75 kz400 carbs rebuilt - idling issue remains

Hi all,

I had my 75 KZ400 in the repair shop to get the carbs rebuilt. When I got it back it ran pretty good with the exception of a little play in the idle that was manageable. Two days later the play got so bad the bike was cutting off as it idled down. It plays between 0 and 4000 rpms for no apparent reason. Adjusting the idle screw does not keep the idle stable. If I get it idled out good sitting and I ride a little way (mile or 2) next thing I know I cannot go below 4000 rpms when slowing.

Is there anything you can suggest I do to fix this? I cannot afford to take her back to the shop right now. I am afraid I am gonna bust my butt as I am a new rider with only a few months experience.

Thanks,
Zoo

zoo wrote:

Hi all,

I had my 75 KZ400 in the repair shop to get the carbs rebuilt...

That sucks. I hate shops. They just dont care enough to take the time to make sure its ACTUALLY working properly.

Well... there are a few things to check/eliminate in order to figure out whats going on.
I would seriously consider getting a service manual. Clymer, or OEM.

#1 would be air leaks. You have to make sure that the carbs are installed absolutely air tight. It is a must! Just tighten all the connections from the engine to the air box snug. You can test for an air leak by spraying some WD40 around those connections. If there is a leak, you will hear it immediately as the WD gets sucked in and the rpm will change quickly.

#2 Fuel height in the bowls. After a carb rebuild, these almost always change, and they are critical, rarely checked by shops. You will need a 1 foot (or more) length of hose, that will fit the small drain spout on the bottom of the carb(s).
1.Affix the hose to the carb drain and hold it up against the carb side, in the center with the opening/end up above the carb assembly.
2. Unscrew the drain and watch the fuel fill the tube up to the fuel height. (It wont overflow, so long as the tube end is above the carbs) This is the height of the fuel inside the bowl. It is supposed to be between 2-4mm from the bowls gasket.
3. If it is not, turn fuel off. Then drain the carb entirely and remove the carb bowl. The plastic floats are easily seen hanging down. The idea is to manipulate the metal tang that pushes on the small pin in the middle of the floats so that more/less fuel will be allowed into the bowl. Its the same concept as a toilet float. Bend the tang upward = less fuel in bowl. Bend it downward = more fuel.
4. Put bowl back on (doesnt have to be really tight!), and repeat step 2. This will have to be done until the fuel heights are correct/spec, and the same between all carbs.

Thats it for now, hope its not too confusing! Hopefully the fuel heights will be correct and you wont have to go in there...

If this doesnt work, we'll talk about vacuum syncronization.

Good luck!