View Full Version : Please help 1999 F100 misfiring at 2000RPM

08-15-2009, 09:07 AM
"Hi there!

I have a 1999 F100 Fourstroke outboard. last season it began to misfire between 2000 and 3500 rpm. Below and above this range the engine ran fine and idled perfectly. I did the following to try and solve the problem;
-The carbs have been cleaned and balanced twice
-New Spark Plugs
-Normal service
-new HT leads
-Coils have been tested and are fine
I am 99% Sure that this is an ignition problem...

This season the engine started to constantly misfire at any revs after i had been trolling for about 15 minutes. I stopped the engine and once it had cooled it was ok apart from the original 2000 to 3000 rpm problem.

Since then the problem has become much worse and now the engine will misfire so badly above 2000RPM that it sometimes completely stalls. It still can rev in neutral up to 5000 RPM but not smoothly.

One thing i tried was putting the engine in gear (in the water), take it to the point it misfires and remove an HT lead from a spark plug. when 1 and 4 were removed, the engine completely stalled. when 2 and 3 were removed there was no difference to the misfire, indicating it was these two cylinders which werent firing. I swapped the two coils and corrosponding wires and repeated the test. Tis time it was cylinders 2 and 3 which stalled the engine... So, it seemed to be the coil. I replaced the coils and we repeated the test and this time the misfiring cylinders fluctuated between all 4 cylinders and was very hard to determin

So can anyone help me here? any ideas or suggestions would be greatly appreciated!