Owners who shy away from rebuilding their own carburetors might be surprised to find it’s often a fairly straightforward proposal. The carburetors on most of our old bikes are quite simple, and experience says that success hinges on two key issues: cleanliness and thoroughness.
It takes more than a simple strip-down and cleaning with a can of carburetor spray to get good results. To do it right, you have to thoroughly soak and clean the carburetor body so that any trace of dirt is removed. That’s sometimes easier said than done, as modern carb cleaners are much milder than their forebearers and less effective at dissolving dirt and other deposits.
Fortunately, ultrasonic cleaners keep getting more affordable, with 6-liter units (the smallest we’d suggest) running around $120. Briefly, an ultrasonic cleaner uses high-frequency sound to create pressure waves. The pressure waves “tear” the liquid, creating millions of microscopic cavitation bubbles, and as the bubbles collapse they release enormous energy, agitating the liquid. Thus agitated, the liquid scrubs whatever surface it contacts, removing dirt and scale. Ultrasonic cleaners are remarkably effective, and they don’t depend on toxic chemicals: The cleaning solution is typically just tap water with an ounce or so of dish detergent to act as a surfactant.
Our project 1970 Honda CB350’s Keihin constant velocity (CV) carbs were particularly nasty, so we took a two-step approach, first soaking the carbs overnight in a can of Gunk carburetor cleaner, followed by a trip through our ultrasonic cleaner (see photo above). This worked well; the Gunk dissolved the major dirt and the ultrasonic cleaner finished the job, leaving the bodies looking clean and ensuring the passageways are clean. If you don’t presoak, spray the carbs with cleaner to remove the worst dirt before they go in the ultrasonic bath.
As removed: Years of sitting had not been kind.
Stripping these Honda Keihin constant velocity carbs isn’t particularly challenging. The important thing is to pay attention to which jets — there are three — go where and, if you’re replacing the CV diaphragms, properly aligning them on the slide during installation.
We got our master overhaul kit from Sirius Consolidated. The kit, which retails for $202.50, includes new floats, new chrome caps for the CV diaphragm, new diaphragms, new float bowl and cap screws, float bowl gaskets, new needle and seats, plus every jet and rubber plug for both carbs. All of the components are made in Japan. At first glance, $202.50 sounds like a lot of money for a carb kit, but the Sirius master kit is remarkably comprehensive and seems to us money well spent.
One note: While this how-to focuses on the major points of CB350 carb overhaul, space restraints always mean skimming over some details. Further, as we were working on a mostly bare bike we didn’t have to remove much to get to the carbs, so you’ll want to consult your manual.
As always, we suggest having a good shop manual on hand for parts identification and to aid in removing and reinstalling the carbs on your engine.
1. The first step is removing the carburetors. Our beater project bike was already stripped of most of its hard parts, so for us, this step was simple. If you’re unsure how to proceed, consult your manual. Note the connecting choke strap; disconnect it from the left carb before removing the carbs.
2. Next, remove the diaphragm cap and spring. The diaphragm is attached to the carburetor slide. Our slides were stuck. To free them, we lifted the diaphragm rubber up and sprayed a little brake cleaner around the slide and let it soak. We came back to it after we removed the float and jets.
3. Remove the float bowl and turn the carburetor upside down. Using a small punch and hammer, gently drive the float hinge pin out and remove the float. If the pin is stuck, clean the area with brake or carb cleaner; it should only take a few light taps to free it.
4. With a pair of pliers, twist the slow speed and needle jets. If they’re stubborn, spray them with brake cleaner. Once they’re loose, pull them out along with the retaining spring clip. Remove the rubber plug covering the idle jet next to the slow speed and needle jets.
5. Next, remove the retaining screw and bracket securing the float needle and seat assembly. Using a pair of pliers, twist and release the seat assembly. If it’s stubborn, spray the area with brake cleaner. It should pull out.
6. With the jets out we removed the slide, spraying it once more to break it free from the carb body. You can see the fuel residue that was holding it tight.
7. With the slide removed, look down into the carburetor body. The two brass points are the needle jet and slow speed jet emulsion tubes. Using a small punch, drive them out through the jet chambers in the float bowl. Again, a little brake cleaner can help break them free.
8. Here you can see the emulsion tubes, knocked loose and ready to fall out. The only jet that screws in is the idle jet, which is under the rubber plug we removed in step 4. Clean the bore with brake cleaner. Unscrew the jet with a small screwdriver; it should drop out.
9. Next, we replaced the rubber diaphragms and the needle. Using a pair of needle-nose pliers, remove the retaining clip inside the slide, followed by the plastic needle retainer and, finally, the needle. Note the orientation of the plastic retainer; it goes in flat-side down.
10. The diaphragm is clocked to the slide, with the diaphragm locating tab as the reference. Roll the diaphragm at the tab over the slide and make a mark on the slide as shown. The tab on the new diaphragm must be clocked to this same location when installed.
11. Next, using a thin metal blade, slowly work your way around the diaphragm’s plastic retainer, loosening it lightly with every pass. After a few passes, the retainer will be loose enough that you can pull the diaphragm out of its seat and off the slide.
12. Slip the new diaphragm over the slide and align its locating tab with the mark you made on the slide. Using your thumbnail, push the diaphragm into its seat. Work around the slide until it’s fully seated. Push the retainer tight.
13. Install the new slow speed jet and needle jet emulsion tubes. This photo shows the slow speed jet emulsion tube being installed. They both drop into place. Screw in the idle jet and cap it with the new rubber plug.
14. Next, give the needle and slow speed jet O-rings a very thin smear of grease. Locate the jets in the slots in the retainer spring and push them home. They are different sized and will fit only one way. Install the new needle and seat and its retainer.
15. Install the new float and check the float height. Our manual specified 26mm. To check, hold the carburetor nearly vertical so the float is touching but not pushing the needle. To adjust, gently bend the float tang where it touches the needle up or down. Install the float bowl with its new gasket and screws.
16. Drop the new needle into the slide, followed by the plastic retainer (flat side down) and the retaining clip. Drop the slide into its bore, aligning the diaphragm locating tab with its corresponding notch in the carburetor body. Install the spring and cap and secure with the new screws.
17. Install the idle/air screw. A rough preset is to screw it in until it lightly seats, then back out one full turn.
18. Reinstall the carbs, making sure the choke connecting strap is properly located and secured. For initial set, loosen each idle screw until it no longer contacts the throttle. Screw it in until it just touches the throttle, then turn it one more full turn. Refer to the manual for final adjustment with the engine running.