I have a 6cylinder 200 johnson, how can I check to see if all 6 are working. I tried pulling the wires on each plug one at a time while at idle. only 2 made the motor shut off, nothing happened with the other 4. how else can i check each cylinder, do i have to put it in gear??? i'm asking because boat runs on land but not underload, and i'm narrowing down problem almost ruling out bad fuel,pumps, etc... any help would be appreciated....thanks in advance 