Been playing with the Mk1 T500 fuel cock - this one had been butchered at some point in its life and as well Suzuki doesn't sell most of the consumable parts you need to repair them. In the photo below, the face seal item 1 is not available, but as Mikuni supplied other brands, I was able to use a Yamaha part (137-24523-00), and also could have used a Honda part (16955-268-020) as they appear to be interchangeable. items 2 and 3 are still available from Suzuki, the screen item 4 is not, neither are the filters (5). The small atmospheric vent gasket (item 6) was not listed by Suzuki but is the same as the Kawasaki part 92065-052 which is also not available, but which you sometimes see on eBay. The diaphragm item 7 can be
re-manufactured by an outfit in Germany at this link. These are the same size as are used on the Suzuki J/K triples, although the pintel in the centre is shorter by about 1.5 mm so if you do go the re-manufacture route, make certain you get yours back, and not one that fits a triple. The o-ring on the pintel itself, item 8, I replace with a viton ring of the same size (and I use the Rick Best (H2Rick) method of using a bamboo chopstick to polish the seat that the o-ring seals to). The spring - item 9 is shorter than the ones used on the triples (to match the shorter pintel) and so are unique to the twins. Item 10 I'll get to in a minute.
This fuel cock had been apart previously ( the screw heads were well rounded
) and someone had drilled out the check valve in the back plate. Till a year or so ago I hadn't realised there was a valve in these - Allan Tucker (Suzsmokeyallan) mentioned it to me in passing as we were talking about methods to get these vacuum valves to seal properly. I'm pointing to the check valve in this photo.
Normally, if the check valve working, if you gently blow or suck on the vacuum line connection you can hear the valve click, plus when blowing it should shut and not allow you to easily blow through the valve. On bikes that have been sitting for years, these check valves set up (corrosion and in one case of mine a spider nest) and get stuck. If you have one handy, look at the hole I'm pointing at in the photo and you will see there is a small - less than 0.5mm - restriction hole visible. On this fuel cock, the check valve had actually been drilled out completely with roughly a 1/8 drill. I suspect the effect this would have is that the fuel cock would be slamming open and closed as there is a lot of pulsation at the carburettor inlets on 2 strokes, which in turn would have affected fuel delivery. As this valve was messed up anyway I took it apart to see what the internals looked like.
In the photo below, the cavity the arrow is pointing to holds the valve. You first have item 3 which is a small rubber gasket, then item 2 which is a small brass plug which normally would have a restriction orifice of about 0.5mm in it. This plug slides into the brass housing item 1, the head of which is what you can normally see when you look at the inside of the back plate. I have a box full of J/K fuel cocks for GT750's - some are working correctly and others are seized up - on this one for the T500, item 2 was actually seized in the barrel (item 1) whether from having been mutilated or corrosion is difficult to say. If the valve is seized the fuel cock should still work - although probably would be slow to open and so restrict fuel flow initially. And if the orifice is plugged with spider web or something else, it won't readily open at all and so starve the engine of fuel.
I released the brass barrel that holds the valve plug by using a Dremel to relieve the punch marks holding it so I suspect that it may be possible to actually repair these in those cases where you want to maintain the originality.
Anyway - on to other problems !
. As I mentioned, from one side the new ones look great - the other side is disappointing because of the lettering on the rim edge, so I've put the side with the lettering on the side stand side of the bike. That way, when its on the side stand the lettering isn't as obvious. Pity Suzuki did that, but live and learn. 
), but it is coming along . 
