A PO has jammed something in the small port that vents the decompression valve to the top of the barrel. It looks like a small bearing ball and was probably hammered in. I can try to drive it down and through. There isn't room, because of the fin, to access it from beneath inside the main valve port. Has anyone else encountered this? Why would someone block that port? Am I missing something? And most importantly, how might I get the blockage out?

The thread for the decompression valve is stripped but I can helicoil that. I should have checked all this previously I know, and actually had the barrel and carby on before I realised. Now I have to make a new barrel gasket as well!

Oh--i must admit i did a similar thing when i bought a new RT3 cylinder & the very last of that model had the huge decomp from the next DT360A. So i fitted a ball bearing big enough & screwed in what i had with that thread & made it so it "just" lightly compressed against the ball at the cylinder wall. Threaded thing was a swing/arm bolt out of a YFM250 huge, about 27mm at a guess. Anyway, not doing myself any favors by telling you my dodgy tricks lol. So to remove your ball bearing is it down the hole to the exhaust port outlet in which case you'll somehow have to removed the blank off up on top that's inline with 'that' hole & where that 'down hole' was originally drilled from. Or am i confused again as usual.

The hole that is blocked is the small hole that vents from the decompression valve up to the top of the cylinder, between the cylinder and the head. So the blockage has been hammered down from above when the head was off. I can see no reason for doing this, if the valve thread was stripped and leaking this would not fix it. If someone wanted to remove the valve the would block the port behind it with a ball and bolt as you have described.

Yes i blocked mine at the cylinder wall but had a good thread to screw something in to press against the ball. Pic of yours would be good if pos, as without looking at mine & if the head was 'off' then on top is something 'factory' blocking of how they originaly drilled the "down" hole to the exhaust outlet. Not sure if the tapped a little bung in that first entry point then glued it over but that would need to come out [not sure how] to enable a straight drive down to whatever's in that hole. Are you sure it'ts not just carbon if you can see it from the port outlet area where pipe hooks on as can get at it with a long drill i think & drill upwards. Not if it's a ball bearing though. Hole will be slightly angled from the lower end by memory--well slightly angled from top & bottom so it ends up close to the outer edge of port where pipe bolts up. I'll go find my spare cylinder incase i'm talking rubbish [again]. Bit busy today though. ps, i've only got one 360 left & not running a decomp but cylinders a bit modified so doesn't kick back at all as i kick it hard right from the top against compression & higher exhaust port reduces the strength of kickback if it catches me unaware. Plus i've probably got the timing slightly retarded for the higher compression i'm running. Anyway--woffling about nothing haha.

If you are referring to the plug where the screwdriver tip is, this is plugged off at the factory. The hole that is drilled down through the top exits into the the exhaust port. The hole in the side of the cylinder is drilled into this at 90 degrees. So the top is plugged to keep the exhaust from making a mess across the top of the cylinder and bottom of the cylinder head, the decomp valve bypasses into the exhaust port. No mess.

Thanks for the pic as i just went down the farm to get my cylinder for a pic & came home without it--forgot key to my shed haha. Ain't fun getting old!!. & you explain it better than me. All good.

The first diagram shows the ball. Arrows show exhaust flow. (Decomp valve not in the diagram).

Thanks, it all makes sense now. I wrongly thought that the plugged drilling was the release port. This site is a great resource for dummies like me!

I went down that route with my RT2 as well!

The material that was smeared over the top that area looked so amateurish that I thought that a previous owner must have been arsing around with it. I rubbed all of this away, but the folks on here put me right before I could do any further damage!