How scary does "rebuild your carburettor" sound?
Well, fear not - it's a piece of cake. You pull it off, take
it to bits, clean
it, put it back together and put it back on the bike. Sure, it's
fairly delicate, but we are not talking eggshell delicate.
This article will show you how, and will demystify the whole
'thing' by explaining exactly what all those bits do to the fuel
and air entering the engine. Here we go:
First you need to remove the fuel tank to gain proper access.
It's probably possible to leave it in place, but it's easier if
it's gone. Lift the seat, pull the toolbox
out and make sure
the carb (there is sometimes a spring clip here, but it's pretty
ineffective and redundant anyway). Block the end of the fuel hose
with something, or hold it with the open end facing up. Remove
the seat post and the 11mm bolts holding the fuel tank on and open
the fuel tap just long enough to wiggle the fuel tank
out of the frame, then close the fuel tap and set the tank aside.
You now have loads of room to get at the carburettor.
To remove the choke, pull the
choke knob out, and grab the cable, and unhook it as you push
the choke knob back in. To remove the throttle cable, manually
lift the throttle arm and unhook the brass cable clamp from its
home (red arrow). Remove this clamp (being very careful not to
lose it) and you can pull the throttle cable out through the adjuster
To remove the carburettor use a screwdriver, socket driver or
spanner (or all 3 depending on how tough it is to move) on the
long hexagonal bolt and shimmy the carb off its manifold.
The complete carburettor (in this case a Dell'Orto SHB 19/19)
and air filter assembly looks like this:
The first thing to do is remove the air filter. On some bikes
there are a couple of butterfly nuts, on others you'll need a spanner.
You'll be left with this:
There are a pair of split pins and some washers holding the air
filter together. Remove thenm and you can see the air filter consists
of a front panel, a drip tray
a few baffles, turns and a bit of wire gauze. Wash everything
in petrol and when cleaning the gauze make very sure there are
no little bits of wire about to drop off and get sucked into the
there to keep the intake noise of the engine down by slowing the
air down. Put the air filter back together when done and put it
to one side.
going to rebuild the carb itself. This carb is a Dell'Orto SHBC
19/19 as fitted to the Smallframes.com ET3, but the following basically
hold true for all Dell'Orto scooter carbs.
The SHB has a sliding throttle, as shown in these 2 pictures.
When closed it looks like this.
When you tilt the lever (which is all the throttle cable does)
the slide moves up, as seen below.
All this does is open the airway up. Air, being sucked through
the carb by the engine, passes through this chamber (the Venturi)
at high speed, and as it does it drags the fuel mix with it. The
off this passage, thereby literally throttling the airflow. You
should also be able to see how the idle screw, located in the throttle
activating arm is a very simple device that physically prevents
the throttle slide from closing fully and completely starving the
engine or fuel and air. The rate of fuel is controlled by jets.
On this type of carb (a constant jet carburettor) the only way
of altering the amount of fuel that gets sucked through is by changing
is a fuel/air mix screw you can see in the bottom of the above
picture. This only affects the mix at low revs, and basically
controls the size of the opening to the idle jet, thus the effect
the passing air has on it, and the amount of fuel it drags through
into the airstream. Screw it in and you get the path is closed,
causing a leaner mixture,
unscrew it and the pathway is opened, so you get a richer
In practice leave it at 2.5 turns out. It will be perfectly fine
for most climates. If you do subsequently find it needs adjustment
don't go crazy - a half turn to a full turn should cover every
Unscrew the throttle assembly and carefully remove the cover.
The throttle spring and slide will come with it. Make sure the
spring doesn't ping across the room!
You can see there is a brown paper gasket on the underside of
the throttle assembly. If it's in bad condition it must be replaced.
It's probably a good idea to replace it as a matter of course.
One of these comes in every replacement smallframe gasket set.
You can also see there is a cork gasket running round the air
intake where it meets the air filter. One of these should also
be in every gasket kit. Some smaller carbs may use a paper gasket
here, but as I don't have one I can't confirm or deny it.
Below you see the throttle spring and slide housing, and the removed
throttle assembly. Clean it all out with petrol. You can also see
the choke cable here. All engaging the choke does is
open a path from the venturi to the starter jet so extra fuel
is drawn into the airstream, making the resulting air/fuel
richer (i.e. rich in fuel). As long as everything is working OK
this fuel rich condition is only required by a cold start, and
your Vespa will flood and run like a lame snail if the choke is
engaged under other running conditions.
Now, back to the overhaul. The incoming fuel mix passes through
a filter. Unscrew it.
Clean these parts with petrol. When you come to put them back
it will be easier to get the fuel line on if you rotate the attachment
clockwise a bit from the recommended vertical position.
Now for our first carburettor jet. In the picture below the screwdriver
is in the idle
jet (aka the minimum jet). This jet controls the amount of fuel
available when the engine is at very low speed.
It should always be screwed fully in. Except for now, cos we
are removing it to clean it out. Unscrew it, being careful not
it or damage it.
Jets are delicate, so don't drop any housebricks on them. Here's
the removed idle jet.
important part is a tiny passage drilled through them through which
fuel passes. The bore (width) of this hole meters the flow of fuel.
The hole can be cleared out with a small-bore drinking straw
and a sharp
puff of air,
or in extreme
about it!') cases a very thin wire. Absolutely the best method,
however, is an aerosol can of compressed air. You can buy these
at computer shops for cleaning keyboards and the like. Just blow
making sure you hold tight and don't blow the jet across the
room. The jet and its passage need to be completely clean.
Once it's clean, replace the starter jet and we'll move onto the
main and idle jets. Start by removing the float bowl by undoing
the two bolts as in the picture below.
You'll see the following once the float bowl is removed.
The large white doughnut is the float which, as you would expect,
floats. Be very careful with this as it is imperative that it remains
float if there are holes in it. The brass screw in the centre
is the main jet, and in the corner is the starter jet. The main
jet meters the fuel available when the engine is running normally,
and the starter jet meters the fuel when the choke is engaged.
Now we need to very carefully remove the float. It pivots on a
pin, which you should remove by pushing it through the holes it sits
Be careful removing the float, because loosely attached to it
is the float needle.
This needle controls the amount of fuel admitted
into the float bowl (and therefore available to the jets) in
conjunction with the float. As the fuel level in the float bowl
rises the float
rises and the needle closes off the fuel intake, opening again
as the fuel level falls. This prevents the carburettor from flooding,
so it is imperative that both the float and needle are in perfect
condition. Treat them very carefully. Check the float for holes
by immersing it (preferably in petrol, but water will do as long
as you clean and dry it off thoroughly afterwards).
Remove the main jet by unscrewing it as shown below.
It should look like this.
Now remove the starter jet...
...which looks like this:
Clean both of these using compressed air as you did with the idle
Clean the float chamber, the float and the needle and blow air
through all the passages in the carb to make sure there is no debris
in there. Replace the jets by screwing them fully into their respective
holes. To replace the float and needle you need to slide the
needle into the slot
and carefully replace the float and needle in their homes. Slide
the pin back into the hinge.
Now replace the float bowl, the throttle assembly and the fuel
intake and filter if you have not already done so. Hey presto,
one rebuilt carburettor. Note in the picture below I have rotated
the fuel intake nozzle just
vertical and makes it easier to remove and re-attach the fuel
line in future.
Reattach the air filter using the hex or butterfly nuts and put
the carb back on the bike. Refitting is the reverse of removal,
as Mr Haynes would say: tighten the collar over the carb and manifold,
hook up the choke cable. and feed the throttle cable through the
adjuster (green arrow). Take this opportunity to screw the adjuster
almost, but not completely, in and put the the brass clamp on the
cable. pull the throttle cable tight and slide the clamp up to
its socket on the throttle arm (red arrow).
You need to unhook the cable again to tighten the clamp up. Note
the correct position, so you can hold the clamp there
when you remove the cable from the throttle arm and tighten
up the screw to hold the clamp in that position. Slip it back
onto the throttle arm and if necessary use the adjuster to allow
throttle arm to close completely (that is, until the idle screw
prevents it closing further) and there is minimal (preferably no)
in the cable before it activates the throttle arm.