50hp 2000 Mercury Bigfoot

[nhvegas]

Not efi. When removing the plugs the top and bottom were dry but the middle two were fouled. Two questions. 1) would this cause rpm's to severly decrease when the boat is in water trying to move foward. 2) what causes it? Thanks

[KURAN]

1st question.
spark plug seldomly fail. U can do spark testing by using spark tester or U can test it by grounding detachted spark plug. If no spark at cyl no 2 and 3, most probably CDM fail, then change CDM.U can do CDM test by shifting CDM no 1 and 4 to suspected fail CDM position (2 & 3), after that run spark testing.If it shows spark, it confirm that U have to change CDM. If not obviously Switch Box is fail.

2nd question
Failure at any cyl whether ignition or fuel system will drop engine performance obviously.

[nhvegas]

Thank You