That air box set up:

I think it was way to restrictive in my opinion and that Engine struggled to breath at WOT (Wide Open Throttle) (Wide Open Throttle).
Here is why (I am being generous here with the numbers):
You got a 500cc Engine that spins 8500 rpm.
So that's 500 x 8500 = 4250000 cc of air.
That's 150 cu ft per minute of air needed.

No goddamn way are you pulling 150 ft through that set up.
I think that was the sputter we heard on that video.

I may be wrong here so feel free to shoot at me but you better show the math.

Even if you did plug chops you would have had to jet down to get there.
CO

I actually like that air box because I can see what you were trying to do there.
But I see a problem with it. I don't think it was supplying enough air with that set up the way it is. It seems way to restrictive to me but maybe others can chime in and correct me.

That carb: I think I remember seeing it here in another thread. We talked about it and if I remember right, never really came to a conclusion. Not sure now as it was a while ago. Unless you got info on it then you may be chasing your tail.
That's a Boyesen super bowl on there I believe. It has a curved interior which turns fuel in on itself, preventing internal splashing/foaming. That's the theory anyways.
CO

PWK72R IS a PWK39. Nice fuel bowl you got there; if it fits

I like the air box... can’t say if it flows enough air or not for certain but if it ran well before, it likely does in certain conditions at least. I can tell you that I had a “modified” banshee that was a beast and one day after preparing for a ride I rolled it off the trailer and putted off to the sand pit through the trails to warm her... one problem, it just wouldn’t rev... after a lot of frustration it turned out it was a different oil I used on the filter.... swapped it out and all was right in the world. Two strokes are particular, some might say picky.
Have you changed air filter or oil recently?

Bought a couple of containers today to try and make bigger airbox

Redskin man, I did some math on filter sizes for you.
Spare every one the head ace (math).
If you have a 2" hole the filter height would be 5.5 inches tall.
If 3" hole 4.16 tall.
These are the minium for you primary filter. For secondary add 1.25 for height. Or second filter with adaqaute square inches.

If you want I'll post the math.

if I read this properly you said your first ride out was with the air box on and it ran good. then you took it off for the rest of the riding that you did. that would of leaned out your jetting..I have been trying to dial my Oddy in and with a 39mm pwk same as yours I was at a 230 main jet and still lean at WFO, with my air box lid off. I have since started over and put the air box lid back on, and I have dropped to 185 main.

if I read this properly you said your first ride out was with the air box on and it ran good. then you took it off for the rest of the riding that you did. that would of leaned out your jetting..I have been trying to dial my Oddy in and with a 39mm pwk same as yours I was at a 230 main jet and still lean at WFO, with my air box lid off. I have since started over and put the air box lid back on, and I have dropped to 185 main.

It looks like the top ring may have rotated past the locator pins? Take the bottom ring off the piston and push it into the cylinder. Measure ring-end-gap and post back.

Also measure your current piston-to-cylinder wall clearance. I'd say if the clearance is <8thou you might get away with a new piston and just a hone, but is doubtful. Think about your piston costs, hone costs, and consider going to 90mm. BTW on a CR500 sleeve you should be able to get all the way out to 91mm on Wiseco. that's a lot of bores/years.

Finally on your O-Rings. This one is easy.

The inner O-Ring you should use measure the diameter of the slot on the dome, and the thickness of the slot. You can refer to various sized O-Rings at http://www.mcmaster.com. You want to be very careful with your selection however, because with the heat I would highly suggest you get Viton O-Rings for chemical and temperature resistance. Look here:
https://www.mcmaster.com/viton-fluoroelastomer-o-rings

The outer O-Ring is going to be EASY. Either you can try to lay it out and measure the diameter, ordering O-Rings 3 times, which isn't a bad idea... OR, McMaster sells O-Ring cord stock where you can make your own O-Rings. Look here:
https://www.mcmaster.com/viton-fluoroelastomer-o-rings
All you need is the diameter, and then cut the O_Ring to length. I'd use Permatex Water Pump and Thermostat RTV at the joining end to attach the O-Ring back onto itself. Clean cut the ends with a razor blade. Hell; I'd use that stuff to dress the entire O-Ring honestly. Let it dry for a few days before coolant is added. This is how we do it with large 2-cylinder JetSki heads with the outer o-ring.

I have a bout an hour of welding experience, as you can tell by my gorilla ugly welds on this pipe.
I’ve seen the commercial for those aluminum welding rods, I am guessing if I used one of those on that pipe it would just get so hot that that weld would just melt and go everywhere? I thought maybe those welding rods had some properties in them that after they were melted onto the object that may be that changed their melting temperature and it would have to be hotter to melt it the second time?

:shock: Well now I gota comment.

Lets start with the deadliest one first.
You have a paper base gasket there. That's a huge NO NO.
It MUST be metal or it will blow out.

Next item.

I am not sure if I just read this right but did you try to weld that pipe with an aluminum welding rod ??
Firstly you can't weld iron pipe with aluminum rod LOL. It don't work that way.
Secondly those aluminum welding rods are complete junk so don't waste your money. You need a proper welder for any aluminum welding.
Third you need a wire welder to weld that pipe because stick welding is way to hot and will blow a hole.
CO

:shock: Well now I gota comment.

Lets start with the deadliest one first.
You have a paper base gasket there. That's a huge NO NO.
It MUST be metal or it will blow out.

Next item.

I am not sure if I just read this right but did you try to weld that pipe with an aluminum welding rod ??
Firstly you can't weld iron pipe with aluminum rod LOL. It don't work that way.
Secondly those aluminum welding rods are complete junk so don't waste your money. You need a proper welder for any aluminum welding.
Third you need a wire welder to weld that pipe because stick welding is way to hot and will blow a hole.
CO

I would suggest going to your local MC shop and getting genuine Keihin jets, the knockoffs will work but it's better to actually know what you are dealing with, I've heard stories that they may not be as advertised so thinking you are gong up a jet size may have really gone down.