Any Outboard techs here.

I have a 4 HP Mercury and it keeps burning out the spark plug. I put a new plug in it and it lasts about 8 hours. I bought it new in 2015 (2014 model) from bass pro. 1st plug lasted 3 years and it just shut off one day while using it. Since that event I have put 4 new plugs in it.

 

Assuming you have a single cylinder 4 stroke 4HP and that you are also using the same spark plug brand /number that is specified in your owners manual. Can you confirm the specifics? Wondering if your outboard is running lean due to adjustment or fuel line (maybe fuel filter) restriction.

 

Yes it is 4 stroke single cylinder and I have been buying the same plug (NGK) as was in it. The last time it shut off I pulled the plug and replaced it with a champion plug. The results were the same. Just seems to give about 8 hours of run time before it quits. Was wondering if the coil could be putting out to much juice and is taking out the plugs. From 1 master retired to another thanks for your service

 

I have not heard of a coil putting out too much juice. I have seen improperly gapped plugs and the wrong plugs cause problems. If a motor runs lean it or using too hot of a plug could cause issues with the spark plug getting hot. Make sure your water pump is always still pumping a good stream when the motor is started and running. If you have the right plug with a good gap, then I would be looking at the fuel system. I had a 4HP four stroke Merc that I traded in last fall when I went for a newer boat and gave the operator and maintenance manual (used from eBay) that I had when I traded it. It was a reliable motor motor. Only once did I have a major issue with it and it was due to letting the boat sit too long with E10 gas. Some things you might look at in order - fresh non-ethanol gas, vent open (on the fuel tank? I did have the motor stall with the vent closed before), fuel filter clean/replaced, fuel line and bulb. I did look through the mercury site and it did not seem to be much help. Good Luck and let us know what you find. https://www.mercurymarine.com/en/us/landing/outboard-care/

 

spark plug / gap.
https://www.marineengine.com/parts/ngk/mercury.html