To change the sprockets on Vee's you need to remove the clutch slave cylinder. If someone botched the job, possible the clutch isn't engaging completely. If the chain is tight, fix it RIGHT NOW. If the chain runs completely out of slack under rear suspension compression, it puts tons(literally) of strain on the countershaft and sprocket. Chain won't last, and will probably do some very expensive damage. If you can't get enough slack with the rear-axle adjusters, either the replacement chain was too short, or the sprockets are non-stock. If a shop did this, take it back.