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 the back plate of the pentagon style vacuum fuel cocks as used on the J/K GT750 and many other late 1960's and early 1970's Japanese bikes - 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 !
