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Have you looked at an Economy Cycle aluminum repro caliper and maybe a pair of Aluminum pistons to put the cherry on top? And then thin the disk rotor and rill it full of holes to drop another pound or so.

You could add Ti or Auminum sprocket nuts and maybe replace the bearing spacer with an aluminum one or fit a TZ250C on rear wheel which has a smaller diameter disk.

OR get a tiny Brembo rear caliper from a Ducati or similar and make a custom caliper mount.

Lots of ways to drop weight.

They are made in China or Taiwan and the die casting is streets ahead of anything that Yamaha made back in the day.

Ports are close enough for a daily driver but the ports in the liner are a little smaller than the aluminum casting and can easily be cleaned up.  Main transfers are much better than OEM, but the auxiliaries need to be cleaned up a little to make them flow.

I'd recommend getting a set of OEM pistons or Pro-X and check clearances, or just get them bored to the next size up with the right clearances and know they are round. 

I have a couple of pairs that I collected for different projects that are not needed now ( I used a TZ750 top end instead)

I cut a set of R5 cylinders apart and that set had cast in liners.  Suzuki did the same thing by casting a rib around the liner to prevent it from moving. IIRC an RD or R5 liner can be bored out but you have to go much further than you would with say a Honda with press in sleeves.

http://erlenbachracing.co/rd-rz%20cylinders.htm  It's not easy to see but there's a "rib" close to the top that locates the liner if you look closely.

But to put in a new liner requires that suck a liner is available with ports already cut or at least roughed out. You could always machine the liner out far enough, port the transfers and then press in a thick wall aluminum liner and then have that bored, plated and honed.  Not sure that's within my budget though.

The Chinese cylinders are still available in MOQ of 100-200 pieces.  It's possible that VITO's bough a stack and are not buying another batch.  The market can't be too large. For some reason I have two sets.  One lightly used (v. low miles) and the other set is new and will be bored and ported and then raced.

They are made in Asia and are available - or were - on the internet.  The VITO's story I heard was that they bought them in bulk and sorted through to get the best ones to sell.

I have had two sets.  The barrel/liner mismatch is slightly better on some than others. The liner always seems to hang into the exhaust and inlet ports and needs to be cleaned up.

The casting is superb though.

Glass beads also peen the surface when used dry.  Sand will cut the surfaces and leave them open.  Fresh glass beads should not remove any surface material.  Not as clean and shiny as vapor blast though.

Try removing the filler cap and see if it flows.  If so you need a breather hole in the cap.

Maybe I was confusing when I said don't take too much metal off. I didn't mean to scare you.

Let's try to be clearer.  Machine both main ports down on both barrels to say 7mm below deck.  Do NOT machine down the auxiliary ports.  Just that mains - like that one barrel.

Then on the bridge between the two transfer ports, just round off the top (when it's upside down).  Perfection is overrated. Slightly rounded off is fine.  The motor can't tell if it's a perfect radius or just slightly less pointy, so go for it.

Ty to radius the transition from the new lower (higher) edge to let the gas get around the corner and up the transfers.  getting the bottom of teh transfers perfect is a lot less important than matching the outlets.

A slight mismatch at the bottom won't be noticeable on an old street motor.  Just do it.

What a little cutie.

Not you.  The bike..

It's not necessary.  Just round it off to take the sharp edge away.  Try not to take too much metal off.

Clean O rings, high temp Permatex and strong springs work for me.  Are the springs strong enough or are they too long to provide proper tension??

Are the jets new genuine Mikuni or used and maybe worn out.  How does a jet get worn out? By owners sticking drill bits and wires through them to get the clean.  And dirty jets need more vigorous "cleaning".  In other words they are now too large.

Jets are cheap and life is short. 

The cylinders sold by VITO's are in some ways a better design than stock OEM.  The main transfers are much cleaner and miss that huge bulge which we have to grind or mill out. The only downside I have seen is that the ports in the liners are not a perfect match to the cast aluminum.

And the die casting is so much cleaner than OEM cast parts, it's not funny. 

Cleaning up that mismatch takes a little time but not a lot.

What I don't know is whether the liner is cast in place like OEM or pressed in after the aluminum muff is machined and painted.

Since it is a very late production head, it may have been a design change that preceded the wider fins on 76 on 400s and 250s.

Don't panic, the knife edge can be made good.

The idea behind that mod is to get air flowing in the main transfers before it starts to flow from the auxiliaries. Both sides on both barrels should be the same. I don't know anyone who has tested sets machined to different amounts of cutaway to measure the effect.  What I'd do is to get all 4 at the same height and straight across - but just on the main transfers.  Do not machine/port the small ports.  Cut them as flat as possible and then just round off the lower edge to improve gas flow.

That won't make a whole lot of difference to power, but will look trick and both sides will be balanced.

As Chuck mentioned, height width and angles of the transfer port outlets are way more important to power and how the bike runs.

It sounds like an ignition issue - which usually means it's actually carbs.

I'd suggest removing the carbs and clean out all the passages and particularly the main air jet passage. If that's blocked, the fuel is drawn in in big lumps and they don't burn well.  Fuel droplets are actually liquid and it's the vapor around the droplet that burns. Air through the main air jet helps to break large droplets down to smaller ones and that provides more surface area and more complete burning.  With poor atomization you can have raw fuel in the exhaust and a mixture that reads lean.  That apparent dichotomy is because not much fuel is burning.

Those carbs should just have a drilling IIRC without an actual air jet, so it should be easy to see if they are clean once the needle jets are removed.