Correct Plug Type - 25hp Merc

[Cordovakid]

I have a 1981 Merc 25hp manual start. The Merc OEM service manual states to use Champion L77J4 (which I understand has been superceded to L77JC4), but on the Iboats Merc plug chart, it states to use Champion L78YC for 25hp and L77JC4 for 18hp. Same exact powerhead, etc, just different jetting, carb and exhaust. The service manual says to use L77J4's for both models. Just would like to know why Iboats is recommending a different plug for the 25hp, and what the difference between the two might be? The service manual can't be wrong, right?? Thanks for any help guys....

 

[Barnacle_Bill]

Re: Correct Plug Type - 25hp Merc

Remember, that manual was written 33 years ago. Things change and are improved upon.

 

[oldman570]

Re: Correct Plug Type - 25hp Merc

Its as bhile has stated. If you look at a 1983 or newer motor, of the same size and manufacture, you will find that it is recommended to use NKG brand BUHW plugs in them. with the Champions you will see that they are of a two electrode style while the NKG is a surface gap plug. I have run both the Champions and the NkG plugs in a 1983 &1985 Merc 25 and have switched to the NKG because I have had better service from those. If you look in the plug manufacture books at a local WalMart store you will find the meanings of the numbers and letters that each plug has near the back. Heat range lenght of thread and electrode, and size of the threads on the plug and wether it is a resistor plug or not. Each plug manufacture has their own desginations. JMO
Oldman570