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1984 mercury 50 hp #3 cylinder plug fouled

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R B

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I have done much testing and other misc repairs. What are some common caused of a fouled plug?

does anyone have a diagram on how to check reed valves?
 

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Post the compression numbers as #4 dont appear to be firing. Have engine overheated? If so I have seen the plastic red dowels melt and allow reeds to shift. Also check spark on lower 2 cylinders..
 
They are all about 120. I swapped coils from 3 to 2 and it remained the same. Maybe I should swap 1 and 4?
 
A plug is fouled in a cold running cylinder !----Reed valves are internal and on a reed block around the crankshaft.---Removing the carburetor allows for partial inspection.---------Cold cylinders could be caused by a ruptured diaphragm in fuel pump / flooding cylinder / poor crankcase compression.
 
I have not tested the fuel pump as I thought if the pump was bad, it would not give fuel. I will look at the manual to check the pump.
 
Sorry, a ruptured diaphragm will put raw fuel into the cylinder, actually bypassing the carburetor.--You need to check this out on both carburetors and repair kits are cheap.----Might not hurt to remove powerhead to inspect bottom main bearing and the 2 seals found in that housing !!!
 
faztbullet said:
Post the compression numbers as #4 dont appear to be firing. Have engine overheated? If so I have seen the plastic red dowels melt and allow reeds to shift. Also check spark on lower 2 cylinders..
Click to expand...

I ran it in the dark and is the porcelain suppose to glow on the plugs? Saw some (very little) spark around the #1 wire at the coil.
 

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I found this about what I am seeing on the spark plugs.

Corona Effect:

In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized. The discharge will occur when the strength (potential gradient) of the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. It is often seen as a bluish (or other color) glow in the air adjacent to pointed metal conductors carrying high voltages. Spontaneous corona discharges are undesirable where they waste power in high-voltage systems or where the high chemical activity in a corona discharge creates objectionable or hazardous compounds, such as ozone. Controlled corona discharges are used in a variety of filtration, printing and other processes.

I came across this on a Corvette forum.
 
Sounds like you may need some new spark plug wires...the corona effect is occurring because the wires are not containing the high voltage.
 
guyjg said:
Sounds like you may need some new spark plug wires...the corona effect is occurring because the wires are not containing the high voltage.
Click to expand...

I replaced the wires and it is still the same. I found a guy that does repairs for a considerably less amount then a dealer. Wish me luck!
 
He suspects two weak coils but states my pump is not pumping water. I recently replaced the impeller and I know water was pumping when I had it running at my house. Maybe running it often off ears could wear the impeller out fast? He's installing two new coils and an impeller so I hope my problem is solved. Will update when the work is done.
 
I recently replaced the impeller and I know water was pumping when I had it running at my house.
Click to expand...
Not defending the mechanic but garden hose water pressure can make it look like the pump is working.
 
Two coils replaces, new pump installed and it runs decent to about 50% throttle. He tested the motor warm and states the switch box is faulty causing #3 and #4 to not fire. Parts ordered for Wednesday. Here's hopping to make my trip on Thursday.
 
He replaced the switch box and says it now runs the way a 50 HP Mercury is suppose to run! Looking forward to my fishing trip now!
 
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