As petenz says as the XT 660 is a big single it gives its chain/sprockets and cush drive rubbers (rubbers in side the rear sprocket carrier) all a hard time.

First check if your cush drive rubbers are ok and the rear sprocket doesnt have too much rotational play, if it does you can either buy new cush drive rubbers, or pack the old ones out with old inner tubing.

Second make sure the chain slackness is set correctly as with a slightly slack chain makes the feeling of this surging very much more noticable than if the chain slackness is seet correctly.

If you still find the surging annoying you now need to start modding the fuel in/out of the bike. Youve already taken the snorkel out and changed the filter so it would be advicable to get a kev fuel mod which allows you to trick the ECU into adjust the fuelling for a better burn/feel. Or you could get a PC111 or similar but they cost ��� and require dyno time to setup properly

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Plough on