(EDIT.. solved. It turns out this is normal and it's done as a decompression thing under 600rpm, to make it easier to pull start.)

Thought this best in General Chat because it's not specifically ATV.. more of a general small engine puzzler

Chap brought me a lawnmower that wouldn't go. He's normally pretty good with engines so was suprised.. he'd already checked spark, fuel & most everything else.

It was banging flame back out the carb though every time he tried the pull cord so I suggested maybe the inlet valve's sticking open, take the head off.. and yes inlet valve seemed to be permanently stuck/corroded open. Nothing obvious holding it open, no damage to cylinder walls.

After freeing off the inlet valve (bit of lube spray & a few twists) we took the piston up and down through several full cycles and watched the valves were opening and shutting when they should.

Suck.. inlet valve open
Squeeze.. inlet valve shut then unexpectedly.. toward the top of the stroke, exhaust opens just a crack, for about 10 degrees of crank rotation, before closing just before TDC
Bang.. both valves shut
Blow.. exhaust open

I said I didn't like what the exhaust valve was doing & maybe the engine would not last long, maybe the camshaft was damaged, but maybe it was normal, who knows..

He was happy to try it anyway despite the risk and.. it ran fine for ages. Couldn't fault it, he's mowing the lawn quite happily.

So is that a normal thing for the exhaust valve to do that sudden slight open at the top of the compression stroke?

It's a one cylinder side valve engine, much like a Briggs & Stratton, maybe 100cc ish. One of those vertical crankshaft ones with the rotary grass cutting blade underneath the deck.

Sort of wondering if it's an emissions or an anti-knock thing maybe?

If lawnmowers do this, is it something one sees on other small engines like ATVs, mopeds etc?