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Joe please help 120 johnson

S

sam

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"I have a 1989 120 johnson tha

"I have a 1989 120 johnson that has stumped me. it has good compression,spark is good will jump 7/16 gap. It will fire up and seems to run perfect. I thought my lack of power was a prop issue until I started it and started pulling plug wires. Ever plug wire I pull off seemed to drop RPM's eccept for the number two, the plug looks like it hasnt fired since I replaced them 2 weeks ago. As I said it will fire the plug while removed from the cylinder and it will just a 7/16 gap but when you install it in the cylinder it seems to do nothing. I have tried a new plug in this cylinder the old plugs in this cylinder and nothing. I'm about ready to throw in the towel and take it my dealer. I also am not sure but when I say number two cylinder I am refering to the bottom cylinder on the left hand bank if you are standing at the back looking forward Thanks in Advance"
 
"Hey Sam...I have the same mot

"Hey Sam...I have the same motor, so I know a little about them...Its going to be one of three areas that the problem is in...electrical, mechanical, or fuel...You have good spark..it runs on at least 3...pretty good fuel...and mechanical seems to be OK...try this:..remove all of the plugs except the problem one...try to start it in the normal manner..pushing the key in for a good dose of fuel...if still nothing, remove the plug, squirt a good shot of fuel in the cylinder, replace the plug and try to start...if it hits then, i suspect the reed valves..either one is broken off, or is standing partially open...Let us know what you find...."
 
"Sam.... Cylinders are numbere

"Sam.... Cylinders are numbered as to how they are in line. For instance, a old inline 4 cylinder Mercury would simply be numbered top to bottom:

1
2
3
4

The OMC V4 models, if you look at the cylinders closely from the back, you'll notice that the top right cylinder is slightly higher than the left bank top cylinder, and that top left cylinder is slightly higher than the bottom right cylinder, and finally that bottom right cylinder is slightly higher than the bottom left cylinder. This translates to the cylinder numbering being:

...1
2...
...3
4...

Also standing in back of the engine, facing the cylinders.... Left=Port....Right=Starboard

Okay, so you're speaking of the port cylinder #4 not firing. If you're sure that the compression is okay, meaning all cylinders are appoximately 100+ and even, and with the s/plugs removed the spark will really jump that 7/16" gap with a strong blue lightning like flame.... a real SNAP, that leaves a fuel problem.

You've described either a fouled carburetor or as Tom suggests, a faulty reed plate (leaf valve).

Do this.... remove the carburetor face plate, start the engine and let it idle. If fuel is being blown out one of the carburetors with every revolution of the engine, then indeed a reed plate is faulty. But if this does not take place, do the following.

Engine not running, pump the fuel bulb up hard. The foremost bottom carburetor.... Remove the brass screw that you see at the bottom front of the float chamber to make sure that fuel is actually in the carburetor. If it is not, the float needle valve is most likely stuck. If fuel is present, put that brass screw back in there.

If fuel is present in that carburetor, The carburetor feeding that #4 cylinder will be the bottom carburetor. With the engine running, stick two fingers into the carburetor throat. If the rpms increase, #4 cylinder is starting to fire and that proves that the carburetor is fouled and will require cleaning, etc."
 
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