CB400 NC31 Idle Adjustment

[mikezs]

Hi all, new here :)

Just got an NC31 that had been sitting for a few years running again. Now onto the business of giving it a proper clean and service. I have a problem when running the engine though: the idle (even when hot) is too low to keep the bike from stalling, so I have to have a bit of choke out at all times when riding.

I think that the idle adjustment screw might be missing, but I don't know where from, and what it supposed to be conencted to (so I can't find it in the parts manual here: http://400greybike.com/files/cb400-Superfour-Parts.pdf). Could someone with a 93 CB400 take a picture of the adjustment screw and where it connects to in the carbs please? :)

[ONDA SF]

No picture but it is hard to miss if the idle adjuster is there, under the carbs smack bang in the middle is a black thum screw. if its gone for some reason i reckon you will need to remove the carbs to fit a replacement.
If thats the case when you take them off all will become clear i am sure.

Colin

[mikezs]

No picture but it is hard to miss if the idle adjuster is there, under the carbs smack bang in the middle is a black thum screw. if its gone for some reason i reckon you will need to remove the carbs to fit a replacement.
If thats the case when you take them off all will become clear i am sure.

Colin

Thanks, the carbs are probably coming off anyway for a good clean. There didn't looks like there was a place for a thumb screw under the carbs, maybe a place above them and further back. I think perhaps the plastic knurled bit has broken off, so just a screw left (hence requirement for a picture).

If you get a chance, could you snap a quick pic for me please?

[bika_37]

Hi Fella

Im in Latchingdon, got a super4 1993.

Idle dropping prob is usually NOT carb related, most likely culprit is vacuum pipe from tank or petcock.

I had the same thing, ur welcome to pop over and i'll show you.

[mikezs]

Might have to take you up on that :)

There's a vacuum tube off the fuel switch that looks properly connected, but the pipe near the seat isn't properly connected, what needs to happen with that?

[bika_37]

There are 2 pipes at rear of tank, 1 (the larger diameter) is a drain feed, the other is a vacuum flow, both really should be connected to the tank, and try blowing thru to ensure there is no blockage, also try filling the tank up past 3/4 full and running the bike on res instead of 'on'.
Sometimes the main(on) feed pipe is blocked and the res is clear? Worth a try.

I doubt it is just a case of turning up the idle as the idle control is pretty stiff and shouldn't work loose, and its unlikely someone has turned it right down???
Prob a fuel issue, but more likely vacuum or feed pipe related IMO

[mikezs]

Theres only a single pipe comming out the back with a T connector and the two pipes going to the carbs/engine. Is this correct?

Seems to run fine with the choke fully on and the fuel switched to reserve, can't ride to a petrol station (no chain at the moment) so only had 5ish litres of fuel put in.

Which way should I blow on the pipes? Into the engine, or into the tank?

[bika_37]

Into tank, there should be 2 tubes at rear of tank where your nuts sit!

One is on left and other on right(small diameter breather pipe).

The petrol thing may not help ,try filling some more fuel.

Also, if you say you need choke(ie making the bike run richer) then its possible the pilot jet could be running too lean, at the bottom of each carb is a brass crosshead screw, bit fiddly with carbs on, but try turning them in half a turn so the bike runs a little richer, but do this as a last resort, try the vacuum pipe and more fuel first, It really is a simple thing that can really f**k the bikes running

[mikezs]

There is definitely only one tube comming from the tank at the back, this feeds another 2 tubes that are connected to the engine, but they both seem to be the same diameter (and connect to a T-pipe that comes from the tank).

From what I remember they're not actually connected (had the tank on and off lots last week!) so I will connect them and see how it runs :)

On a semi-related note, what's the best way to disconnect the fuel tank from the bike? Disconnect the tube between fuel tap and fuel filter?

[Superjack44]

First turn the petrol tap to the off position, there two10mm bolts holding the rear of the tank down remove them, now lift the rear of the tank this will allow you enough room to remove the vacumm pipe and the petrol pipe from the petrol tap (there may be a small spring clip holding them in position).
Now its a simple job of lifting and easing the tank back towards the rear of the bike.
There are two pipes at the back the larger is to connect a rubber pipe too to act as a drain from the petrol cap filler recess, the other smaller one is a breather pipe for the petrol tank this should also have a rubber pipe connected to it ,incidently while the tank is off no harm in making sure this is clear by blowing through it if there is any restriction it will interfere with the amount of petrol getting through to the carbs.