kz400 carb issues

Thanks to everyone who helped me get my bike rideable in time to enjoy the last couple of weeks to nice weather.

Actually riding reveals there's still a couple of things to be worked out :

Warm, it'll idle varying between 1200-1500, and occasionally drop lower and stall, unless I blip the throttle. If I give it more idle screw, it will instead occasionally take off and idle stably at 3000 rpm. The pilot screw is 1.5 turns out from all the way in. Changing it doesn't seem to change anything.


Below 3000, it doesn't want to take any load. It pulls strongly between 3000 & 4500, but falls off after that - it won't go above 6000 even with full throttle.

It has pod filters and a 2-2 exhaust with crossover tube from a 440. Carbs are stock, don't know about the jets. I haven't tried a plug chop, but after a normal trip, the plugs are sooted.

I get equal but low vacuum readings from both carbs - 11" at idle. However, my gauge has the restricting valve right before the gauge itself - I'll have to try this again with a valve all the way at the carb end of the hose, if that matters.

The carbs looked squeaky clean when I had them apart, and the float levels were fine then. One diaphragm had a 1/4" tear that I patched with Plasti-Dip that appears to have not fallen off yet.

As far as I can tell, other things I need to do are :
Recheck float levels
Check compression (have to find a gauge to borrow)
Check ignition timing (have light, but no dwell meter)
Check valve clearance

What else can be going on ?

almost sounds to me like you need to do a carb sync. Just make sure they are pulling together, you know?

Electric Monk wrote:

I get equal but low vacuum readings from both carbs - 11" at idle. However, my gauge has the restricting valve right before the gauge itself - I'll have to try this again with a valve all the way at the carb end of the hose, if that matters.

It matters. The restrictor, ideally, should be right at the carb port. (I assume you are using the small threaded ports on the carb-block with a threaded pipe?) The vacuum is in pulses and the tubing to the gauge affects the pulses. The air in the tubing will move too much if the restrictor is at the gauge. Putting the restrictor at the engine will prevent the excessive air movement in the tubing.

But then the problem is that the restrictors affect the reading. To correct this you need to calibrate the restrictors to make them equal. That's easy to do. You hook up a Y splitter to one vacuum port on the engine and hook up the manometer to the Y. This way you know both gauges are getting the exact same vacuum, so you adjust the restrictors until both gauges give the exact same reading.

The cheaper manometers use a pin-hole restrictor. The only way to adjust it is to poke the hole with a needle to open one up slightly. Be careful, it's pretty sensitive so you don't want to make the hole too big.

Post edited by: loudhvx, at: 2006/11/10 12:10

Regarding the carbs, that loss of power on the top end usually is a slide that is not opening on those carbs. Pull off the filters and look into the carbs while revving. Both slides should go all the way up at the same time. If one doesn't, I suspect the diaphragm is leaking, but there can be other possibilities. I also put ONE drop of oil on the slides in case they are binding. BUT DON'T GET TOO CLOSE, STAND IN BACK OF THE BIKE TO VIEW THE CARBS!!! A 3-FOOT FLAME CAN SHOOT OUT OF THE CARBS IF IT BACKFIRES!!!

Not having an airbox will usually require a re-jet, but that is trial and error. I did it on a 76KZ400, but it had diferent carbs (piston slides, not diaphragm slides).

Thanks !

I threaded a plastic tube and stuck it into the normally covered ports on the carb. I suppose I could use some teflon tape to make sure it isn't leaking there. I'll try again with the restrictor on the end.

Electric Monk wrote:

...after a normal trip, the plugs are sooted....

Definitely do the checks you mentioned, but when it comes down to it, you're running rich. So to focus on the fouling plugs, make sure you adjust the pilot screws, and float levels (lie you mentioned). You may also consider making sure the coil has good power, perhaps with the "coil mod" (direct power via relay from battery). Check the filebase for details...

As Biquetoast said, it is rich. The slides not moving up can cause that too. Definitely pull off the filters and verify that the slides move up all the way and in unison.

Ok, after freezing my fingers half off yesterday afternoon I know know that :

- Float levels are correct.
- The patch on the right carb diapgragm cracked again, leaving a 1/2" long slit.
- Both slides rise to the level of the choke plate at 5000rpm and about 3/16" over that at 6500. That's about all my neighbor's dog would take I can't see both slides at the same time, without getting too close to one.
- Spark is present, but nothing to write home about either. Plus by the shock I got, the plug wire insulation isn't all that either.
- While checking the spark on the opposite cylinder, the engine started & idled smoothly on just the right cylinder , but dies right away on just the left. Adjusting the sync screw to produce an equally bad idle on just one results in a much smoother idle on both. Except that it runs away to 3000rpm and stays there with just a throttle blip.
- Stepping on one of the fluid differential manometer end fittings doesn't do it any good. oops
- A vaccum gauge on either carb reads 3" hg at idle. I have to think this is a gauge problem - could the engine even be able to run at all if it's leaking that badly? I'll try the gauge on a known vacuum source and see what it says.

It's supposed to warm up in a couple of days, so I'll get a chance to repatch the diaphragm, check the timings and sync.

Is NRP the only source for diaphragms ? And I have to send them my slide to have the diaphragm fitted to it ? In that case it may just be a good idea to put the bike away for the winter, send the slide off and take care of it all in the spring.

Also, screwing the pilot screw in is the direction of leaner, right ?

Thanks !

If the slide is holed, the bike will run fine at idle but once the slide should be raising, and it doesn't, the needle will be lower in the seat than it should be and will be progressively LEANER I think since it isn't coming out of the needle jet when it should. I think you may have multiple problems with these carburetors... if the slide is torn, they will never respond correctly or be properly synchronized once off idle. At idle, when the idle hunts around, this is caused by an air leak. The most common source of air leaks on a Keihin assembly is worn orings UNDER the pilot mixture screws... could also be a vacuum cap or intake manifold but the little fine orings under the mixture screws do deteriorate... there is a spring which provides tensioning for the mixture screw and a washer under the spring. Under that washer a small oring is what keeps air from leaking into the pilot system. Also, on the early Keihin carbs, they have been known to leak air where the butterfly rod goes through the carb body... if the rod wobbles around and you suspect a leak, you might want to visit Odd Ivar's KZ400 web site and look/ask about how to fix that problem.

wiredgeorge wrote:

If the slide is holed, the bike will run fine at idle but once the slide should be raising, and it doesn't, the needle will be lower in the seat than it should be and will be progressively LEANER I think since it isn't coming out of the needle jet when it should.

Yes, the needle doesn't come out too far, but with the slide down, it's getting no air. It's like having the choke on. Then you also have very high engine vacuum pulling through the idle circuit as well. Carbs are generally designed such that you get a rich pulse when you crack the throttle. When the slide stays down, it's sort of like being in that rich pulse continuously.

When you pull the choke, a plunger unblocks a passage way to apply manifold vacuum to the bottom of the slide to keep the slide down.

Post edited by: loudhvx, at: 2006/11/17 13:49

Multiple problems wouldn't surprise me at all.

O-rings under the pilot screw ? If I pull out the screw, the spring falls out but nothing else - either I'm missing the washer & o-ring, or they're in there and stuck to the bore.

The butterflies seem firmly fitted. Different sites seem to say different things about whether testing with WD40 will eat the holder ?

My vacuum gauge appears to be reading correctly. Drat. Now to find a compression tester.

The slides are supposed to go all the way up even with no load ? They rise about 3/4 of the way right now. And that's with the engine pulling 1/3 the vacuum it should.

The petcock went an dumped a quarter tank of gas on the ground, but now that that's taken care of, at least one day this weekend looks good for wrenching.

Thanks !

Yes, the slides should move up all the way if you give it wide-open-throttle. It takes a moment for the slides to move all the way, but they should be up before the engine hits redline.

Maybe the air jets need to be bigger when not using an airbox. My friend's 76 400 is using the #40 air jets drilled out to #60 in order to use pods.

As a test, take out the air jet so that it gets more vacuum. Your carbs are different from his so I don't know for sure if your air jets are removable. On my friend's bike, they are at the top of the carbs, just under the slide pistons. (You have a diaphragm instead of slide pistons.)