Permagap spark plugs

I just bought a Mercury 7.5 HP 2-stroke outboard that needed fixing. First thing I did was pull the spark plugs. They are Champion L78V’s. I noticed the electrode design was different from anything I’ve seen before. The center electrode is flat, almost flush with the end of the threads, and the spark jumps from the rim of the plug to the center electrode.

Because there is no way to adjust the gap, it’s a “permanent gap” plug, hence “permagap.” Has anyone tried this on a car engine? Maybe not practical because the plug tip doesn’t extend down into the combustion chamber. I don’t know. I just found it an interesting design and wondered if anyone had tried it in a car engine. The gap does look to be more than is typical in a car engine.

Those are called “surface gap” plugs and were tried as a method to reduce 2-stroke plug fouling…In order to work, the engines that use them usually have a hot capacitive discharge ignition system…You find them in some snowmobile engines too…They didn’t use them for very long…In practice, they didn’t work as well as in theory…

Thanks, Caddyman. Most things do seem to work better in theory. :wink:

A similar design is used in some race car applications also. But it would be a mistake to assume that anything used in race cars is transferrable to daily drivers.

The issues are complex and could consume lots of typing so to be brief- their primary limitation in general automotive use is that they actually reduce the available surface area of the discharge arc. Half of the arc is shielded from use as it migrates in direct contact across the insulator. They tend to arc over at less voltage so the peak spark energy is actually lower unless the ignition system is designed to “over power” them. Hence high capacity CDI type ignitions work best. Their anti-fouling design fails miserably if run cold (relatively). Way too many downsides to be effective in general automotive use…

Thanks, @TwinTurbo. I just discovered that the plugs in my outboard are one heat range too cold, which would explain the fouling I noticed when I pulled them. Am going to replace with the correct (hotter) plugs. Hoping that will make for less fouling and easier starting.


I have a 150hp BlackMax that, among the other sins imposed upon it by the prior owner, had the wrong heat range plugs in it. Changing to the correct ones made a world of difference.

It took a while to undo all the self-inflicted problems he introduced. The engine ran fine the first two times I had it out. Third time, same lake, couldn’t prime it. I mean the bulb was like a rock. I even tried stepping on it in desperation. We resorted to trolling for the day. I discovered he omitted the clips that connect the needles to the floats in the carbs. ??? He also put in those viton tipped needles. So driving down the road, the floats pounded the needles into their seats and there was nothing to pull them back out. I couldn’t believe how hard they were set into the seats. Replaced with OEM metal needles and clips (obviously) and solved that issue.

He also omitted the thermostats for the only reason I could imagine- this engine design is strange in that the pee stream only runs after the engine is warm. Totally ridiculous design IMO. So pulling the stats gives immediate indication of proper water flow but the engine almost never heats up. That exacerbated also using the wrong heat range plugs BTW.

Good luck in your endeavors!

I bought a set of spark plugs (Champion, I think) back in the early 70’s that claimed they would never need re-gapping. They were right, in a way, because they kept fouling out a few weeks after they were installed. I put them in the garbage can and installed a “real” set of spark plugs.

Thanks TT. This is my first experience with outboards. It’s just a small 7.5 HP Merc auxiliary for a sailboat. Carb was badly gunked up from previous owner and I feared a rebuild need…but just cleaning out the bowl and shooting the carb full of cleaner got it back to running fine. The exhaust manifold cover was cracked, too, and I got a used one on EBay. And the lower unit had no oil in it.

Yes, it does seem like half the people in the world are stuck fixing the hack jobs done by the other half.

One bit of advice- if you haven’t done so already, change the water pump impeller. They don’t last forever and failure is often catastrophic. I assume your season is coming to a close? Check for water in the lower unit before storage…I empty mine and fill fresh at end of season. I know guys who wait until spring without realizing the output shaft seal was/went bad, the lower is filled with water and then end up with a cracked lower…

Previous owner said the impeller had just been changed. I ran it at idle for an hour in a trash can and the telltale stream was strong. Am also flushing/filling the lower unit for storage.

Thanks for the tips!