Hi, here's my story so far:
Bought my gtr A9 with 87k miles, rode it home, used daily then parked up for winter. Recently decided to give it a bit of a tidy up, ready for summer.
I replaced the fuel tap diaphram, it never leaked but thought it a wise move...
I refitted the tank, two days later I pressed the starter and it locked up 
Luckily the engine didn't turn over and I got away with it. Not sure why this happened (I'm 54 years old and have a lifetime of automotive building experience), but hey ho, bought another genuine kwak diaphram kit, and replaced it, all good.
Just been out today, maybe a month after above, and the airbox is full of fuel again!
Took the tank off, and the fuel tap is working perfectly, not even a single drip escaping  :
Seems to be that the bike is still 'sucking' on the vac pipe even after turning off.
Anyone had this before please?
Has anyone found a replacement on/off/reserve fuel tap and blanked off the vac tube?

Call Murph's.
The rebuild kits are not reliable.
Being cheap...I mean frugal I installrd a Briggs and Stratton inline shutoff valve. Works a charm.

Jacksdad, I agree the petcock problem should be solved, but you have another/worse problem.
That problem is; one or more of your carbs is allowing gas to overfill the bowl.
Regardless of the petcock problem, a carb should never allow gas to overflow...
Normal cause is; the needle valve is not sealing, or incorrect float height.

NOTE: I'm dealing with the same problem right now, myself...
          Gonna pull my carbs when I get a chance...

On yours, a couple of questions.
  Are you sure the petcock wasn't accidentally left in the prime position?
  Was the bike on the side or center stand?

Best of luck.
Ride safe, Ted

Hi, thanks for the quick replies!
The bike is on the centre stand, the fuel tap is on the ON pos, about 2 gallons in the tank.
The airbox was leaking fuel out of the air filter lid, not a huge amount, and no fuel has got into the engine, I removed the spark plugs and it bone dry in all 4 pots.
Number one spark plug is black with carbon, the other 3 plugs are the correct colour, all plugs are brand new.
The tank has been off all day and not leaked a drop, so its not the tap.
As cylinder one is running rich, and I've not touched the carbs yet and its done 87k, I think a carb rebuild is needed.
I can easily strip the carbs, done many 4 pot bikes in the past, and rejetted after fitted bellmouths/K&N pods etc, but can I buy a rebuild kit of some sort, rather than ordering parts one by one?

Murph sells the kits,

Also I'll reiterate it takes 2 things in sucession, for that leak... one of those things is the float valve needle failing to seal....

Keep I mind, if you do replace them, (I would, and id do all 4), you will need to verify float level again, as they will all have changed due to differences in the plunger springs on the replacement float needles.

"I refitted the tank, two days later I pressed the starter and it locked up 
Luckily the engine didn't turn over and I got away with it."

What locked up?  If you had a piston full of gas, the engine isn't going rotate much more than 1/2 of 1 revolution.  While you have the plugs out, you should probably do a hydrolock test just to be sure.  You should also check your oil level to see if it's overfilled from gas seeping pass the piston(s).

Hi. Pistons heights are perfect, engine is running well, so I think all is well...However!
The repair to the head behind the outer RH exhaust stud has failed, its blowing oil onto the exhaust and smoking me alive 
I've got a head, which I'm going to strip and recon.
Can anyone tell me the head bolt sequence and torque settings please?
Also got a new genuine Kaw camchain to fit, I'll strip the carbs whilst I'm doing the work, will be done for good then!

Replace the starter chain too! Also go to the Shoodabeen website and read a little.  Lots of good information there.
You need Overflow tubes  right away and  before you push that starter button again.  You got lucky once. Do not count on that happening again.

Thanks Mike, I can see me having a rebuilt motor before long...
I wanted Shoodabeen camshafts but he won't ship outside the USA, so I've got a good used pair to replace my pitted ones, the head looks amazing considering the high miles its done, the valves look perfect. I'm going to re-grind them all, and give the ports a little dressing with my Dremel, something I've done on my 900's, just to clean things up, nothing major.

Update: The fuel tap is still working perfectly, and I've fitted a used head, which I rebuilt.
I then turned to the carbs, stripped them completely and cleaned everything and replaced a few bits from a second set of carbs I bought.
The reason number one spark plug was fouling up was soon found...the Jet Needle 16017-1313 was missing! The jet holder had been screwed in and it was compressed and bent out of shape, and this one carb had a larger main jet fitted, obviously whoever reuilt the carbs failed to notice the missing part 
Now running perfectly, once the snow clears I'll get the mot test done, then I'm on the road