Australia, who recently purchased a River (1996 model). Im doing a few things to the bike, including looking at "de-restricting" it, and I will keep you posted, feel free to email me at ross_bentham@hotmail.com for any questions about the bike and any modifications possible. In the mean time, look at improving the exhaust system and putting a pod filter on to give a bit better top end power (as syngle cylinder engines suffer from lack of top end power) I would also be interested in knowing any information you have on the bike too, maybe any modifications you have made, the differences they have made and if you are able to get more technical information on the bike, I would be appreciative if I could get more information on this as they are EXTREMELEY rare in Australia and finding data on them ( like valve clearances etc.) is very hard to get. In the mean time, to de-restrict the bike, look at anything in the air box before the carburettor that may restrict air flow, and also if you can, pull the muffler apart to see if you can remove anything "restrictive" inside. I know that many of the single cylinder dirt bikes come to Australia with restricted air boxes and VERY restricted exhausts on them. Anyway, enough babbling on, let me know how you go and if you have any questions, email me and I´ll let you know how I go.