gpz750 uni-track won't start (B reg)

Hi

I have a gpz750 that I bought about 2 months ago. It ran fine at first, although occasionally took a while to come down from high revs at idle. After a while it started to stall when idling and then get stuck on around 7,000 revs (constantly not jumping about) once the throttle was opened.

I have pulled out the carbs (I think they are the Mikuni BS34 models) and cleaned out all of the jets etc. Today I put the carbs back in and now it won't start at all, it turns over but that's it! No fuel is coming from the tank even with the tap set to 'prime'. I think it may be a vacuum leak - any ideas? I've also checked the exhaust system for leaks and there aren't any. Also the previous owner but a ZX-6R Ninja exhaust on it, would this make it run too lean?

Thanks

If you are getting no fuel flow in the PRI position, then I don't think the problem is with the petcock diaphragm. From what I understand, the PRI position bypasses the diaphragm valve and allows the fuel to flow with no vacuum.

So I'd try removing the petcock and checking the filter screen to see if it is getting blocked.

You have either a major air leak in the intake system or the throttle butterflies are sticking open some. The former can be anyplace there is rubber such as the carb holders, vacuum lines/caps, orings around the mixture screws, etc. The latter can be binding of the throttle butterflies or shafts OR the cable needs adjusted. If the bike isn't starting, you should check to see if the carburetors are getting gas from the tank and then make sure the pilot and choke circuits are clear and clean internally.

sounds like its time to take the petcock apart, something aint right with it,
Until you foigure out what is wrong with it you can suspend a gas can with a hose on it to start the bike.
I bought a small shipion hose from napa, set the tank on a shelf and ran a hose to the carbs, a few pumps and gas flows

Thanks I have finally got the bike started - cleaned the fuel hose. It turned out that one of the vacuum hoses had a big split in it so I have changed the hose and the bike starts!

Only problem now is it still has the same problem as before - idles ok but if I rev it, it gets stuck at about 4,000 rpm and won't come down. I've checked and lubricated the throttle cable and it isn't getting stuck. If I turn the main idle screw really far out it stalls unless I use throttle, but can be ridden ok (doesn't get stuck at 4,000 revs) as long as the throttle is kept open a bit. As the bike has a different exhaust could this cause it - maybe needs re-jetting to match the larger exhaust flow?

Your idle is stuck at 4K rpm... I have already mentioned that your throttle cable is holding the throttle butterflies open (PUT SLACK IN CABLE!!!) or your throttle butterflies may be damaged and not shutting completely (TAKE OFF CARBURETORS AND LOOK TO SEE IF THEY SHUT COMPLETELY). If you do both these things, and the problem remains, you have an air leak.