CG 125

Hi there,

I'd be really grateful for any suggestions. I have bought a CG 125 off
my brother that hasn't been run for about a year. It use to run ok and
my brother ran it daily, but now I can not get it to start. I've got a
Haynes manual and have tried a few various things, each time trying the
engine before moving on and trying something else.

I checked the spark plug, cleaned and set the gap. I have checked for a
spark with the plug out and turning the engine over and it is ok, so I
am inclined to think that the coil/points/wiring is working.

I checked the valve clearances and they are 3 thou at TDC on
compression stroke as they should be.

I checked whether any fuel is getting through from the tank to the carb
and it is.

I checked and cleaned the airfilter.

I opened the carb and checked the float and checked for any dirt or
blockages in the jets. Everything seemed fine.

I drained the old petrol and put brand new petrol as I heard that
petrol can go stale.


After all this the bike does actually start, but only very roughly. I
tried to ride it round the block and it was dying on me all the time.
It is unrideable really. I think that it is probably the carburettor or
the ignition. What does anyone think? Has anyone had the same trouble?

I've started to lose heart on it now. It used to run really well and I
was wanting to use it as a cheap means of transport to and from work as
my car is too heavy on the juice.

 

It will be the carb, which will be gunged up. Depend on it. get a good
used one off a breaker rather than try and sport it out - it'll be
easier and cheaper (shoudn't cost more than a tenner).

Try Making The Sign Of Holy Pushrods as well.

 

wrote in @g10g2000cwb.googlegroups.com:

Take a good look at the pilot jet - might be clogged.

 

Your carburetor probably has gum and varnish in the idle mixture
passages and the idle jet. You have gone through the obvious
troubleshooting steps that worked on older motorcycles, but most riders
are not aware of what environmental agencies like to EPA and CARB have
required the manufacturers to do to clean up the atmosphere.

They have mandated the constant vacuum carburetor (1) which has a round
dome on top of it with a diaphragm underneath that will only allow mid
range mixture to be enrichened if there is enough vacuum when the
throttle is opened.

And, if your idle jet and idle mixture passages are plugged up, the
engine never makes enough vacuum to raise the vacuum slide.

I recommend mixing 2 or 3 ounces of Berryman's B-12 Choke and
Carburetor Cleaner in a full tank of gasoline and going for a slow ride
to clean out the idle jets. B-12 comes in aerosol spray or in liquid
form. It contains xylene, acetone and methyl alcohol.

You can also find Gum Out and STP aerosol sprays that have the same
stuff in them, but I like the B-12 because I can get it in the
non-aerosol form.

For a really quick carburetor clean up, you can also drain the float
bowl and squirt a 50/50 mixture of B-12 and gasoline down the fuel
hose, filling the float bowl. Leave it in the carb for about half an
hour and start the engine and it will run on the mixture and clean the
carb out.

You also need to be aware of the fact that constant vacuum carburetors
rarely have a real choke anymore. A real choke has a plate that makes
engine vacuum increase when the plate is closed.

Constant vacuum carbs have a starting enrichener device which is just a
valve in the side of the carburetor. When you put the "choke" lever in
the ON position, you're just opening that little valve. If you turn the
throttle handle while trying to start the engine, you are actually
causing the vacuum in the carburetor to DECREASE, so the extra fuel you
need to start the engine never gets sucked into the intake tract.

The procedure for starting an engine with a starting enrichener is to
AVOID twisting the throttle grip until the engine starts. If the
carburetor is clean and properly adjusted, the engine will start and
fast idle on just the starting enrichener.

Where riders get into trouble is the carburetor begins to get dirty and
the engine is running too slow and stumbles and stalls when they come
to a stop sign. So they adjust the idle speed knob up a little bit and
the engine idles at the correct RPM when it's hot. So they forget about
what happened. They park the motorcycle and don't ride it again for
months.

Then, when they try to start the engine, it won't start because the
vacuum in the carburetor is too low with the butterfly too far open.

The strategy of starting an engine for the first time after it has been
sitting for months and months is to find the idle speed knob and turn
it all the way counterclockwise so the throttle butterfly is closed.
Then, when the engine acts like it's going to fire, the rider can
control the idle sped manually by turning the throttle grip.

When the engine is warm, the idle speed can be adjusted to the correct
speed, which is usually on a sticker under the seat.

(1) Motorbikes used to have older style slide valve carburetors where
the cable-operated slide was the throttle. Riders would use too much
throttle and "bog" the engine in higher gears by opening the slide too
much. When the rider rolled off the throttle, any gasoline left in the
combustion chamber would blow out the exhaust pipe
and pollute the atmosphere. Constant vacuum carburetors overcome the
bogging problem, but the tiny orifices which make the carburetor so
much more responsive to small changes in throttle setting are
susceptible to getting plugged up.

 

<snip the usual>

This is a CG125. It doesn't have a CV carb, FFS. And it's an good bet
the OP is in the UK.

You really, really need to do some homework ;-)

 

Thanks for the tip. I'm glad you suggested that. I really didn't want
to open that carb up again. I'll get a new carb from a breakers as you
say. And thanks to every one else who replied.

I'll get this little bike on the road yet!