Project Honda CB450 Cafe Racer: How To Sync Keihin CV Carbs If You’re Cheap Like Me.

How To Sync Keihin CV Carbs

Don’t let that picture fool you, this method can all be done with your carbs on the bike!

How To Bench Sync Keihin CV Carbs

Before you begin messing around with Keihin CV carbs, you need to start from a baseline sync, preferably done with an $30-$70vacuum gauge kit, with which you must also modify your carb boots to allow for a vacuum port. Right now I  don’t feel like doing that, or spending the money, as I’m really trying to shake out any other issues before I really tune this baby up. With my luck, I’d mess up a boot with my vacuum port install and spend extra time just sorting that out. If you’re running a 4 cylinder beast, you should probably invest in the vacuum gauge or get a shop to do this. I have no idea how to do it on 4 carbs! Sorry! To sync the carbs by eye on the bike and get it running, I did the following:

  • Make sure the carbs have smooth action. The springs that return the butterfly valves should operate smoothly, they can sometimes stick. You can apply some grease to the spring and actuator mechanisms to keep grit from binding them up.
  • Back out your idle mixture screws back out so they won’t touch their stops at all. The idle mixture screws are the screws on the “outside” of the carbs, if you’re sitting on the bike and looking down at your carb, the top of the screw is facing you.
    • With the idle screws backed out, you’re eliminating them from the equation.
  • Now, check that your throttle cables are set evenly. This takes time and a good eye. Firstly, if these are new cables, or shortened cables if you’re running clubman bars, you’ll probably need them maxed out since they haven’t stretched.
    • Maxed out means they are at the lowest end of their adjustable range, so they are seated in the cable slide holders at the bottom of the threaded portion. This is how mine wound up being set.
  • Adjust each so that allow the spring-return levers to fully return to their relaxed (technically engaged) position, or “bottomed out”. They can’t travel downward anymore.
    • For mine, I just finagle an 8mm wrench in to turn it ever so slowly, or try to snake my fingers in there (not to be done if you’ve got a warm bike!)
  • Now, screw the threaded cable holders back “up” until you take up any slack in the line. What you want is that when you pull your throttle grip back, you’re not also having to wait to take up a bunch of slack down here before your carbs actually start moving.
    • Do this for both sides.
  • Now, you’ve got to sync the carbs, meaning, you have to match them so that their movement and actuation is, well, synchronous. They move at the same time. When you say “Go”, they both say “On it!” at the same time.
    • Yes, motorcycles speak to me. Don’t they for you?
  • Get on the right side of your bike. Put one hand on your throttle grip and position yourself real close-like to your carbs. What you’re trying to do is line up your eye so that you can see the cable closer to you while also being able to see the throttle cable on the opposite side at the same time.
    • It helped me to look at the far side cable, while keeping the closer one in my peripheral vision. If you’re work space is well lit, light should shine on the cable strands to help you pick up on movement.
  • Slowly twist the grip and watch for movement in your throttle cables. You want them to move at the same time and same speed. If one’s moving fast than the other, turn them up or down accordingly.
    • This takes time, and probably a few tries of thinking you got it right, and then realizing you haven’t. It’s not easy, but its cheaper. Remember that.
  • Once you’ve got them moving in a sweet sexy dance all romantic-like, tighten down the lock nuts so they don’t turn themselves out.
  • Now, you’ve got the carbs pretty much bottomed out but running smoothly. If you turned the bike on now, it wouldn’t idle unless you held the throttle back a hair. Go back down to your idle screws, and turn each of them in until they are just touching their stops.
    • Do this for both sides.
  • Now that they are just touching, turn each equally in a full turn to start.
  • Turn your fuel on now. Then start the bike.
    • You may find it doesn’t really want to start, try the choke.
  • Once it’s running, adjust your idle screws until you’re idling in the range as specified in the shop manual.
    • I try to get mine withing 1,000-1,500 rpms. Since my bike is still running kinda wonky, it ranges in there but usually holds steady once it’s warmed up.
  • Try to keep your screw adjustments even on both sides. If one gets a quarter turn, the other does too. This will help maintain equal response out of both carbs when coming off from idle.

From here on, you’ve established a rough baseline for your bike to sort out other issues. Of course, using a vacuum gauge is the best method for doing this, as it gives you a vacuum read out with which you could adjust your settings precisely across each carb. I may pick one of these up now, since I could pick up a cheap kit for my twin for about $30. But, I may wind up bringing my bike into a shop for a real pro tune up once I’ve got most of the issues solved or identified by myself first. (Yes, it takes a while, but at least I’m learning. And that’s part of why I’m going through all this heartache!)

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