3rd cylinder will not fire

[tshaljr]

I have a 1980 70hp Mariner ob. Runs but has a miss about every 3 seconds. If I pull the plug wire off of the top plug I can hear it arcing and if I move far enough away, about 1 inch, the motor bogs down. Putting it back and the motor comes back. Same thing with the second plug. However, with the third plug I can hear the arcing but if I move it further away until it stops arcing, there is no change in the motor. It continues to run with the miss. Putting the plug wire back on and it does not change. What is causing this to happen? One local guy says it is the switch block. Another says it might be the bottom carburetor. Any ideas. (Sorry, this is the second time I posted this because I didn't think I did it right the first time.)

 

[Scott Danforth]

You have spark. My guess you DONT have fuel.

That would point to the carb


If you don't have fuel, you also dont have lubrication in that cylinder

 

[racerone]

If the motor has 2 carburetors the bottom carburetor feeds # 3 cylinder and also 1/2 of # 2----The top carburetor feeds # 1 and the other 1/2 of # 2 cylinder.

 

[tshaljr]

I took the cover off of the front of the motor so I could get to the carbs. Using a spray bottle I sprayed some fuel into both carbs. The motor ran much better but not 100%. Do I need a complete rebuild of the carbs or are there some adjustments that could be made?

 

[racerone]

It could be an issue with reed valves for # 3 cylinder.---Could be the bottom crankshaft seal.------And what does a compression test show ?