Modern Vespa

Yesterday I was riding my 300 GT in dense traffic and gunned the throttle hard but instead of the expected 'hyperboost' of acceleration, it had the opposite effect. The engine suddenly shut down and lost power like a deflating balloon.

My first thought was the spark plug cap, since I recently replaced the spark plug, so I reached into the engine with my leather glove on and tried to press it in, but it seemed normal so I ruled it out.

Now, when I replaced the spark plug some time ago, I noticed it was the NGK CR8EK version, which has a bare stud where it connects to the cap. The dealer told me the correct plug was the NGK CR8EKB version, with a nut on the connector.

Both plugs will work, but only with the right cap. Apparently, someone had replaced the cap on my 300 with an aftermarket NGK that works with a bare connector, so I couldn't use the stock spark plug with it.

Long story short, I couldn't start the bike so I took it to my regular mechanic, who also suspected the spark plug cap, only to find that it was missing the metal retainer inside, and had in fact popped off from the spark plug, causing the engine to shut down. When I first replaced the spark plug I should've noticed that the cap did not make a hard click when I fitted it on.

I think because of the nut on the connector, the factory cap holds on better, so I decided to fit original equipment and prevent this from happening again.

In case this happens to someone else, the factory spark plug cap part number for the 2008 300 GT is 969348, and this works with the NGK CR8EKB that has a nut on the connector.

Similar issue to be aware of.

I'm running the Denso Iridium IU24 with the screw on nut and factory cap. Twice now I've had the nut unscrew off the plug and the cap fall off. Loctite doesn't work, tried high temp epoxy last time, so far so good.

What happens with the Loctite? It's always worked well on sparkplugs caps for me.

Another trick is to use pliers to slightly damage the threads, just so there's some interference when you screw the cap back on.

Yeah, tried that too, with the Loctite. If it fails again will source a different plug, IU24 only comes with nut.

That's interesting because with the aftermarket cap still on the bike, I tried unscrewing the nut from the factory spark plug but apparently they come glued so it's impossible to remove them (or at least I couldn't).

Put a small nick halfway down the threads with diagonal cutting pliers, force the cap past it and fully seat the cap. It will stay put now.