Sounds like the cam has some flat lobes on it. If they are exhaust valves that aren't opening correctly then it will back fire under a load.
Timing chain is probably worn, but won't have any effect on how much the rocker arms move. If a few aren't moving as much as the others then you have a cam problem.
Last edited by Roy Rodgers; 05/17/2014 10:14 PM.
