By Marc Malagelada Duch
GS500 owner from Girona, Catalunya, Spain
Editors note: The GS (and many other
bikes) is known to collect rust in the gas tank which gets into the carbs.
These rust particles can clog up the jets and inhibit proper float action
causing the bike to run poorly. The procedure shown will help you tackle
this job yourself .
A friend of mine was selling his old GS500 '91. He could not afford the insurance and had his bike abandoned in a parking for about 2 years.
I got it. It had 39000Km on the counter and a lot more of problems that have been being solved since I got the bike in september2000.
The bike had a dead battery that came back to life after a good charge (can't believe it!) but refused to start. I'm sure it was because of the old fuel in the tank. So I went, got the Clymer manual, and started some maintenance on it.
The carburetor cleaning, valve job and camshaft chain inspection I did on October2000 was repeated in
March 2001 because the bike was not working properly. The photos shown in this web were taken during this second 'rebuild'.
Aspect of the bike, after the seat and side covers removal.
Somebody installed a wrong battery (breather in the opposite side). The vapors coming out the battery rusted (nearly
ate) cables connectors and fasteners of the black box. Not a pretty sight to see.
Remove the tank. Only 2 screws.
Close the cock under the tank and remove hoses.
Larger tube (right in the photo) goes to L shaped intake in petcock.
Shorter tube goes to general intake in petcock.
Remove air filter. 2 screws at each side of the frame,
and 2 that
will link to the carburetors.
At this time, you might want to start the bike and see how both
carburetor slides work as you wind the throttle.
This will give you an idea if they are both working OK.
You might see the "slides jumping spectacle": At low rpm,
when the engine sucks air, slides lift up because of the vacuum generated.
As soon as valve closes the air flow, they will fall down again.
Soon you will run out of fuel.
2 screws will hold carbs to the engine intake manifolds.
Remove gas cable and choke cable.
Remove hoses coming from the petcock:
upper T: goes to lower part of the bike: fuel overflow.
lower T: fuel intake coming from the petcock.
vacuum tube that will automatically enable fuel to travel from
tank to carbs through the petcock. PRI position does not require
vacuum to let fuel reach the carbs.
Remove carbs assembly.
*click for larger photo
Upper rubber diaphragm. Inspect for holes or cracks.
Do not miss any o-ring. Replace if missing or dried out.
Unscrew jets. Before removing main jet, hit with a hammer: this
will move needle jet out of its place for easy removal.
There's a o_ring hidden under the slider support.
Inspect needle valve.
Inspect main jet (120 or 122.5)
Inspect jet needle (5DH9 or 5DH8)
Inspect needle jet (0-3)
Inspect pilot jet (37.5)
Do not mix parts from one carb to another.
See how dirty the float bowls are.
See the needle jet with little holes full of dirt (rust coming from
There was a thorough clean work done here, and it only needed 6000Km
to show again this dirty aspect. Guys: keep rust from getting your tank.
Clean carb bodies thoroughly. I used diesel and a
brush. Then rinsed with soapy water. Some diesel and a thin wire (a tine
off a wire brush works in a pinch) will let you clean all these
little holes where dirt tends to hide.
The hole shown in the photo feeds air to the needle jet.
(remember to clean needle jet holes too)
The hole next to it, feeds air to the mixture screw circuit.
upper "D" shaped big hole is the vacuum circuit that
will lift the slider.
Clean thoroughly the choke circuit (other side of the carbs)
Clean too these little 4 holes in the air enrichment circuit
(mixture screw circuit).
Clean both fuel intake holes and needle valve seat.
Just installing the needle jet.
You need to install the slider support (while plastic), and check
that the o-ing at the bottom is in good condition.
Then slide the needle jet (see photo). It must be inserted in a
position: inspect carb body (needle jet housing) to see that there is
a small notch that will prevent the needle jet from being installed
the wrong way.
I lubricate parts with 2-stroke oil.
Installing the main jet.
Carburetor assembly rebuild finished.
Mixture screw set to 2 turns out (default). You can play with 2 to 4
Jet needle in the middle position, (some carbs have only one clip
position) might want to lift it by 1 notch to get a richer mixture.
(or install a .020" shim (an M3 washer should work)). The GS500
is known to be jetted lean at high RPM)
Tighten bolts securing carbs to intake manifold.
Reinstall throttle cable. Move steering to ensure that there is a
little play in all positions.
Reinstall choke cable
Now it's a good opportunity to
install a fuel filter.
Install a small piece of hose between the lower "T" of the
carbs, attach the filter to that, and build another piece of hose to
connect to petcock output.
Install air filter box. A new air filter element is about
20$. Replace it if it's dirty or too old.
Do not forget to:
connect petcock vacuum hose to left carb.
connect battery breather hose.
Ensure that all cables are properly routed through the frame.
Ensure that breather hoses are routed to floor (canister in
California), except valve cover breather that goes to air box.
If I had a vacuum meter (used to synchronize the carbs), I would check
if carbs are both working fine.
Reassemble everything else, pull choke and crank the
It should start and you will need to reset idle screw (middle of the
carbs) to set it up to 1200RPM.