Motorcycle Owners Club

Honda CG125 but also some universal motorcycle information

Honda CG125 Idle Screws

< Servicing

Idle Mixture Screw

The following applies to the front disc brake model.
I would advise you to also try it with the front drum model,
but do not expect to have to adjust it by such a large amount or have such a warming up problem in the first place,
since it does not have such strict emission regulations.

Tools required,
a small flat headed screwdriver bit
(it must not have a handle since it would hit the bike and so not fit)

If the Air Filter is clogged up, it will affect the mixture, so if you are in any doubt that the filter is clogged, clean it,
see Air Filter in Servicing page.

The fuel mixture is set incredibly lean when idling by the manufacturer (to reduce emissions).
This results in the engine taking forever to warm up
as well as hesitation when opening the throttle (even when fully warmed up).
The engine also needs the choke on again to start the engine even when it has been switched off for a short time.

The dealer may have adjusted it if you're lucky.

If not, fully warm up the engine = 15 miles or more with the last 10 miles at 50mph or faster
(40mph for 5 miles does not warm it up).

Make sure the outside temperature is between 14c and 30c and you're near sea level (not up a mountain).

Look under the carburettor and find the screw
(right hand side of the bike,
it's the screw sticking out and is not tight),
adjust it until you get the fastest idle.

1/4 turn will make a difference,
adjust the screw and then wait a few seconds for the engine to react,
blipping the throttle is sometimes needed.

When you have found the fastest idle,
you will find that even opening and closing the screw 1/4 turn,
will slow the engine down.

If you have trouble hearing if the engine has increased or decreased in speed,
ask someone to stand the other side of the bike or behind the bike (listen to the silencer)

The screw would originally (front disc brake model) be set to about 1 and 3/4 turns open.
I ended up setting mine to 3 and a 1/4 turns open in order to get the fastest idle.

After setting the idle mixture screw,
you may need to adjust the Idle speed screw (see below) if the idle speed is to high,
but whenever you adjust the idle speed screw, recheck the idle mixture screw setting again.
Because it may need adjusting again.

If you open the Idle Mixture Screw to many turns, it will fall out,
you can simply push it back in (it does have a spring and washers inside that will fall off if turned upside down).
If the screw is on the fall out turn part, vibration may make it rotate slightly and drop out.
I have had no trouble at 3 and a 1/4 turns open, it has still got over 1 whole turn to go before it would fall out.
I class one turn as 360 degrees.

Idle Speed Screw

The Owner's Manual shows a picture of where it is.

On the front disc brake model,
look at the right hand side of the bike,
look at the right hand side panel,
look half way down the side panel
and then just to the right of the side panel.

See that small black plastic box.

There's a small flat headed screw
on the left hand side of it
(almost touching the side panel),
that is the Idle Speed screw.

Fully warm up the engine = 15 miles or more with the last 10 miles at 50mph or faster
(40mph for 5 miles does not warm it up).
Slow the engine idle speed down to the point where it starts to thump during one of its cycles,
then speed it up until it stops thumping,
then speed it up a fraction more.

The whole idea about what speed to set the idle, on any motorbike engine is the following.
To slow and the bike will stall, nearly fast enough and the engine will splutter or stall now and again.
Too fast and the engine will overheat in a traffic jam and burn more petrol than needed.
The gear change is also effected by idle speed;
to fast or slow will affect how smooth it is or even stop you changing gear in extreme cases.
The difference in overheating speed and stalling speed is large, so you only have to roughly set the idle speed.

< Servicing