KZ400 Idle issues. I'm stumped.

Im hoping some one here can help me out, Im scratching my head on this one
1978 KZ400 Deluxe. an odd breed, having the early 400 engine but the later Kehin CV34 carbs.
I have gone through everything. adjusted timing, valve clearance adjusted, carbs completely broken down and cleaned, then rebuilt with new carb kits, new plugs. new air filter. manifold boots carefully inspected for cracks, as well as the carb boots. new battery

It starts up easily and idles fine at first but as it warms up the idle climbs to about 2500RPM and wont come down unless I turn the throttle back so the decell cable pulls. The idle adjust is all the way backed out. It seems to be running a little lean by the plugs. I have a gunson color tune, and i put that in and determined the air screws seem to have little if no effect on the combustion. Screwed in all the way made no difference. The color of the combustion was only blue, i could never get it to go orange even when snapping the throttle open. I thought it was an air leak, so i sprayed brake cleaner on the carbs an boots and it made no difference to the engine speed. The throttle cable setup is a bit strange to me, you're supposed to have slack on the decel cable, so if you turn it past where the spring stops it, you can actually bring the RPM down.

What the heck is going on?

It starts up easily and idles fine at first but as it warms up the idle climbs to about 2500RPM and wont come down unless I turn the throttle back so the decell cable pulls. The idle adjust is all the way backed out

Sounds like a cable issue, personally I never use the deccel cable, just the opener against the return spring tension. Try removing that fist, you may need to change the deccel cable.

I thought it was a cable issue too, but i disconnected the decell cable and the problem persists, with loads of slack in the acell cable. The air screws not having an effect makes me think that perhaps they aren't seating correctly or something, letting too much air in and causing it to run lean. When i got a new carb kits i did a cursory look over to make sure they were the same, and they appeared so but i didn't scrutinize them to be sure. Other things i am now considering are; float heights, and the jet plugs leaking letting too much fuel in.

Did you comapare the replacement jets with the stock jets?

Hi Vic, sounds to me that you need to vacuum sync the carbs. By the symptoms, appears that one of the carbs is bottoming out while the other is still a bit open pulling revs up. When you apply closing force you are slightly bending certain components forcing the "opened" carb to close and bringing the revs down.
Cold and on choke this might not be as noticeble.

As a side note, if the Colortune is showing a blue flame, then the mixture is spot on.

Doc, I've been meaning to ask, what is that on your foot?
Steve

Z650Spirit wrote: Hi Vic, sounds to me that you need to vacuum sync the carbs. By the symptoms, appears that one of the carbs is bottoming out while the other is still a bit open pulling revs up. When you apply closing force you are slightly bending certain components forcing the "opened" carb to close and bringing the revs down.
Cold and on choke this might not be as noticeble.

I figured i couldn't sync the carbs until i figured out why it was changing so much. I will do that and report back.

Z650Spirit wrote: As a side note, if the Colortune is showing a blue flame, then the mixture is spot on.

I thought a straight blue was lean. Ideal color being a whitish blue, just above the yellow/orange.

Kidkawie wrote: Did you comapare the replacement jets with the stock jets?

Yes. They are the stock jets, with stock exhaust and stock filter in the airbox.

swest wrote: Doc, I've been meaning to ask, what is that on your foot?
Steve

Lol. I was in West texas in the spring on my KZ1000 years ago, I'm scooting along and i see this brown mass on the road, as i approached i realized it was a shit ton of tarantulas. I stopped to take a photo and when i did this one crawled on my foot.

[quote="DoctoRot" post=678023
I thought a straight blue was lean. Ideal color being a whitish blue, just above the yellow/orange.
[/quote]

Anything in the blue range would be OK. Slightly lean and the blue flame disappears from sight completely, meaning you won't see anything in there.
The onset of a blue flame is a green one and a tad rich at idle; as soon as it turns really blue would be the sweet spot.

We have black ones here. They're aren't dangerous but very friendly.
Steve

Got a link for the color tune?
Steve

No eBay APP ID and/or Cert ID defined in Kunena configuration

swest wrote: Got a link for the color tune?
Steve

This would be the correct one for your bike, Steve.

I had the same issue with my KZ1000p. would idle too high, and it would idle on two cylinders instead of all four. I first thought I had a bad ignition coil. If I rode away, from idle to higher revs, the motor would suddenly run stronger and it ran on four cylinders.

I had done the same as you, took carbs off a few times, cleaned them, adjusted everything put it all back together and would end up with the same thing, rough, high idle.

In my case, I went into the carbs yet again and, changed out the pilot jets, which control the idle circuit. Done deal, all is well now! Good luck.................