Well I pulled the valve out and found pitting on the stem. Same valve bent last year so I had a valve job done and it ran fine the rest of the year. The mechanic didn't tell me why the valve bent, maybe it stuck first. Coincidence that it should happen to the same valve? What causes that? Is there something wrong with the head / guide?

What brand valves are you using, are they Inconel ?

That's what I was told.

Did you stick a magnet to it to see what it does, you might want to look at all of the valves.

What should the magnet do?

Nothing if it's inconel.

It's not magnetic.

Inconel doesn't rust.

I didn't notice any rust. I could put it in water and see what happens.