Reply To: 7-1/2 Merc75 Idle Problem

[Don]

Yeah, doesn’t seem like a carb issue if it is actually dropping the same cylinder all the time…Carb issues are going to affect both cylinders. I don’t know much about Mercs but where is the fuel pump plumbed into the crankcase for its vacuum/pressure pulses? Perhaps the fuel pump diaphragm is leaking and fuel is being flooded into the offending cylinder….
Like Bill says, start with basics, hope it is ignition or fuel pump related….
PS Just because it sparks on both while cranking, doesn’t mean it is sparking under load. You might want to try those inline neon spark testers or a timing light to monitor spark while running…Speaking of basics, did you replace the plugs? Just because they are new, doesn’t mean they are any good. A quick test for a leaky fuel pump diaphragm is to pull the fuel hose and let the engine run out of fuel…If the second cylinder seems to kick in when most of the fuel is out of the engine, then perhaps this is a fuel pump issue. Just don’t get confused, because engine RPM usually picks it up a bit as the engine leans out while running out of fuel…