78 kz650 carb screw question

there's a screw going into the bottom of each carb towards the front of the carb... what does it adjust and how do you set it as a starting point? I was told it is fuel mixture... so out is more fuel? or or out is more air? I am running no filters, open exhaust and 102 mains over the stock 90 mains. any help is appreciated.

Those are "pilot" screws which control fuel at idle. I believe the starting point is 1 1/8 turns out from lightly seated.

so is there a main mixture adjusting screw? and thank you for the info!

Sounds like you have Pilot-Screw type carbs.
They have the screw on the bottom of the carbs, in the centre, near the front.
Pull off the pilot screw extender (plastic cap).
Turn the pilot screw until it seats lightly, and then back it out 1 1/8 turns.
Push the extender onto the pilot screw with the ridge pointing to the front. This allows you to adjust the extender (pilot screw) either way if needed.
Adjust idling speed to 950~1050 rpm by turning the idle screw.
Check to see if the engine rpm rises when the pilot screw positions are altered. If it rises, repeat the last two steps alternately until the engine is idling saiisfactorily, and engine speed cannot be increased by further adjustment of the pilot screws.


Gotta go shopping now.
The pilot system adjust fuel and not air if I'm not mistaken. Air screw type carbs adjust the air ratio and the screw is located on the side of the carbs pointing upwards near the back of the carbs.

Yes they are the pilot screw type. Turning out=richer mixture. Due to the extreme taper of these screws it's real easy to damage the tips and/or damage the screws themselves and the seats. It's real hard to say turn out x amount of turns really as I noticed on all the KZ650 carbs I have with this type carbs they are all over the place when lightly seated, so out x amount of turns is still all over the place. Guys here might remember my saga with these carbs when I was removing them every 500-1000 miles because the pilot screws were getting clogged up causing a lean mixture. I think I got mine fixed by turning the pilots out like 3 turns. Don't remember exactly, but apparently my seats were all fouled up and setting the screws to lightly seated was actually way more closed than anticipated hence turning them out 1 1/8 turn was in turn way too lean. Engine would run fine for 500-1000 miles then gradually lean out till it would run like crap. I got 4000 miles now since the last problem since I turned the screws way out. I like the air adjust carbs much better as the taper is less and much rougher to foul up.

so is it only idle mixture or is it to overall mixture setting? btw mine are comeletely cleaned out and brand new screws. right now they are 1 1/2 turns out and it idles fine but it's running slightly lean according to the plugs ( clean white electrode resistor ) thanks

The pilot mixture screws are connected to both the pilot air jet and pilot jet and both air/fuel come through... Out is richer.

sorry but I still don't know if this controls overall air/fuel mix or just the idle air/fuel mix... I'm confused I think.

It basically just controls the idle circuit, other circuits, jets take over above idle.

They control the low speed mixture, but if they are really messed up it will even effect highway running. Like I said before, when I had the problem where my screws were clogging every 500-1000 miles once they clogged up I could no longer ride the motorcycle on the highway even as it would surge so bad it made riding unbearable.

Kingcobra wrote:

sorry but I still don't know if this controls overall air/fuel mix or just the idle air/fuel mix... I'm confused I think.

Pilot screw controls A/F from approximately from 0-1/8 throttle opening, from there appr upto 1/3 slides and needle jet control and the last circuit is the main jet up till WOT. The accelerator pump can be adjusted to work on different areas of throttle opening. My (Mikuni TM33´s in a 1109cc ELR) acc pumps are activated from 1/2 -throttle up. Usually you should refer to throttle opening since that is the relevant info when adjusting, speed is not so relevant. All these 3 circuits should be dialed in correctly for best performance.

Post edited by: Z1109R Fin, at: 2007/10/09 08:57

Post edited by: Z1109R Fin, at: 2007/10/09 08:58