KZ400S Engine Races on Start-Up

I just got my '76 KZ400S running, and after tweaking it a bit - I set the points and valve clearances - I'm happy to say it started on the first kick!  But it immediately jumped up to a VERY high RPM, so I shut it down and consulted the original shop manual I have and tried adjusting the idle that way, but No Good. I turned the idle screw on the side of the carb almost all the way out, to the point it nearly came out, but it still revved very high - like, 'Holy Crap!' high!  I turned out the fuel screws to 1 1/2 turns out, but no luck there either. (But at least it runs! Yay!) 

What would make it do that?  Air leak, maybe...?? (It'd be a freekin' BIG air leak!) It doesn't act 'air leaky' - there's no slow climb up, it just jumps immediately into what sounds like 5,000 RPM...any ideas?
 

Check the routing of and the slack in your throttle cable. If it's binding or if there is no slack in the adjustment, it will hold the throttle open; adjusting the idle screw, etc. will not affect this.

Put the idle and fuel screws back to their recommended settings, disconnect the throttle cable from the carburetors, and start it again.

Also did you have the choke on or off when you started it? If you started it with the choke on, try starting with it off or on just a little bit.

Either throttle cable, or vacuum leaks, would be my guess.  And please, don't be fooled by the thinking that spraying stuff on the carb boots will always find vacuum leaks, because it doesn't work that way.

Yeah, it was the throttle cable - I had it adjusted all the way out and must not have dialed it back in when I installed it.  (DOH!)   I have it idling well now, letting it run and getting new oil to all the nooks and crannies. I was really psyched when it started on the first kick; and still kicks right over, too!   Nice.  

Btw, what's the setting for the fuel screw? I have mine at 1 1/2 turns out, it seems to be doing fine....and just offhand, while I have you guys here, can you run synthetic 10W-40 in these bikes? I read somewhere that synthetic oil may be too slippery for vintage bikes...

The shop manual should have the fuel screw setting. Is it a Kawasaki manual or something else, like a Haynes? There are manuals available in the KZRider file storage area.

Synthetic is fine but you need the kind that is intended for diesel engines. Typical synthetic oil for cars contains friction reducers that will make your clutch slip.

It's the official Kawasaki shop manual, from Kawasaki Heavy Industries. I'll have to go do some digging, but where I have it set now seems pretty good. The official shop manual is very clear/well written compared to a lot of other manuals I have. I'll pick up some oil for conventional transmissions. The oil I drained out was black, but I of course refilled with fresh 10W-40 conventional (not synthetic) I had on hand, and I expect to have to drain this very soon as soon as it 'rinses' the engine. Thanks for the tip. 

It sounds like you have found a good location for the screw setting. If you have access to a color tune or like tool you can check and see if your setting is correct.

Not sure this will of any help but I attached a table I pulled down from the kz400 site.  You will not that the setting is noted as pre-set, meaning they did not want you to alter it

You do not want transmission oil. Ideally, you want an oil that meets JASO-MA specifications. What you do not want to use is automotive engine oils that have lots of friction modifiers because they can make the clutch slip. Most engine oils designed for diesel engines do not have the modifiers and some actually have the JASO-MA rating.
Beyond that, you want to change the oil and filter at least as often as the factory manual recommends.