On a motorcycle with a multi-carb setup, each carburetor must have identical settings for the bike to run properly. Dialing in all these adjustments so that your carbs are the same and the bike is running smoothly is called synchronizing or “syncing” the carbs, and it’s one of the most important things you can do to your bike to improve its performance. In this article, and the accompanying video, we’re going to explain how to sync the carbs on your Honda CB175 / CL175 / SL175 / CB200 / CL200 motorcycle to get the most out of its engine.
The Keihin round slide carbs used on the 175 and 200-series bikes are simple, completely mechanical units—no fancy rubber diaphragms or vacuum-operated slides for these bad boys. This makes syncing them relatively easy, although it requires paying a lot of attention to tiny details along the way. Syncing these carbs is so easy, in fact, that all but one part of the process can be done with the bike off and the engine cold. We’ll call out the one instance where you probably need the bike running later in this article.
Prepping the Bike for Carb Syncing
For the purposes of this article, and its accompanying video, we’re going to be focusing on two adjustment points—the idle speed adjustment screw and the throttle cable adjuster screw. While both of these adjusters are on the outside of the carb, they can be tricky to get to if the tank is on the bike. So, to make your life easier, we recommend pulling the tank before getting started.
If you haven’t done it before, removing the fuel tank is pretty easy. First, make sure your petcock is turned off and remove the fuel lines. Then flip your seat up, slip the rear tab on the tank out of the rubber cushion that holds it down, and wiggle it backward off the rubber studs just behind the steering head. Make sure to set it down on something soft in a safe place where it won’t get kicked, stepped on, or knocked over. Now, with the tank off, you have free and easy access to the carbs and throttle cable.
A Quick Note On Throttle Cable Routing
Figuring out the right way cable is surprisingly tricky. The bulky plastic splitter and the unequal-length lower cables make it difficult to find the right place for the cable. In addition, the lower cables need to be positioned just right so that there’s no undue stress on the cables themselves and so that they’re not pulling or stretching and adding preload on the carburetor slides.
During the research and development we did on these bikes, we tried numerous ways of routing the cable until we found one that worked. From the right-hand switch box, we run the upper cable down behind the headlight ear, around the front of the steering head, and under the left-hand rubber mount for the fuel tank, resting the splitter tight against the frame’s backbone just above the front engine mount. Then, we take the lower cables and run them up over the backbone to get each one as straight a shot as possible into the top of each carb.
With this routing, there’s no binding or excessive wear on the cable with the tank on, and there are no sharp angles where the cable’s outer coating can crack. In addition, the cables aren’t stretched and don’t pull on the slides or the carb itself, which has a seriously negative effect on the bike’s performance and the longevity of both the cable and the internal parts of the carburetor.
Idle Sync - Phase 1
Setting your idle sync involves adjusting the carbs’ idle speed screws to position the slides in the same position when the bike is at idle.
The Idle Speed Screw
If you watched our videos on breaking down and reassembling the 175 and 200-series carbs and/or read the accompanying articles, you’ll remember we talked a bit about the idle speed screw already. It’s a long alloy or brass screw threaded into the port just above the carburetor’s float bowl. It has a thick, knurled head with a slot for a flathead screwdriver cut into it, a short stack of fine threads, and a long shaft with a rounded end. It threads into the carb and fits into a slot in the carb slide where it dictates the slide’s height and, along with that, the idle speed.
Original equipment Honda idle speed screws, along with the screws we offer in our Honda CB175 / CL175 / SL175 / CB200 / CL200 Carb Repair Kits, are stamped with a T-shaped mark between the threads and the head. The crossbar of the T runs perpendicular to the screws body and is close to the threads while the shaft of the T runs parallel and points back toward the screw’s head. This is an indexing mark used to set your carbs to a base idle speed before syncing and to help you with a reference point while performing a sync.
Throughout the years, Honda used screws with various different lengths and T-mark placements. Even our own CMC idle speed screw is unique.
What does matter—what is of paramount importance, in fact—is that your idle speed screws are the same in both carbs. If your idle speed screws are different lengths or have their T-marks in different places, you’ll never be able to dial in your idle speed correctly. Having mismatched idle speed screws is like wearing two different but similar-looking shoes. They may look the same, but they’re mismatched where it counts and their small differences make for big problems.
To set your idle speed, pop the spring over the idle speed screw shaft and thread the screw into the port cast into the side of the carb body. Tighten the screw down until the horizontal section of the t-shaped zeroing mark, the one that runs perpendicular to the screw body, is flush with the lip of the port. From there, adjust the screw slightly until the vertical mark, the one parallel to the screw body, is lined up with the pointer cast into the top of the port. Make sure the idle speed screws are in the same zeroed position on both carbs before moving on to the next step.
Progressive Sync - Phase 2
When adjusting your throttle cables for progressive sync, you’re making it so that the slides in your carbs are in the same position and move up and down at the same rate when the throttle is twisted. You do this by adjusting the throttle cable adjusters on the carburetor top caps until each cable has the same amount of slack. When properly synced, the slides move together and provide smooth, even power under acceleration.
Throttle Cables and You
Before we dive into the progressive syncing process, though, let’s talk a little about the throttle cable. The throttle cables on Honda’s 175 and 200-series bikes are fiddlier and more complex than a normal throttle cable. They’re mult-piece assemblies that consist of an upper cable with the typical connectors for the throttle grip, a tubular plastic “splitter” that separates the upper and lower assemblies, and two lower cables—one for each carburetor.
The lower cables each have a stiff rubber boot that slides down over the throttle cable adjuster when the cables are installed in the carburetors. These aren’t just dust boots, although they do a good job of keeping the adjusters clean. No, these boots are crucial to the bike’s performance, and if your throttle cables don’t have them you’re going to have a hard time keeping your carbs synced.
Why are the boots so important? Take a look inside. See how they have a hexagonal shape to them? These are locking boots designed to hold the cable adjusters firmly in place once you have your progressive sync dialed in. They do this by holding the adjuster tight and then slipping over a lip cast into the carburetor top cap to lock it firmly in place. Once locked into place, the boots prevent the cable adjusters from moving, thus keeping your sync in place.
Another thing to be aware of when working on these bikes is that they’re extremely sensitive to throttle cable problems. Cracks, kinks, a dirty inner cable, or anything else that impacts the cable’s proper operation can throw your carb sync off and negatively affect the bike’s performance. Before you sync your carbs, make sure your cable is clean, well-lubricated, free from damage, and operating smoothly.
Trying to sync your carbs when your cable is old and busted is a fool’s errand. If you have a bad cable—either a knackered OEM item or one of the many, poorly built, ill-fitting aftermarket options found on the internet—we highly recommend replacing it with one of our throttle cables for the Honda CB175 / CL175 / SL175 / CB200 / CL200. Our CMC cables are well-designed, and we worked hard to make sure that they fit your bike as well or better than the original cable.
Okay, with cable talk out of the way, let’s get to adjusting your progressive sync.
Lower Throttle Cable Adjustment
Adjusting the progressive sync is simple, it’s just using the throttle cable adjusters on top of the carburetors to take up slack in the throttle cable. To do so, back the adjuster out of the carb until there’s no slack in the cable, but right before the cable starts pulling on the slide.
Once you have all the slack out of the throttle cable, make a reference mark on the adjuster—we like to color in one of the flat faces on the adjuster with a sharpie. Using your reference mark, screw the adjuster back into the carb two full turns to add a little slack to the cable. Do the same to the other carburetor, and you should have your slides perfectly in sync with one another. Twist the throttle grip a few times to ensure that any slack is out of the cable and double check the cable play on each carb, do a fine adjustment if needed.
Upper Throttle Cable Adjustment
While the upper throttle cable isn’t actually part of the syncing process, it’s always a good idea to have a properly adjusted cable. The upper cable has two adjustment points, the jam nut that locks the elbow into place at the handlebar switch box and the adjuster nut used for taking up slack in the cable. Knowing how to properly adjust both is important to maintaining your bike and keeping it in tip-top shape.
Let’s start with the jam nut and elbow. Using a 14mm box end wrench, break the jam nut loose at the switch box. Once it’s loose, let the cable come to rest naturally. This is where the cable wants to be, and securing it in this position will reduce wear and tear on the cable and ensure smooth operation. Once the cable finds its ideal resting place, go ahead and tighten the jam nut to keep it there.
Next up is the upper cable slack adjuster, which consists of a long metal ferrule with a hexagonal cross-section and a jam nut threaded onto the metal elbow screwed into the switch box. First, slide the rubber dust boot off the ferrule and loosen the jam nut. With your fingers, adjust the ferrule until there’s about ⅛ to ¼ of an inch of play in the throttle. Once you’re happy with it, and this is definitely a “by feel” kind of operation here, put an 8mm wrench on the ferrule and a 10mm wrench on the jam nut and snug them together. They don’t have to be super tight, just enough to keep them from moving.
Setting your Idle Speed
With your idle speed set to the factory-indicated baseline, start your bike and let it warm up. You’ll have to find a way to get the bike fuel since you removed the tank, but you can either feed it from an auxiliary fuel bottle or gently set the tank back in place and reconnect the fuel lines. However you do it, once the fuel is flowing and the bike is idling, let it come up to operating temp and check the idle speed.
The 175 and 200-series bikes should idle between 1200 and 1500 RPM. If your bike idles within that range with the idle speed screws in the base sync position, that’s great! If they’re not—and they probably won’t be, you’re not that lucky—you’ll have to bring the idle into spec by adjusting the idle speed screws.
The idle speed is dictated by the location of the slides within the carb bodies—the higher the slide the faster the idle. To increase the idle speed you turn the screws clockwise to lift the slides, and to decrease the idle speed you turn them counter-clockwise to lower the slides. When you make an adjustment on one carb, we recommend doing so in half-turn or quarter-turn increments, you need to make the same adjustment to the other carb. IE, if the idle speed is too high, back out each speed screw ½ turn. Once you make your adjustments, give the bike a quick rev and let it settle back to idle. If it is now too slow, bring each screw in ¼ turn, rev the engine again and check the idle speed. Repeat the process until you have the idle in that 1200-1500 RPM range and you’re all set.
If at any point during this process you get mixed up or make a mistake and the carbs end up out of sync, don’t panic. Just go ahead and set the idle mixture screws back to their base setting and start over. After a couple of tries, you should be able to dial in the idle setting like an old pro.
So, there you have it. At the end of all this fiddling around your carbs should be perfectly synced and your bike should run better than ever. If you’ve been following along with this series since we took the carburetors apart, your fuel system should be as good, or better, than it was when your bike left the factory.
Thanks for reading along, and happy motoring. 🏍
CB200T Vacuum Sync Kit - Coming Soon