I got this from a forum member (can't remember who) some time ago and it helped me with this particular problem. You don't have to remove the boiler:
"The switch is located in the rear motor frame and the axle that has the cam is the second one in from the rear trailing truck, the same axle that the main con-rod is connected to.
I made an error in that you don't use the box spanner provided to remove the con-rods, you need a small allen key, I used the box spanner to remove the nut screws from the rear wheels to spring the cam wheel assembly out.
All this can be done with the engine upside down, boiler still on, but be careful with the front bell as it tends to rest on it and if in the wrong position it will spring itself out. I placed it on a thick 'throw' that my wife uses for covering furniture, when she wasn't looking.
There are three large screws that are in line on the keeper plate, you can remove these first and there are two much smaller screws opposite each other, 'tip', don't remove them, just slacken them off about a 1/4" or less, just enough so the keeper plate comes off OK, if you take them out you will need a bit of patience to get them back in again.
I made up a little tool with a notch in the side so that I could adjust the arm on the switch and 'set' it in the new position. I can't remember if I set it lower or higher now, but adjusting it a little bit at a time and then listening for the switch to click every quarter will tell you if you have done it. In all it took me about an hour from beginning to end.
I came to the conclusion that the lobes on the cam are not equal in size and one doesn't operate the switch correctly."
I would also check that the switch is firmly mounted. If the screws are loose it will also create problems.
