VM26 carbs with Accelerator pump-Won't idle down.

:S
Okay...I need some information, please.
I'm having a problem tuning my newly rebuilt KZ900. The bike runs like a charm, starts, idles great and no miss fires or stumbles.
The problem, like many others I've read about in this forum, is the carbs do not want to come back to idle. I've checked and rechecked all of the obvious issues (new rubber manifolds, new Kawasaki clamps, non adjustable ignition advance) and I'm still having the problem.
My question is this. These carbs had the aluminum plugs covering the Pilot Air Screw. I removed the plug during the cleaning process. There are no O-rings on the Pilot Air adjustment screws. Is it possible that additional air is coming in via the Air Pilot Screws and now allowing the card to idle down correctly?
Any help would be appricated.
Thanks,

Buzz

That seems like a logical place to start! In my experience when the bike won't drop to idle there's an air leak somewhere, or the valves are out of whack. Being newly rebuilt I would assume that valve clearances have been checked and spec'd, but if not check those for sure. I'd get some o-rings that fit those air screws and reinstall them and see if the problem goes away before digging into the valves though...just my $.02

Thanks anarki650,

Yeah the engine is new including valve job done by a shop that does all of my race heads. Valve adjustment is at three thousands for all valves. Leakdown is good as is compression.
I'm an old dragracer and normally don't build street engines or tune stock carbs so this all new to me, can't say I care for it much!
Maybe I'll try to find a set of Air Pilot Jets that were made for 0-rings and install them.

Thanks again for responding.
Buzz

KruZer wrote:

:S
Okay...problem tuning my newly rebuilt KZ900. The bike runs like a charm, starts, idles great and no miss fires or stumbles...carbs do not want to come back to idle. I've checked and rechecked all of the obvious issues (new rubber manifolds, new Kawasaki clamps, non adjustable ignition advance) and I'm still having the problem.
My question is this. These carbs had the aluminum plugs covering the Pilot Air Screw. I removed the plug during the cleaning process. There are no O-rings on the Pilot Air adjustment screws. Is it possible that additional air is coming in via the Air Pilot Screws and now allowing the card to idle down correctly?....

With timing light, should be able to watch the advancer move back and forth as rpm is varied. May need to clean and lube the advancer to assure it isn't tending to stick in the more advanced position when it's supposed to quickly move back to the idle rpm position (F mark alignment).

The aluminum plugs are to prevent tampering by us tree-hating reprobates. Toss 'em and good riddance, or use 'em in a sling-shot.

The o-rings on the oem Pilot Air adjustment screws should be readily available from the dealer, as they are common to many Kaw models up through some 2007 models (per kawasaki.com).

Here's the diagram from kawasaki.com

#33 is the 0-ring, and iirc fits in a grove around the #32 pilot air screw.

It doesn't show any plug, and I don't believe any oem KZ900 Mikuni VM26 carbs had such plugs. My purchased new KZ900 for sure doesn't have them.

Good Luck!

[Click on image to enlarge view]

oem Mikuni VM26SS carbs on 1976 KZ900-B1 LTD
(Same as standard model KZ900)


[Click on image to enlarge view]

Here's the advancer that deserves periodic cleaning and lubing.

I really appricate all of the input and advice, but I still haven't cure the problem.

I'm running a Dyna S ignition with a billet aluminum advancer, no springs and static timed @ 38 degrees btdc. So I know the advancer is not the issue.

All of the carburetor float bowl vents and overflow hoses are ran correctly and the two carb vacumn ports are plugged as well as the vacumn ports on the rubber intake manifolds.

The throttle cable has been checked and double checked and has ample slack to allow the slides to return to the idle position. There is no binding in the throttle linkage and each carb slide has been checked for sticking, no issue there.

I've removed the intake manifolds (which are new) and applied silcone sealer and torqued to specifications.

Valves are adjusted at .003 both intake and exhaust and the head has a fresh valve job and the cylinder has a fresh hone and new rings on the pistons.

I'm not sure what I'll try next....maybe a large hammer!!!!!

Thanks again for the help....

Buzz

KruZer wrote:

...haven't cure the problem...advancer is not the issue...All of the carburetor float bowl vents and overflow hoses are ran correctly and the two carb vacumn ports are plugged as well as the vacumn ports on the rubber intake manifolds...no binding in the throttle linkage and each carb slide has been checked for sticking, no issue there...removed the intake manifolds (which are new) and applied silcone sealer and torqued to specifications...not sure what I'll try next....maybe a large hammer!!!!!

It's tough coming from drag racing to good streetable lower rpm performance. :laugh:

Am perhaps misinterpreting, but will suggest the following:

float bowl vents -- are to open air, usually via tubes routed to under the seat or under the bike (over the swingarm). The float bowl vent tubes are designated by red lines in the illustration shown below. They should not be capped or plugged.

overflow hoses -- one from each nipple at bottom of float bowls, usually all four being gathered and routed over the swingarm to under the bike.

vacumn ports on the rubber intake manifolds -- nipples always plugged/capped except when used in syncing procedure. Sometimes 1 of the 4 vac ports is used to provide vac to a vac-operated petcock. And sometimes there's a dedicated 5th vac nipple for providing vac to a vac-operated petcock. Unless used for a vac-operated petcock, this 5th vac nipple should also remain plugged/capped.

the two carb vacumn ports are plugged as well -- what two carb vacuum ports are being referred to here? Am wondering if these are actually the float bowl vents which are supposed to exit to the atmosphere, which aren't supposed to be plugged. If plugged, the fuel level is adversely affected.

Here's an illustration for the connections.

[Click on image to enlarge view]




Am thinking the carbs at hand don't have the dedicated vac port (yellow) for a vac-operated petcock. And likely have two fuel pipes instead of only one T-joint as shown in the pic (blue) where fuel line connects to carbs.

Good Luck!

You are soooooo right about the difference in tuning for top end performance verses tuning for good street performance......and all this time I thought I knew what I was doing. I guess we're never to old to learn...

I can't believe there's an air leak anywhere because I've been very diligent (I'm kinda anal in that regard) in using new parts, checking torque specs and reviewing my work as I go along.


Anyway thanks again for the suggestions. I've checked all of the vent and vacumn hoses (about ten times now, just in case) and they're all okay. I'm going to remove the carbs again this weekend and completely go through them, again.....

Anyway I guess this is a test of my fortitude so I'll keep pluging along and see if I can't get to the bottom of this. I told myself I wouldn't work on my next project (turbo KZ100 dragbike) until this street bike was 100% complete.....maybe someone is trying to tell me something!!!!

Thanks again,
Buzz

Am thinking the carbs at hand aren't same Mikuni 26mm carbs that came factory equipped on 1976 KZ900 models. Because the KZ900 didn't have aluminum plugs, nor an accelerator pump (uncertain about the Calif model), and did have o-rings on side located pilot air screws (see parts diagram earlier posted).

So am guessing the carbs at hand are Mikuni 26mm from some later year when the EPA got more involved, and accelerator pumps were needed because of the leaner mixtures, and anti-tampering aluminum plugs were also added.

Questions -- Is it known exactly which version Mikuni 26mm carbs are at hand, or from which year bike and model? Could a pic be posted?

The statement, "Maybe I'll try to find a set of Air Pilot Jets that were made for 0-rings and install them" raises question of whether the fitted pilot air screws are the wrong parts for the carbs at hand (perhaps from some other type carb using non-o-ring pilot screws). Is there a groove around the pilot air screws presently fitted that would accomodate an o-ring?

The statement, "All of the carburetor float bowl vents and overflow hoses are ran correctly and the two carb vacumn ports are plugged as well as the vacumn ports on the rubber intake manifolds" indicates the carbs at hand are equipped with two additional vacuum port nipples besides the usual 4 carb holder vac port nipples. Am curious about that, because thinking a dedicated extra vac port (yellow) is to provide vacuum to a vac operated petcock , and don't know exactly what a second dedicated vac port would be used for (except possibly a second auxilliary petcock in a dual-petcock situation). However, the 1976 KZ900 came factory equipped with a manual petcock, not a vac-petcock.

Depending on the exact carb involved, an air leak might be through a faulty seal on the choke plunger, but that's just a wild guess perhaps inapplicable to the particular carbs are at hand.

I don't rely soley on "static" timing for ignition (only use "static" as ball park), and much prefer strobe light "dynamic" timing as being more accurate and also allowing actual viewing of the advancer moving back and forth as the throttle is blipped to vary rpm. With the Dyna-S ignition, be sure the F mark aligns with the permanent case mark at idle rpm (same as with the old points system).

Good Luck!

You're correct. The carbs are not the carbs that were on the bike from the factory. They are just like the carbs in your previous picture except there are no ribs on the slide housing.

The carbs on the bike did have an aluminum plug covering the Pilot Air Jet (which I removed) and the Pilot Air Jets did NOT have o-rings installed on them. Since the Aluminum plugs were in place when I received the carbs I can figure they are the factory install Pilot Air Jets and that they did not have o-rings install from the factory. There are no grooves on the installed Pilot Air Jets to accommodate an o-ring.
When I said the two carbs vacumn ports were plugged I really meant that all of the carb vacumn ports were never drilled, just a boss on the carb body where they could be drilled. You’re correct; these carbs are for a manual petcock application.

I did notice that all of the other Kawasaki carb diagrams I look at all have o-rings on the Air Pilot Jet, that's why I thought it might be the problem. I would think that with the aluminum plugs removed that air could work its way around the threads of the Pilot Air Jet and maybe cause my problem.

Interesting comment about the choke plunger. I have not even looked at the chokes or removed them from the carbs....That will be my first focus when I remove the carbs this weekend.

I static time my engines because I don't use stock advancers on them. I time them using a degree wheel because I feel I get better accuracy. I use billet aluminum advancers that do not really advance??? No springs on the advancer, see picture.

I really do appricate your time and input. The choke thing sounds interesting and I'll look at them closely.

Thanks for your time...

Buzz

Not sure why the advancer picture did not attach correctly.

Just double click on the white area under my reply and it will bring up the picture.

Buzz