Have the engine compression tested for a start.
Did you check the spark plugs?
Remove all the spark plugs and lay them out according to the cylinders. Note if any is different from the others, in terms of color and carbon deposit, if any.
Have the ignition timing checked and valve clearances adjusted.
Repair would depend on what needs to be done.
Monday, December 19th, 2011 AT 4:33 PM