Step by step: From general maintenance to complete restorations, we share tips and tricks for working on classic bikes.
We’ve previously shown you how to rebuild the forks on a 1973 Honda CB500. This time we’ll show you a carburetor rebuild. While our step-by-step should make the process pretty clear, we highly suggest a good Chilton or Haynes shop manual to help sort out the many details in removing and installing the carbs. A picture, as they say, is worth a thousand words.
Detach the throttle and choke cables, loosen the clamps on the insulators between the carbs/engine and the air cleaner bellows. Remove the carburetor assembly from the engine. A manual helps here.
Turn the assembly over so you can see how the carbs mount to the throttle assembly plate. Number them one through four so that they go back on the throttle assembly plate in the same order.
Detach each carburetor from the throttle assembly plate. You have to loosen them all before they’ll come off separately. Watch out for the pin that holds the throttle spring, positioned between carbs 2 and 3.
What you have now are four dirty carburetors. We’ll walk you through how to rebuild one: Simply repeat this process for the other three.
Begin by flipping the carb over and removing the four corner screws that attach the float bowl to the bottom of the carb body.
Here’s what to expect once the float bowl is off. First, remove the leaf spring off the main jet.
To remove the float, use an awl to push the float pin out of the posts. Be careful if the pin is sticking, as the posts can be fragile.
Here we’ve already used a large Phillips-head screwdriver to remove the screw and clip plate that holds the float needle seat. Using a pair of needle-nose pliers, remove both the float needle and the seat.
Next, use a large blade screwdriver to remove the slow jet at the lower left. Use your needle-nose pliers again to remove the main jet, which sits just to the right of the slow jet.
Next, remove the fuel-air mixture screw on the side of the carb body. Note and remove the T-type fuel pipe between carbs 1-2 and 3-4.
Flip the carb over and remove the screws that attach the carb tops. Straighten the tab of the tongued washer and remove the bolt from the shaft. Remove the throttle shaft and the plastic spacer. You can now reach in and grab the link arm and pull out the slide.
Here are all the pieces of a carb laid out. Next, we removed the float bowl gasket, then took the float bowl and carb body, and ran them through an industrial parts washing machine at a friend’s garage. You can also soak these pieces in a carburetor-cleaning dip solution; follow the product instructions.
Remove the two Phillips-head screws that hold the link arm on the slide. Note the position of the link arm on the slide and remove it. The accelerator needle will now fall out. Note the position of the C-clip on the needle. Put the new C-clip on the same spot on the new needle that came in the carb kit. Drop the needle back into place, line up the link arm and reinsert the Phillips-head screws.
To clean the yellowed clear-coat off the float bowls and carb bodies, we sprayed them with carburetor cleaner, followed by a copper brush to remove the clear. We then used a buffing wheel and jeweler’s rouge to bring the finish on these pieces to a nice shine.
Begin the reassembly by screwing in the new idle jet (left). Next, lightly grease the O-ring on the new needle seat and push it in by hand (right).
Do the same with the main jet; needle-nose pliers help to position it.
Install the new float needle. Reinstall the float and the float pin, again pushing the pin carefully so as not to damage the posts.
Now it’s time to check the float height. This is done by turning the carb body over and seeing how high the float sits when at a gentle rest against the needle. Correct float height for these carbs is 22mm. We used a set of measuring calipers to make a mark 22mm from the edge of this wood block, then set it against the carb body to check that they match.
If the float needs to be adjusted, pull the float pin back out. Use needle-nose pliers to carefully bend the small metal tang on the float up to lower or down to raise the float height as needed. Reinstall the float and the float pin.
Reinstall the leaf spring on the main jet.
The new float bowl gaskets in our rebuild kits were just O-rings and weren’t formed, making it hard to set them in place on the float bowl. To make things easier, fill the groove in the float bowl with white lithium grease, press the gasket in, wipe off excess grease and install the float bowl with its four screws.
Put the slide and the needle back into the top of the carb body. A guide pin in the body locates a recess in the slide, so it will only go in one way.
Slide the link arm over as shown, then install the throttle shaft, placing the plastic bushing between the two. Reinstall the 6mm bolt and bend the tongued washer back around the bolt. Put the carb top on with its two screws.
Install the new fuel-air mixture screw and spring into the side of the carb body.
After reinstalling the T-type fuel joint, this carburetor is now finished! Following these same steps on the other three carburetors nets us a fully reconditioned set.
With the carburetors loosely attached to the assembly plate, reinstall the throttle return spring pin that fits between carbs 2 and 3. Finish attaching the carburetors to the plate.
Once all four carburetors have been reattached to the plate and screwed down tight, check that all the throttle plates work equally. That’s it, they’re ready to be mounted back on the bike!
Last issue, we announced plans to have the wheels on our CB500 rebuilt by Buchanan’s Spoke & Rim, Inc. (www.BuchananSpokes.net) in Azusa, Calif. After a quick call to the company, they told us since they’ll be rebuilding our wheels with new rims and spokes, they only want our wheel hubs and bearings. This meant taking apart our old wheels. A small set of bolt cutters and a few minutes later, we’d cut each spoke, separated the hubs from the rims and removed the remaining pieces of spoke left in the hubs. Since the entire surface of the hub is now easily accessible, we’ll give our hubs a light polishing and have new bearings pressed into them before we box them up and ship them out.