Carb settings

[cb400joe90]

Hello again all, I recently removed the carbs and gave them a good clean. When I open the choke the bike starts and runs. But if I turn off the choke the bike quickly dies. Im am aware that its a fuel / Air mix problem so I just need the correct setting for a starting point. Lets start with the Idle screw, Should I wind this fully anti-clockwise or fully clockwise and start from there?

Next is the Air pilot screws ( 4 of them ) I have them turned 2 3/8 from seated. Is this the correct starting point.

I do have a tester but I want to sort the choke issue first. The bike is a 1994 cb400 superfour NC31

I have the converted Russian manual but I cannot find the info I need in it.

[Superjack44]

Mine is a 92 superfour I stripped all 4 carbs and rebuilt them , to set them up I inserted a 0.005 ml feeler gauge under the butterfly and adjusted the slide until I could just barely feel movement then did the same on all for carbs and then coupled them up.
Once coupled up I screwed the mixture screw in ( not excessively so) fully home and then backed out about 2 turns as a base setting it fired up and ran, then twiddled about with them until I was happy with the running of the bike.

[cb400joe90]

Thanks superjack. Just to clarify we are talking about the 4 brass air mixture screws under each carb?

I didn't disassemble each carb so I shouldn't need to make major adjustments.

I did accidentally turn the idle screw so ill have to play with that alright.

[Superjack44]

Thanks superjack. Just to clarify we are talking about the 4 brass air mixture screws under each carb?

I didn't disassemble each carb so I shouldn't need to make major adjustments.

I did accidentally turn the idle screw so ill have to play with that alright.

Yes they are under each carb , the idle screw adjuster is also under the carbs , I knurled plastic nut, not the little cross headed screw that you can just access on the top of the carbs

[SevenThreeSeven]

When I open the choke the bike starts and runs. But if I turn off the choke the bike quickly dies.

This means you are having to compensate for one or more (or all 4) idle jet circuits that are not functioning properly. I'm a GSF400 Bandit owner and we have Mikuni carbs, but I also own a Kawasaki EX250 which uses Keihin carbs.

Next is the Air pilot screws ( 4 of them ) I have them turned 2 3/8 from seated. Is this the correct starting point.

If the idle jet circuit in each carb is clean (flowing both air and fuel as they should) then 2 3/8 should be enough to get the bike to idle.

On both types of carbs I've seen situations where the owner is saying he's adjusting the idle mixture screws and it's having no effect on the engine, there's no change at all when he adjusts them.

This almost always means one of two things: either the fuel pickups for the idle jets are not working properly (the tiny little emulsion tubes are clogged up, so there's no fuel for idle), or the pilot air jet passageway that admits air into each carb's idle circuit is clogged up (so there's no air for idle).

More often it's an air supply problem instead of a fuel supply problem. People concentrate on cleaning the fuel related things and forget about the little passageway the admits air for the idle circuit.

The owner swears he has cleaned the carbs, including the idle air circuit (from the orifice that is vented to the airbox side of the carb all the way to the idle circuit's end at the idle mixture screws), but in the end it usually turns out he didn't insure that the entire length of the idle jet air passage was clear. The idle air passage is the longest passageway in the carb, and the hardest to completely clean.

[cb400joe90]

Thanks 737, Plenty of detail there for me. I should have mentioned that the carbs were working fine before removal. I removed them for cleaning for more of a learning thing more than anything else. I boiled, carb sprayed & high pressure blasted the carbs so im hoping the are fairly clean and clear now. I set up the gauge tool I bought and play with it today. Thanks all

[cb400joe90]

Hello again. This still has me stumped lads and I really need the bike running well for the riding season. The bike does run but I just know I dont have the 4 mix screws adjust correctly. I have them at 2 turns out from seated . I have bought the special carb tool with a 90 degree angle to adjust the screws but im unsure of the procedure.

Lets say the bike is warmed up and idling at 1300 rpm. Whats the next step?

The carbs are balanced as I connected up a carb sync tool and they looked fine.

Also has anyone any good link to guides and setting regarding the Keihin carbs.

When people say these screws just adjust the low idle does this mean that once the rpm goes above 2000 rpm it does not matter when the settings of the 4 screws are?????

[Superjack44]

Hello again. This still has me stumped lads and I really need the bike running well for the riding season. The bike does run but I just know I dont have the 4 mix screws adjust correctly. I have them at 2 turns out from seated . I have bought the special carb tool with a 90 degree angle to adjust the screws but im unsure of the procedure.

Lets say the bike is warmed up and idling at 1300 rpm. Whats the next step?

The carbs are balanced as I connected up a carb sync tool and they looked fine.

Also has anyone any good link to guides and setting regarding the Keihin carbs.

When people say these screws just adjust the low idle does this mean that once the rpm goes above 2000 rpm it does not matter when the settings of the 4 screws are?????

Those four screws are for the individual mixture settings for the four cylinders they have nothing to do with the idling speed of the engine.
The idle adjuster screws are on top of the carbs and difficult to see in situ these should be adjusted with a manometer in place and once set left alone the main idle adjuster is a plastic knurled knob accessed under the carbs from the left hand side of the engine.

[cb400joe90]

Ok superjack. The bike is running so I should just leave the 4 mixture screws alone? Ill pop the manometer on again but they all looked in balance before. No.3 was slightly different from the others.