KZ900 Carb issue

[oceantools]

Hello All. Any carburettor gurus out there? My 76 KZ900 A4 has a bit of an issue with tickover. I set it to around 1250rpm with the bike nicely warmed up. I take her for a ride and the tickover tends to jump to around 3000rpm. Give it a blip and some of the time it will drop back to 1250rpm.

Plugs have been cleaned. Carb bowls removed, jets etc taken out and cleaned. Air filter is new. Checked plugs after ride and very sooty. Mixture is correct (AFAIK - screwed fully in then backed 1 ⅜ turns).

So, I suspect it could be a jetting issue? If anyone could advise on how to check the jets and set them I'd be very grateful.

[kz900B1]

Have you checked the throttle cables ? I've had issues in the past with them seizing up or being frayed .

[zed1015]

Two possible issues.
Most common are.

Hanging idle =
Maladjusted or worn/sticking throttle cables especially the return being too tight .
Air leak on inlet boot.
Sticking ignition advancer.
Over lean mixture.

Sooty plugs ( on idle unless you have done high speed plug chops ) =
Too rich on the pilot screw.
Float heights too high.
Perished 'O' ring on the needle jet tube.
Choke plunger seal leaking.

Also tick over is a little high, average is closer to 1000 rpm.
There shouldn't be anything wrong with the jets unless they are blocked which would cause different issues to what you have.

[oceantools]

Many thanks for the input - much appreciated. The throttle cables are new and appear to be working really well. They snap closed very nicely. I don't think there is an air leak as sprayed carb cleaner about the place and no rise in revs. I'll have a look at the ignition advancer - that's a good call.

[kev edwards]

I had something like this years ago where a previous owner had adjusted the slide heights, probably to balance and they were pretty much as high as they could go and it would do this, adjusted to mid travel, bench set and then balanced when on bike cured the problem. If all else fails it may pay to look at this.

[oceantools]

That's an interesting one about the slide heights. I'll have a look at those later. Many thanks.

[z1bman]

follow this guide it will give you some useful info on the engine revs hanging back ? pay particular attention to the r/h side of the page highlighted in yellow
http://www.aloh.co.uk/renovation/

[oceantools]

z1bman - thanks for that - very interesting.

So, took the carbs off again, gave them the once-over, fitted new bowl gaskets and lightly lubed the boots with a silicon grease before assembling and checked the boots were sat nicely before tightening the clamps. Fitted new plugs. She started beautifully. The revs creeping up issue appears to have gone away so maybe it was an air-leak. Took her for a test-ride of about 8 miles and had her up to 85 (not saying if mph or kmh!) and felt really strong. She seems to be running slightly rough on tickover and hesitates when blipping the throttle. Going on the advice kindly offered so far I'm definitely thinking needle height. So that's going to be the next job - carbs off, strip down and check the needles.