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Twin Mikuni carbs hanging high 02 May 2021 11:26 #847874

  • Pagala
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Hi everyone

I've got a 1981 KZ440, and have fitted Mikuni VM-type round slide carbs, as the original carbs were damaged beyond repair. The air filter was missing, so it's running pods.

I've got a whole bunch of spare jets for these carbs, plus a Colourtune. So my plan was to get them working just well enough to begin the carb tuning process.
I've constructed a 1-into-2 split throttle cable using a dedicated kit. It definitely isn't catching anywhere.

The problem I've got is, I can get these carbs to idle and even to run, somewhat. But once the revs reach past, say, 3k or 4k rpm, it will hang. Then it will take a purposeful blip on the throttle to bring the engine back down to idle.

I've spent all day today, until well into evening, trying to chase down this problem. Here's what I've done:

1) I've made sure the throttle cable is completely smooth, with no kinks in it.
2) I've checked (by hand) that the slides open exactly when I begin to wind on the throttle.
3) I've set the (air side) mixture screw to 1 1/4 turns out, which I believe to be factory.
4) I've made sure the idle speed screw is set exactly the same on both carbs, by winding them both out until the screw was just touching the slides, and then out 1.5 turns.
5) I've made absolutely sure there is no vacuum leak.

I've checked all the above several times. I've opened up the throttle switchgear and inspected and lubed the throttle cable from there. I've also checked it at the 1-into-2 joining part. It's fine. The springs in each carb snap the slides down nicely.

My head is fixated on the idea that it's a carb  imbalance problem, but with my best hand sensitivity I have adjusted these as finely as humanly possible so the slides begin to move up at the same time.

While inspecting other things, I came across what appears to be a semi-chewed wire going to the ignition points. Plus, I noted that I haven't yet checked that the automatic centrifugal ignition advancer is opening freely. Is it possible that the poor running could be down to one of these conditions and not a carb issue?

I know for a fact that the main jet in each carb is far too large, but at least they're the same size, and I was hoping this would give me a starting point.
Plus, the symptoms don't seem consistent with an excessively large main jet.

Could this be caused/helped by lowering the needles in the carbs? Perhaps they're simply getting too much air-fuel mixture all at once when the main jet circuits are active.

Any help/tips much appreciated.
1̶9̶8̶3̶ ̶K̶a̶w̶a̶s̶a̶k̶i̶ ̶G̶T̶5̶5̶0̶ ̶(̶U̶K̶-̶o̶n̶l̶y̶ ̶m̶o̶d̶e̶l̶)̶,̶ ̶a̶l̶s̶o̶ ̶k̶n̶o̶w̶n̶ ̶a̶s̶ ̶t̶h̶e̶ ̶Z̶5̶5̶0̶ ̶G̶1̶,̶ ̶r̶e̶g̶i̶s̶t̶e̶r̶e̶d̶ ̶i̶n̶ ̶1̶9̶8̶4̶.̶
1981 Kawasaki Z440 (KZ440C1)

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Twin Mikuni carbs hanging high 02 May 2021 17:08 #847895

  • Nessism
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A hanging and/or creeping idle is a sign of a lean mixture, typically from an air leak.  Did you remove the rubber carb boots and inspect them closely while flexing them for cracks?

Needles don't cause a problem like you mention.  

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Twin Mikuni carbs hanging high 02 May 2021 17:23 #847896

  • Pagala
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Yes, the carb boots were split. I've put brand new ones on. Tested while running, by spraying carb cleaner all around them. It isn't the carb boots - I've been very careful to check for air leaks.

I have a feeling it might be something to do with the springs over the slides in the carburettors being too weak to push the slides back down all the way in the face of the great pumping vacuum generated by the engine at slightly higher revs. The throttle cable snaps back and the slides do close at idle, but it's a comparatively weak spring action. The round slides themselves might be sticking slightly in their bores. I noticed that the hanging action was especially like to occur just after changing up to second gear when running the bike up and down the road. It seemed less likely to occur when revving the bike when the engine was not under load, although it would happen in that situation too.

I have a couple of Amal springs on order, and I'm going to augment the springs in the carbs by adding these when they arrive. I spent hours redoing and rechecking everything on those carbs.
1̶9̶8̶3̶ ̶K̶a̶w̶a̶s̶a̶k̶i̶ ̶G̶T̶5̶5̶0̶ ̶(̶U̶K̶-̶o̶n̶l̶y̶ ̶m̶o̶d̶e̶l̶)̶,̶ ̶a̶l̶s̶o̶ ̶k̶n̶o̶w̶n̶ ̶a̶s̶ ̶t̶h̶e̶ ̶Z̶5̶5̶0̶ ̶G̶1̶,̶ ̶r̶e̶g̶i̶s̶t̶e̶r̶e̶d̶ ̶i̶n̶ ̶1̶9̶8̶4̶.̶
1981 Kawasaki Z440 (KZ440C1)

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Last edit: by Pagala.

Twin Mikuni carbs hanging high 04 May 2021 08:46 #847989

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What carbs are these again?

The VM24SS carbs that were on my KZ650 do not have springs over the slides.
CV type of carbs do have springs over the slides.
Rob.
CANADA
forums.kz650.info/
'78 KZ650 C2, 125K kms, Rickman fairing, Delkevic ex, EI, CVK32, ATK fork brace, steering damper, braced swingarm, 18" Z1R front wheel.
'00 ZRX1100, Ohlin's shocks, fork brace, steering damper, ELS +4, Corbin seat, Gimbel bugspoiler.
'11 Ninja 250R

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Twin Mikuni carbs hanging high 04 May 2021 09:59 #847992

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2 x VM34s

I've tried a lot of different things with these now. The bike will run, but the revs will take a long time to come down to idle. There are NO air leaks and no snagging cables. This situation is very, very slightly improved by winding the mixture screw (which is air side) almost all the way in, making it as rich as possible, along with masking tape over both pod filters. The pilot jets in there are huge - near the upper end. I would say they could be 50s.

I think it might just be too much carb for the engine.
They work, and on mains they pull like a train, but obviously the hanging high is a dangerous condition and will need to be sorted out.

I may decide to go down to just one VM34, and use that joining piece sold by 11th Hour Industries. Would be better than buying a manky old pair of original CV carbs.
 
1̶9̶8̶3̶ ̶K̶a̶w̶a̶s̶a̶k̶i̶ ̶G̶T̶5̶5̶0̶ ̶(̶U̶K̶-̶o̶n̶l̶y̶ ̶m̶o̶d̶e̶l̶)̶,̶ ̶a̶l̶s̶o̶ ̶k̶n̶o̶w̶n̶ ̶a̶s̶ ̶t̶h̶e̶ ̶Z̶5̶5̶0̶ ̶G̶1̶,̶ ̶r̶e̶g̶i̶s̶t̶e̶r̶e̶d̶ ̶i̶n̶ ̶1̶9̶8̶4̶.̶
1981 Kawasaki Z440 (KZ440C1)

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Last edit: by Pagala.

Twin Mikuni carbs hanging high 04 May 2021 19:23 #848011

  • Nerdy
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2 x VM34s
 


On a 440?

Yeah, that's far too much carb for that engine. Let's put it this way: some people believe that VM30s are too big for a stock 440 and insist that VM28s are the way to go.
1979 KZ400 Gifted to a couple of nephews
1967 Yamaha YCS1 Bonanza
1980 KZ440B
1981 Yamaha XT250H
1981 KZ440 LTD project bike
1981 GPz550
2013 Yamaha FZ6R
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