1982 KZ440 Hanging idle, seemingly no vacuum leaks

Hello all, let me start this by saying I have been working on cars, motorcycles and dirtbikes my whole life, and have been a certified mechanic for 6 years. Im going to list the entire string of things Ive done to try to fix the problem with no success, and be as detailed as I can. It will be a long read.

We picked up this bike a couple months back, its a 1982 KZ440 Belt drive. I have gone through the carbs twice now. After the first clean, it started and idled consistently, I was able to sync them, but under power it leaned out. So for the second clean I dissassembled everything I could, and soaked the carb bodies in Pine Sol for 8 hours, followed with a spray of carb clean and blowing all the passages with compressed air. After putting it all back together, it starts, idles, and runs with all the power I suspect it should have. However, if you snap the throttle from idle, it will hang around 4000 rpm or more (tach cable is broken so going by ear). Kill switching it for a split second will let the rpms drop, and it will happily fall back to idle and sit there and idle smoothly all day long, untill you snap the throttle again. Sounds like a vacuum leak right?

One of the very first things replaced were the carb holders, new reproductions from eBay. They are fully coated in rubber, didnt think they would need a gasket between them and the head, seller stated they didnt need a gasket, but maybe they do? I should mention, it runs the stock airbox, the boots between the box and carbs are soft and pliable, with no visible cracks even when you bend them trying to find cracks. The stock vacuum petcock has been replaced with a manual petcock that I had around (from a DR350, fit right in place). And the throttle cable is moving freely and returning normally.

Ive been using carb cleaner trying to find a vacuum leak. Sprayed carb holder to head, carb to carb holder, airbox to carb boots, carb tops, throttle shaft area, vacuum port plugs, cylinder base gasket, cylinder head gasket, everything you could possibly think of, with no change in rpm. My next thought was maybe a lazy slide. Popped the tops off and they are moving freely. I popped the carbs off to feel the spring pressure and how they returned with the spring pressure on them. I thought they seemed a little slow, so I grabbed a pair of springs from a junk set of Mikunis I had around, they were the same diameter. I stretched them a little and tried them on the bike, with no change in the problem.

Next thought was the ignition advancer. It was seized up pretty bad when we got the bike, and I managed to unseize it and make it move freely at the time. Thinking, maybe the springs are weak, I stole the springs and weights from another advancer that came off a 1980 KZ750 (Ordered off ebay cause it was cheap, didnt realize it rotated the opposite direction, otherwise it looked identical). I changed the weights and springs, no change, timing is dead on.

Back to vacuum leak checking, I made a smoke tester that a fellow on youtube came up with, involving a rubber glove, bike tire pump, a mason jar, soldering iron, and baby oil. Carburetors off, I put each cylinder one at a time on the intake stroke, put my smoke tester on the carb holder, and tested. Not even the tiniest bit of smoke out of either cylinder. I also did each carb individually, throttle plate slightly open, choke open, the engine end of the carb duct taped closed, and my smoke tester over the inlet. Again, not the tiniest bit of smoke. Started thinking about throttle shaft seals, I remember seeing on the internet that alot of keihin carbs use a felt seal for the throttle shaft, mine had nothing at all. A plastic spacer with an aluminum washer that rested on the carb body, but nothing between that and the throttle shaft bushings (which are tight). So, I made a makeshift seal using 5/16th vacuum hose, thinking it couldnt hurt anyways. Put them back on and no change.

I also want to mention, It got new plugs in it at the same time as the carb holders. They have 75kms on them roughly, they appear normal in color, maybe slightly on the black side. Seems inconsistent with my issue, that really seems like a vacuum leak? I also would like to say, I have checked valve clearance, and Ive done a compression test. Cold, #1 makes 140 PSI, #2 makes 165 PSI. Id like to see them a little closer together, but I dont think that would attribute to my problem, but that was also checked the day I brought it home, I have not checked since its been run a while.

That brings me up to today. Ive run out of ideas in my head of what to look at next. Should I try making a paper gasket to go between the carb holder and head? Maybe there is just something horribly wrong with the carb bodies themselves? ( have a set of mystery mikunis on my workbench that have Identical inlet and outlet diameters, Im thinking BW34s, they have XS400 written on them?) Im just not sure where to look next.

Cheers!

Just a thought. I just put the carbs on my 440 and the idle would hang abut 3500 and then slowly come down. I checked for vacuum leaks and all was good there. I then blipped the throttle while watching the carbs. All seemed good. The cable was slightly loose when throttle was released with about 3mm of play. Then I tried loosening the throttle cable a bit more and the idle came down like it should. You might want to try that.

Does the idle eventually go back down after a little while?
If so, check for pin holes in the diaphragms. The normal wear pin-holes can be hidden right up close to the slide piston. The lip of the plastic retainer can cover it a bit. You have to stretch the diaphragm a tiny bit to see them. You will see wear spots where the diaphragm folds through normal use.

The pin hole alters the movement of the slide and can cause a bit of fuel to accumulate in the port off idle, then at idle there will be more fuel than normal. After that extra fuel gets used, the idle drops back to normal.

Another related symptom is that the fuel mixture screw might become less responsive or less repeatable.

Sorry for the very late reply, life gets in the way sometimes.

Ive been playing with the fuel mixture screws from time to time, I can get it to where sometimes it returns to idle ok but also sometimes hangs up. When it does hang, sometimes it settles itself out, other times it doesnt. I just went out and checked, I cant see any pinholes in the diaphragms.

However I was playing with it today and it seems ok, but ill have to try it a couple other times and see what it does. I think I am at ~3 turns out on each mixture screw, my manual says it should be 2-3/4s. Sometimes when its sitting there idling it will just sputter once and shut off, I can only think to describe it as seeming like the ignition timing suddenly became very advance for that one instance, and stalled it. But that is also very inconsistent.

Shot in the dark, perhaps the timing chain is sloppy?

Cheers

Sounds like carbs. Did you vacuum sync? You can go up to 3.5 turns open on the pilot screws before they become ineffective, at which point it's best to stop.