KZ750 sometimes starts up but climbs to 7000 rpm

Hi! Im a new member to kzrider and new to motorcycles in general. I just bought a 1980 KZ750, previous owner said that it doesn't start but was running a little rough a couple years ago when he had it started last; and that it would probably need the carbs cleaned. I put in a new battery and tested all the plugs and petcock flow and everything looked good, but it would't start it just kind of acts like its gonna but doesn't fire up.
My next step was to pull the carbs and give them a good cleaning. I cleaned the carbs following the steps in the Clymer guide. I had a heck of a time getting the carbs back in, but was finally able to after seeing a tip online about using a hair dryer to soften the boots. I put the choke on, put it on run position and it fires up but immediately climbs to 7000 rpm, I shut it off thinking the throttle cable must be stuck or something but it looked fine. I go to start it up again and it doesn't start at all and sounds the same as before I took the carbs off.
So that's pretty much where Im at, Ive gotten it to run a couple more times the last few days but it does the same thing either starts up and quickly jumps to between 6000 and 8000 rpm's or it doesn't start at all. Im hoping to be able to do as much work as I can myself but Im getting pretty frustrated any help is greatly appreciated.

sounds like a vacuum leak to me which is when air is entering the fuel mixture where it shouldnt. make sure the carbs are seated properly in the boots and are good and tight tight.

Have you adjusted the idle screw on the bottom of the carbs? You should be able to turn it down with that.

Did you test the fuel level in the float bowl?

Mine was doing this too once after I put the carbs back on. What happened was a screw from the carb holder clamp was in the way of the throttle linkage keeping the carbs open even though the throttle cable was resetting all the way. Twist the throttle open and closed and watch the front of the carbs. Check to make sure everything is smoothly sliding up and down and the carbs are all closing completely.

Nice bike!!

If it's been sitting for some time the carb boots on the cylinder head could be leaking. Also make sure all the the ports on the boots have caps or if hoses on some make sure they are not split.

Spectre1982, I did try adjusting the throttle screw but it didn't seem to make any difference, I will make sure the carbs are properly seated and check the float bole levels this weekend. The other thing I noticed was that the bike is starting up no problem when cold but when I shut it off and try to run it warm it doesn't start up, would this also be explained by the vacuum leaks and the float bowl issues that were mentioned.

-Thanks for all the great advice!

I am watching this with great intererest as I am having the exact same problem.

I'm still new to it all myself, but I do know that the boots should be soft enough you shouldn't have to heat them to get them back on. Also did you make sure to meticulously clean all the jets in the carbs and attempt to bench sync them before you put them back on? My bike was having similar problems revving extremely high even with the choke off it had to do with the butterflies in the carbs being to far open in their idle position as well as some plugged up jets. Another thing you might want to check is that the carbs are actually resting on the idle screw when they are in the idle position and not on the screws that adjusts their sync. I just went through the same issue on my 750.