KZ400 Idle issues. I'm stumped.

If you still have the original jets I'd install them. Aftermarket carb kit jets are typically inferior quality compared to the original Japanese made parts.

Spraying crap around the carbs is a poor test for vacuum leaks. Only gross leaks can be detected this way. A creeping idle when warm is a sure sign of lean running, and air leaks are the prime suspect.

I'd try opening up the pilot screws to at least 3 turns and vacuum syncing the carbs as the next step. After you restore the stock jets that is. While the carbs are off you might want to spray carb cleaner through the pilot circuit and confirm the passage is open all the way to inside the carb throat too.

Lastly, make sure your airbox is installed, filter installed, and everything is 100% sealed. Any air leaks in the system can cause lean running.

Good luck

I only have some of the original jets, so i figured it was better to have all matching aftermarket than mismatched stock and aftermarket. Many screws and jets on these carbs had been stripped in some manner by the PO. I ended up drilling and dremeling out something like 7 different screws and jets. Some poeople...

The more i thought about it the more is seemed like an air leak to me, so i pulled the carbs and found the tiniest hole in the diaphragm. This very well may be the culprit. I sure didn't see it the first time around, but as i let the bike idle more, the problem seemed to get worse, so perhaps it was getting bigger. The good news is there is a company making reproduction diaphragms out there so i have ordered those.

jbmindustries.com/Keihin_diaphragm.html

I'm absolutely sure the hole in the diaphragm is not related to the idle issue. In fact CV carbs will allow for perfect idle even with the top caps and diaphragms out of the carbs.

Thats interesting. The diaphragm would need to be replaced anyway so once that gets here i will tackle this again.

So I got the new diapraghms in. When i had the carbs off i realized i did not do a very good bench sync, corrected that and to be safe i shot some carb cleaner thorugh the pilot circuit. Got them back on and the bikes turns over very nice, however once it gets warm it will not idle below 2k rpm. The idle does not creep up like before, it just sits at 2000 with the idle adjust backed all the way out. If i blip the throttle it goes right back to 2k. I tried to do a carb sync, but i can't even calibrate the thing because it is pulling too much vacuum. It just tries to suck the fluid all the way out.

suggestions?

BTW i carefully inspected the manifold boots and they don't appear to have any cracks and are quite supple still. I took them off and put a very fine coating of 1194 on the mating surface to make sure there was no leak. THe carb boots seem to have a good seal but they are kinda hard. The airbox and stock filter is in there properly.

So, not sure if you mentioned that you have played with the pilot adjustment or not? when I had the same issues you are faced with, I adjusted the pilot screws and I would get the idle close to where I wanted it, 1000rpm.

I still had the issue of the bike running on two cylinders until you rolled the throttle to higher revs. I ended replacing the pilot jets and all is good.

I have the pilots set at 1 & 1/4 turns out per the FSM. The manual says to adjust the pilot screws until the highest possible idle has been achieved without going past the limiter, which i don't have. I will try turning down the pilot screws, but it seams like that might be only a temporary solution and not solve the real problem. This may seem dumb but is a "turn" 360, or 180?

A turn = 360°. Keep in mind that closing the fuel screw leans out the idle mixture.

My mantra for fixing bikes should be "When in doubt start throwing new parts at the problem". I got some intake manifolds off a later kz400 and filed the mounting holes to fit as the they were otherwise exactly the same. I also i discovered i may had been overtightening the airbox boots causing them to buckle because the rubber is hard. Regardless, i threw the carbs back on and it seemed to idle very nice around 1200, but I didn't want to leave it running very long as it was so late (neighbors) I will attempt to sync the carbs tomorrow.

seems like i have just scratched the surface of the problem. I wasn't able to sync the carbs because on a previous attempt when the vacuum was pulling too strong it sucked some of the sync fluid out. But it was seemingly running well enough i decided to take it for my first spin. It bogs down around the 4500 mark and wont go over 5500-6000 when under load. When its sitting in neutral it will rev till redline. after riding the bike the revs wouldnt come down past 3000, but if i let it sit for 30 minutes it will start right up and idle fine even after letting it warm up and blipping the throttle.
I checked the plugs and was very surprised. number 1 is lean and number 2 is rich. I checked the compression because the head gasket is leaking pretty good on the left side (which apparently is common) , and both cylinders have 170psi. I just got some OEM pilot jets so i guess i will put those in even tough i don't think thats an issue anymore. Also gonna check the float levels and the timing advancer. Any suggestions?? this thing is driving me crazy, but from first impressions it seems like a fun little bike, especially when it has an even power band.

DoctoRot wrote: ... number 2 is rich... I checked the compression because the head gasket is leaking pretty good on the left side (which apparently is common) , and both cylinders have 170psi. I just got some OEM pilot jets so i guess i will put those in even tough i don't think thats an issue anymore. Also gonna check the float levels and the timing advancer. Any suggestions?? this thing is driving me crazy, but from first impressions it seems like a fun little bike, especially when it has an even power band.

....

Perhaps the fuel level is too high in #2 carb and causing an excessively rich mixture.

If not already done, would perform the clear tube test to assure fuel levels are within specs.

Good Fortune!

I had a chance to mess around with the bike. The rich cylinder's float was at the upper limit of the acceptable range so i adjusted to to be in the middle. I also repaired my sync, so i was able to sync the carbs.It still has the idle issue once it gets hot! Although i took it for a brief spin around the neighborhood and it seems to run much better after the float level and carbs were synced.

I wish i could get some new carbs boots for this thing so i could take that out of the equation but i cant find any. Also im not sure i'm setting the timing right. I am setting it per the FSM but when i check it with a timing light it appears to be slightly off at idle and when i rev to 3500 it doesn't align with the right of the two marks. it aligns with the left. I pulled the points plate off and checked the advancer and lubed it. it seems ok, the springs are a bit soft it seems but everything seems to work, and i don't think this would contribute to the high idle

any suggestions for carb boot replacements?