hi, just a small problem that hopefully someone can shed abit of light on, should fuel flow to the carbs when the tap is turned on, or is it only when being turned over via vacuum pipe, as my fuel runs out when the tap is turned on, could this be a problem with the tap diaphram, thanx jon

When you start the engine it creates a vacume which opens the fuel tap , so it only flows fuel when the bike is running and shuts the fuel off when engine is turned off.

Another idea...

There is an HRC mod that removes the need for the vacuum to open the diaphragm so fuel will flow when the tap is set to ON. I would bet that a fair few previous owners may have done it to their bikes without needing to because it was done by HRC and therefore it must make my bike faster! I have this on my race bike and have a dry break fitted to the fuel line as well as a precaution.

I'd whip the tank off, drain the fuel and take a out the fuel tap. Be very careful opening it as there is a spring in there, but check to see the assembly is modified from standard or simply perished. Rick O does a fuel tap rebuild kit, though I'd check in with him to see what it consists of.

It's not uncommon for fuel taps to stay on even when the engine (vacuum) is off.
I have modified quite a few succesfully over the years, I have never seen a perished diaphragm.
Split the tap & you will see a small alloy insert on the inside of the endcap where the vac pipe attaches to. Use a small bradawl to remove the insert & little rubber flap behind it.
Reassemble & the tap should now work perfectly.
It seems that with age, the rubber flap prevents the chamber filling with air (closing the tap) when the engine is turned off.

Phil

If it's a standard tap, check the vaccum line is connected properly and not split or leaking.

I've seen a few older taps where the rubber expands slighty, they all work perfect but are slow if not impossible to close due to the expanded rubber.

hi, thanks for replies & info, will check vacuum pipe and rubber, plus the alloy insert & rubber flap behind it asap, thanks again, jon