intake problem?

'79 KZ400 - novice with motorcycle maintenance but slowly learning

currently i can start up the bike and ride it around the block for just a few minutes but all of a sudden the RPMs gradually increase no matter what gear you are in and i have to kill the engine. You can even put it back down to Neutral and the RPMs are still at around 4k. I'm almost certain it's not a problem with my throttle. Would this indicate a problem with my carb holders or would it be something else? When I bought the bike they already had electrical tape wrapped around them and they seem pretty cracked. Will new carb holders likely fix the problem or is my problem more serious than that?

del

Cracked carb holders (manifolds) is the first issue to deal with. Air leaks here will excessively lean out the motor and make diagnosis impossible. Get new ones asap.

Changing intake manifolds should be standard procedure on any vintage bike. I think the problem may not be in the intake manifolds. I would first adjust the throttle cable so some visible slack is present. There is a barrel adjuster at the right switchgear. Loosen the locknut and turn the adjuster towards you till you see some slack. If that doesn't do the trick, replace the orings UNDER the washers under the spring under the pilot mixture screws. If that doesn't do the trick, remove the carbs and look at the butterflies. If the carbs had been disassembled, these MIGHT be binding a bit and not closing fully all the time. You may have to loosen mount bolts and retighten on the brackets that hold the two carbs together. The last thing I can think of is that the butterflies are on shafts that sit in indentations. The indentations have small caps I think and these can leak. If they leak, you will get erratic idle and I would probably try some epoxy or JB Weld on the ends (outside and don't get it on butterfly shafts). I haven't done this myself as a repair but a fellow named Odd Ivar (Ibsen20000) documented this as an issue. He has a KZ400 web site and I suspect you could contact him through that web site for help doing this chore. I just remember that the carbs leaked (vacuum leak) and that is a fix he came up with. There is likely a link to the KZ400 web site somewhere on this forum.

i took the boots off last night and they were incredibly cracked. new question - will a boot for a different year or different engine size fit my bike? i have a '79 400 but these things are so rare im having difficulty finding one for that exact model. z1enterprises doesnt have it and cant order it according to their customer service.

for clarification: intake manifold = intake boot = carb holders. they are all the same thing with just multiple synonyms?

Sounds like the choke is on. Are you starting it with the choke on and then riding? If so, as the engine warms up the rpms will increase and you will have to slowly close the choke until the engine is fully warmed and the choke is all the way off.

I had that happen on my old KZ650 ('78). I had the carbs kinda gummy and had the idle adjustment cranked up very high so it wouldn't stall. As it warmed up it ran better and so the idle would move up until it was over 3k. But turning the idle down mean I couldn't start it if it was cold.

May/may not be your issue.

bikebandit.com has intake manifolds for your bike. They are called "Carb Holders". Look at the fiche for the cylinder head... they will be the same for all the 400s I think but double check by the bikebandit part number which is DIFFERENT than the Kaw P/N. Change the "Carb Holders" ASAP!

Kawasaki parts number: 16065-1002 HOLDER, CARBURETOR

Bike Bandit parts number: 560471-001 HOLDER,CARBURETOR $17.02

The same holders was used for the 1978 KZ400B1 and C1, and the 1979 KZ400B2 and H (LTD).

If Bike Bandit can't supply them, try Z-Power in England, or Klasmo Bikes in Germany. The different European Z400 models had them all the way up till 1984.