NGK are good plugs with a long life. After 14 yrs and 800hrs of use, the Plugs that came in my 1984 115 Merc, were still in it when I traded it in 1998, on a 150 FICHT. In the 4 yrs I had that one, it must have went thru 42 plugs, all Champions
First thing to do if it passes the Compression, Ignition, and Gearcase Seal tests, is to drop the Lower Unit, and replace the Water Pump Impeller, or if need be, the complete Water Pump.
A bit of Ballast in the Bow, may allow the boat to plane, and the top speed will be higher, so sometimes a bit more weight, in the right place can mean more Speed
Here is a link to a List of Aluminium Boat makers. These Brands are usually Planing Hulls, but you might find a Displacement Hull in there too.
http://www.fiberglassics.com/library/index.php?title=Main_Page
What is the Gap on your Inline Tester? If less than 3/8", it can spark all day, but there is no Guarantee that once the Compression Rises in the Combustion Chamber, that will deliver a spark strong enough to jump the gap on the Spark Plug
Scott, and myself, have both recommended engines of 6 hp or less as max. However, you are free to put something bigger on if you want.
I've seen people put 25 hp on boats like that. Not very smart, but they did it. I don't know what happened afterwards...
I replaced the Gasket 3 times before the Service Manger took pity, ordered a new cover and gave it to me no charge. They warp if they get too hot, and even a new gasket leaks after a few runs. I installed them dry, I installed them with sealer, the Dry lasted longer, but the real fix was the new...
