Exactly what kind of Cam is in this thing that would warrant a 2.5 powervalve? Mine gives no trouble being a stock 6.5 and I only produce 10 inches of vac at best when idling. I agree to pull it apart and put a different PV in it but unless the 2.5 is already on the bench I wouldn't waste my money. Check the carbs over(with them off), put a new PV in, set the throttle blades as advised. See how that works and go from there.
