Okay, here is the deal, I changed from 33" tires to 35" tires on my suburban, and lost some power...expected that as I only have 3.73 gears....no problem, still ran fairly well. Well, in the last few days, power has dropped off more, now the truck does not really make power untill I get her up to 2600 or more RPMs from 2600 on up to 4000...power is fairly good, for a stock 350 w/quadra junk. I have set the timing to 10* with vacume disconnected and plugged off. I have checked for vacume leaks using eather at base of carb, and all vacume lines hooked to the carb, no leaks found. Truck has 160K miles SHOWING on the motor, I say showing because I found out the truck used to be a state fish and game truck, and then a sheet metal company truck, so I am betting mileage is higher....but do not know. Anyone have any ideas what might be wrong with my poor truck? Could it be the timing chain getting loose? ? ? ? Also, gas mileage has dropped off BAD in the last few days, I used to drive to Denver (100ish miles one way) and back on 1/2 a tank with the 35s, tonight it took me 1/2 a tank to drive to Ft.Collins and back (45-50miles oneway).