Welcome to part two of our series on rebuilding the Keihin round slide carbs found on Honda 175 and 200-series bikes. If you’ll recall, in the last article we tore down a carburetor off a CB175, discussed all the various components, pointed out some interesting facts, and generally had a good time. In this article, we’ll reassemble that CB175 carb with one of our new Carb Rebuild Kits for the Honda CB175 / CL175 / SL175 / CB200 / CL200 and get it ready to pop back on the bike.
A Quick Note On Carburetor Cleaning
Before we get to the meat of the rebuild, we must know—is your carb clean enough? For a proper rebuild, a carb body needs to be clean enough to eat off of. You know all those little passages and venturi and chambers in the carb that move fuel and air around? Those have to be totally clear for the carb to work right. Even the smallest obstruction can affect performance, which is one of the reasons why carbs can be so fiddly.
Even if you think it’s clean enough, double-check. Then check again. Make sure you blow carb cleaner through every nook and cranny, then chase them with a guitar string or a set of tiny carb cleaning brushes/wires. Seriously, taking your time to make sure everything is spotless may be tedious, but it’s not as tedious as trying to troubleshoot a bike that won’t run right.
Having said all that, are your carbs clean enough? Are you sure? Okay, good. Let’s get to work.
Float, Float Bowl, and Jets
We’re going to split the assembly work into two parts—The top of the carb and the bottom of the carb. In this article, we’re going to start the rebuild with the bottom, which includes everything covered by the float bowl. Assembly is, essentially, just the reverse of the disassembly we talked about in the last article, but there are some important things to watch out for—like the proper orientation of the emulsion tube discharge nozzle—that we’ll discuss in detail.
Discharge Nozzle and Jets
To start off, flip the carb over and set it on the bench so that the bottom is facing up. Now find your shiny new discharge nozzle, it’s the short brass fitting with the ridge around the outside. See how the ends are different? One end has a broad opening with a bevelled edge and the other is flat with a smaller opening. When installed, the broad, bevelled edge faces the top of the carburetor where the slide assembly is, and the flat end with the small opening faces the float bowl. Pay attention here, and don’t get your ends mixed up.
Before you install the discharge nozzle, we recommend cleaning up the lip of the broad, bevelled opening with some fine grit sandpaper. Occasionally, these components come off the lathe with extra sharp edges and tiny burrs on that lip which makes installation more difficult. If you gently sand around the bevelled edge before you drop the nozzle into place, you can soften the edge and knock down any burrs, saving you a potential headache.
Once you’ve smoothed out the bevelled opening, put a little lubricant—we like Marvel Mystery Oil—on the nozzle and drop it into the long, central port cast into the carb body with the tapered opening facing toward the top of the carb. Place the carb on a flat surface and, with a small punch and a lightweight hammer, gently tap the nozzle into place until you feel it seat itself. When correctly installed, the nozzle’s lip protrudes slightly into the carb’s main venturi.
Next, it’s time for the emulsifier tube. Lube up the threads with a bit of oil and gently thread the emulsifier tube into place over the discharge nozzle you just installed. Use a 7mm wrench to gently snug the tube in place. Don’t worry that you have exposed threads, it’s supposed to be like that. Once that’s in place, lube the threads on your main jet and screw it into the emulsifier tube until it’s snug.
Our carb rebuild kit comes with three sizes of main jet—#90, #92, and #95. When installing the new main jet, make sure it’s the same size as the old one you’re replacing. The jet sizes we include in the kit cover the stock jet sizes and allow for tuning if you have aftermarket exhausts or air intakes.
Finally, we’ll install the idle jet or pilot jet in the little port next to the emulsifier tube/main jet port. Like the emulsifier tube and main jet, apply a touch of oil to the idle jet’s threads and screw it into its port. Make sure it’s just snug, you don’t have to really crank down on it with all your strength here. These things are brass and they’re going into aluminium threads. Going whole hog with the screwdriver is a great way to strip your threads and ruin your carb.
Float, Float Needle, and Float Needle Seat
With the discharge nozzle, emulsion tube, and jets all squared away, it’s time to install the float, needle, and seat.
Needle and Seat
The first part, installing the needle and seat, is bog simple. Put a drop of oil on the seat’s threads, pop the fresh new aluminium crush washer on the shaft, and with an 8mm socket, tighten the seat down. As with the jets, only use enough force to make it snug because it’s real easy to strip these threads.
With the seat in place, go ahead and drop the needle in. Make sure that the pointed end is inside the seat and the spring-loaded pin is facing up. The pin rests against a tang in the middle of the float and helps with the regulation of fuel flow into the bowl. If you put the needle in backward, which happens more often than you think, the carb will leak like a sieve and will never work properly. Measure twice, cut once, and make sure the needle is in the right way before installing the float.
How’s your float? Is it okay? Any old fuel, carb dip, or other cleaning solvents sloshing around inside? If so, you’re going to need to replace it. The soldered joints on these old, 50-year-old floats can crack and degrade over time, allowing fuel to seep in. When this happens, the fluid inside weighs the float down so it doesn’t, well, float in the bowl like it’s supposed to. This compromises its ability to meter fuel flow properly and makes trying to get your bike running properly an absolute nightmare.
As affordable as new floats are, trying to solder up an old, leaky float (which in itself is a huge pain) doesn’t make sense from a time or money perspective. If you have a leaky float on your hands, we recommend just replacing it with a new one. It’ll ultimately save you a ton of time and effort.
Whether you’re reusing your OEM float or installing one of our new replacements, installing the float in your freshly cleaned carb is a breeze. Or, well, it should be. Just line up the float’s central support with the holes in the towers cast into the carb body. Slide the pin through, and voila, one installed float. To ensure you have the float installed right side up, make sure that the adjustment tang that touches the float needle is facing down.
Setting Float Height
Float height is crucial to properly operating carbs. Proper float height is the measured distance between the highest point of the float and the float bowl gasket surface. The proper height varies by bike, so consult your shop manual for the right measurement for your bike.
To measure your float height, turn the carb slightly on its side so that the float’s adjustment tang is just touching the spring-loaded float needle pin. If you measure while the float is actually resting on the float needle, the float’s weight can compress the needle slightly and throw your measurement off.
With the carb properly situated and held still, measure both floats from the float bowl gasket surface to the highest point of the float, roughly the top middle. Make sure not to touch the float as you measure, because you’ll throw off your measurement. If the floats are close to the right height, within a millimeter or two, you can adjust them by carefully bending the horizontal support until they’re correct.If they’re way off, you’ll need to remove the float from the carb and bend the adjustment tang to bring the float in line.
The final part you’ll need to install to wrap up the bottom of the carb is the float bowl. Before installing the bowl, though, make sure it’s spotless. Maybe give it one more splash of carb cleaner just to make sure. While you’re at it, inspect the little brass tube in the center of the bowl for any cracks, holes, or looseness. This is the overflow pipe that vents fuel if the carb floods, so if the pipe is loose or cracked it’ll leak fuel everywhere. You can polish up the tube with some fine steel wool to make any small cracks easier to see.
If your float bowl is clean and the overflow pipe is good, go ahead and place the float bowl gasket in its groove around the bottom of the carb body. Then place the bowl over the float and, using the spring clip, secure the float bowl in place. Lastly, go ahead and install your freshly cleaned (or new) drain plug. If you have an early-style “small head” drain plug, use the aluminum crush washer included in the CMC rebuild kit. If yours is a later-style “Big Head” drain plug, use the small rubber o-ring to seal the plug.
There, all set. The bottom of your carb is all squared away with new components and you can move on to the next part of the rebuild—the slide assembly.
Idle Screws and Slide Assembly
Reassembling the top and exterior of the carb is, like the float bowl end, just a reverse of disassembly. With new parts and a clean carb, there aren’t too many pitfalls to look out for. Things are, thankfully, pretty straightforward. There are a few things to look out for that are unique to these little carbs, and we’ll talk about them in the following sections.
Idle Mixture Screw
The idle mixture circuit on the Honda CB175 / CL175 / SL175 / CB200 / CL200 carbs is an air bleed-style circuit, meaning that you add or remove air to the idle mixture to adjust it. Throughout the 175 and 200-series production, the carbs were equipped with two different kinds of mixture screws—solid body and hollow body. Solid-body screws were used on earlier 175s, and when screwed all the way in they completely shut off the airflow to the idle circuit. The hollow body screws found on late model 175s and 200s are drilled out so that even when screwed all the way in they allow at least some air into the idle circuit.
We provide both kinds of screws in our CMC Honda CB175 / CL175 / SL175 / CB200 / CL200 carb rebuild kit. When installing new idle mixture screws in your carbs, it’s important that you use the same type of screw in both carbs. Honestly, you can’t go wrong with just replacing your old mixture screws with identical new ones. If you somehow lost your old mixture screws, however, or your carbs were missing them altogether from the start, you can use either type in your rebuild with no problem as long as they match.
Whichever screw you go with, installing them is simple. First, dab a little oil on the threads to make things nice and easy. Then slip the spring over the screw body, slide it into the idle mixture screw port (the one closest to the air cleaner when the carb is installed on the bike), and just screw it in with a flathead screwdriver. Thread the screw in gently until you just feel it seat. Once it’s seated, back it out one full turn. This gives you a base setting from which you can fine-tune the carb once it’s on the bike.
Idle Speed Screw
The second of the two adjustment screws mounted to the outside of these carburetors, the idle speed screw does just what it says on the tin—it controls the bike’s idle speed. To do so, the blunt rod on the end of the screw protrudes into the carb body and fits into the angled slot in the side of the slide. How far it’s screwed into the carb body dictates how much the slide opens—the further in the screw is turned the faster the idle, and, of course, the further out it is the slower the idle.
To install the screw, simply lube up the threads, slide the spring over the shaft, and thread it into the port. See that t-shaped mark etched into the screw just below the head? When installing new idle speed screws for the first time, you’re going to use these marks to set the base idle speed sync. To do so, thread the screw in until the line that runs parallel to the screw lines up with the pointer cast into the top of the screw mounting port and the perpendicular line sits flush with the port opening. The screws won’t stay there permanently, but setting them up like this gives you a solid base from which to start your tuning.
Intake Manifolds and Choke Linkage
Let’s go ahead and pop the intake manifolds back on and reconnect the choke linkage. The manifolds can be tricky if you’re not paying close enough attention, and it’s easy to install them incorrectly. This doesn’t necessarily hurt anything, but it is funny.
To install the manifold, put a fresh o-ring in the groove on the carburetor body’s intake flange. The manifold has two faces, one with a groove and one without. Place the side of the manifold without the groove over the carburetor, line up the holes, and screw in the screws. We recommend you dab a little anti-seize on the threads before threading the screws in to save you (or some future owner) a headache if you have to take the carbs apart again.
Once the manifolds are in place go ahead and reinstall the choke linkage. To do so, just line up your carbs side by side and place the eyelets on the end of the linkage over the mounting posts on the choke arms. Then slide your washers over. To secure the linkage in place, Honda used tiny cotter pins, which you took off and likely mangled during disassembly. If you have appropriately sized cotter pins lying around, that’s great! Go ahead and use them. If not, you can use tiny hitch pins or safety wire. Whatever you use, make sure it’s secure and won’t unwind or vibrate off once the carbs are on the bike.
The slide assembly is the final major component of the Honda CB175 / CL175 / SL175 / CB200 / CL200 carbs you’ll need to deal with for reassembly. Once finished, your carbs will be complete and you can slap them back on your bike and proceed with syncing. Before that, though, there are some real fiddly bits to deal with.
Just a quick note, first. Putting the slide assembly back together and installing the throttle cable is typically done with the carbs bolted to the engine since the throttle cable is an integral part of the slide. Since we have the carbs on the bench for this video and article series, we’re going to just pretend they’re on the bike. If you’re doing this at home, we recommend having the carbs mounted to the bike first and assembling the slide and top cap on the bench. Good? Okay, let’s dive in.
Slide and Needle
If you remember from the previous article that covered the disassembly of these carbs—and you should, because why would you start in the middle of a project?—the slides on these 175 and 200-series carbs aren’t identical, they’re left/right specific. The problem with this is that, if you’re not paying close enough attention, you can put the slides in backward. Putting the slides in the wrong way makes these carbs an absolute bear to tune and the bike will refuse to idle without some throttle input. It’s an easy mistake to make, and one that even seasoned professional bike mechanics can fall prey to.
To avoid installing your slides the wrong way around, take a good close look at them before reassembly. Specifically, look at the bottom of the slide. See how one half is flat and the other has an angle cut into it? When the slide is inside the carb, the flat half faces the engine and the angled part faces the air cleaner. The little cutout in the slide allows air into the idle mixture circuit even when the slide is completely down. No air, no idle, which is what happens when you put the slides in wrong.
First, choose the correct needle for your bike out of the rebuild kit. The needles included in the kit are marked 175 or 200 for your convenience, so just pick out the ones you need and keep the others for a future project. Install the little retainer clip in the same groove as the old one you took out of the carb during disassembly. If the old clip or needle got lost, just go ahead and put the new clip in the middle of the five ridges on the new needle. That’ll give you a solid starting point for future carb tuning.
With the clip in place, drop the needle into the slide. Then gently grab the v-shaped retaining spring with a pair of needlenose pliers and slide it down into the slide over the end of the needle and the retaining clip. Once it’s in the slide, use a small flathead screwdriver to seat the clip and adjust its position.
When you install the v-shaped spring clip, make sure the open end of the v faces the long, narrow slot cut into the side of the slide. This groove is where the throttle cable goes, as well as an index slot that goes over a tab inside the carb body. If you put the retaining spring in wrong you can foul the slot and make it so that you can’t get the throttle cable in. Again, patience and close attention are the key here.
Top Cap Assembly
Assembling the top cap is one of the simpler tasks on these carbs. First, dab a little anti-seize on the throttle cable adjuster threads and screw it into the top cap. Thread it all the way down so it sits flush with the cap, this will give you a good starting point for future tuning and adjustments. Finally, fit your new top gap gasket over the locating tabs on the bottom of the top cap. The gasket might be a little snug or fit a little funny at first, but it will eventually relax and seat fully.
Now it’s time to put all this together. First, take the knurled top cap retaining nut and thread it onto the throttle cable. Slide the end of the throttle cable through the throttle cable adjuster on the top cap, and then slide the spring over the end of the throttle cable that protrudes from the bottom of the top cap. Compress the spring and pull the throttle cable out as far as it will go. Now we’re ready for the slide.
Locate the long, vertical slot on your slide. That’s where the throttle cable goes. See that round depression on the bottom next to where the needle comes out? With the slide in one hand and the throttle cable in the other, put the metal knob on the end of the throttle cable in that depression, pull the cable through the long slot, and let the spring fall into place inside the slide. At this point, the slide assembly is ready to go into the carburetor.
Put some anti-seize on the threads at the top of the carburetor where the slide goes in. Since it’s so exposed, that bit likes to corrode and will jam up the top cap ring so a little anti-seize makes good insurance against future carb-related headaches. Once that’s done, feed the slide into the carburetor, making sure you have it on the correct side with the angled bottom half facing the air cleaner. Now seat the top cap over the top of the carburetor. See how the throttle cable adjuster isn’t centered? Make sure that it’s toward the engine when you put the cap on.
To finish, bring the top cap ring down and screw it over the top cap until it’s snug. Once you have that done, slide the rubber boot down over the throttle cable adjuster and click it into place. There, all done! Now you can sit back and admire your work.
Look at that, two shiny, clean, reassembled carbs with all new components inside. Feels good, doesn’t it? Go ahead and take a moment to soak it all in. If you did things right, if you followed this guide and its accompanying video to the letter, your carbs are as good as they were when the bike left the factory and they’re ready to be installed on your engine.
In our next video, we’re going to teach you how to sync these carbs to get your bike running as well as it can.