Carb Sync

Work in Progress


Syncing, AKA balancing, the twin Keihin CVRD 43 side draft carburetors is a snap. The KTM 950 is probably one of the easiest “faired” bikes I’ve had the pleasure of doing in my forty-seven years of tuning. The carbs should be checked for proper sync every 15,000 km. I also check the sync anytime I make any changes to the carbs, adjust the valve clearances, or the engine “feels” rough or otherwise is not running smoothly.

It’s really no mystery as to what you’re trying to do here. All you are doing is adjusting the linkage between the front and rear carbs so that both carbs’throttle plates are opening the same amount at the same time (same amount of air flowing through each carb). If this isn’t happening, the cylinder with the highest flow will be straining to pull the other one up to its speed. The result can be rough running, higher engine temps, poor economy, and poor throttle response, to name a few.

You will need some type of vacuum gauge set or flow meter to compare the airflow of the two carbs. There are a number of ways to do this. Here are just a few available. The mercury “tube type” shown above measures vacuum in the intake manifolds which is indicative of the airflow.

The KTM Repair Manual shows a dual analog gauge 600.29.011.000 which also measures vacuum. No matter which type vacuum gauge you use, it must have adequate damping of the tubes/gauges or the task can become a chore. Also, with these multiple dial/tube type tools, be sure to compare the two dials/tubes to one another by connecting both to the same vacuum source and adjusting the damping screw (if applicable). The readings should be identical.

Another option is a direct airflow meter like the STE Flowmeter shown above which measures the actual airflow though carbs or F.I. throttle bodies. The correct model for the 950/990 is the 6537 AKA the “SK”. It fits snuggly in the air horn and doesn’t need to be held in place (though if you touch your finger to it lightly, it will make reading the gauge easier with the “normal” V-Twin engine vibes). It’s the most accurate and easiest to use tool of the bunch, as no vacuum connection is needed. Click on the image above for a link to more info on this tool.

Above image shows how the STE SK fits in the air horn. Note that you will need a gasket between the airhorn and the carb when the air filter is removed. I simply removed a pair from an old filter, but you can make your own out of closed cell foam or an appropriate size rubber gasket.

You can even make a “penny tech” version of the vacuum tube gauge for …. well pennys. Take a look at this thread on the Advrider Forum if you are so inclined.

To start with, remove the glove box, air box cover and air cleaner element. The fuel tanks need not be removed for this procedure (if you have previously routed the vacuum hoses to an easily accessible area or are using an STE Flowmeter). The arrow points to the screw you turn to sync the carbs. Its on the left of the rear carb as you face the front of the bike. You should be looking at something like the above photo.

The above image is a close-up of the adjustment screw with the long shaft Philips screw driver engaged. This screw adjusts the rear carb’s relationship to the front carb. Turning the screw ccw increases the rear carb’s opening.

  • Ensure that valve clearances and idle mixtures are in spec (you will have to re-sync after adjusting clearances/mixtures).
  • Make sure the engine is warmed up to operating temps and make adjustments a small bit at a time.
  • Insure that the choke is fully off.
  • The Repair Manual says to adjust for equal readings between the two carbs at idle speed. I get ~7 kg/H at an idle speed of 1400 rpm with the STE Flowmeter. I also like to check the balance at the RPM I cruise at. I haven’t found an appreciable difference on the 950, but many of the four cylinder bikes that I’ve tuned did.

I’ll describe the adjustment procedure using a set of mercury vacuum tubes in the next few photos. The same procedure is adaptable to the other tools with a bit of common sense.

Save yourself a lot of hassle and take this opportunity to route your hoses to a place where you won’t need to remove the tanks in the future. All the info you need to do that job can be found in the Canisterectomy article elsewhere in the HOW. The image above shows which intake manifold ports to connect the vacuum lines to the gauges to. Leave a short section of hose connected to each manifold as shown by the red arrows. Then route the tubes up and ziptie them to the frame in a location that allows connection with the tanks mounted. Plug the ends with a suitable sized screw. Then, next time you want to check your carb sync, all you need to do is remove the glove box, and connect your gauge with double male unions.

Above is an image of the double male you need on the end of the gauge hoses. This one incorporates a damper.

Here is the procedure from the Repair Manual for those who haven’t re-routed your vacuum hoses yet:

“NOTE: before you start with the maintenance work
described below, remove the engine guard, seat and both tanks
(950/990 Adventure
– Disconnect the vacuum hoses from the air filter box and from the
secondary air system valve and run up on the side between the
frame and the air filter box.
– Mount both tanks and connect the fuel lines, open the fuel taps.
NOTE: before you use the special tool 600.29.011.000, always check
both dial gauges for synchronism:
– Connect both dial gauges to one cylinder with the vacuum hoses and
a T-fitting.
– Tighten the knurled nuts on the special tool by turning them in a
clockwise direction almost to the stop.
– Start the engine and unscrew the two knurled nuts until the dials
barely begin to vibrate. You should still be able to read the gauge
clearly. Both dial gauges should indicate the same value. If not, the
special tool is damaged and should not be used.
– Turn off the engine, disconnect the hoses and the T-fitting.
– Connect each dial gauge on the special tool to a cylinder. You can
also use the on-board hoses (see photo).
– Tighten the knurled nuts again (also adjust stiff damping).
– Start the engine and unscrew both knurled nuts until the dials barely
begin to vibrate. You should still be able to read the gauge clearly.
Both dial gauges should indicate the same value. If not, dismount the air
filter and turn the synchronization screw 3 on the carburetor linkage
until the dial gauges indicate a value of +/- 0.03 bar.
NOTE: before making the adjustment, make sure the cold-start system
(choke) is completely closed and the engine is at operating temperature.
– Remove the vacuum gauge and connect the vacuum hoses to the
secondary air system valve or to the air filter box again.”

Warning: Be very careful about reving some engines with mercury “tube type” gauges. The high vacuum generated when the throttle is suddenly closed from high RPM or “blipping” the throttle may suck some of the liquid mercury into the engine through the intake valve. This is not a problem with all of the gauges out there, but it has been on some. ‘Nuff said.

In the above image you can see that the two columns of mercury are not identical. This is actually “pretty close” and most dealer mechs would leave it as is.

I’m a bit of a stickler when it comes to carb syncing, so I will fiddle with the adjustment until it is perfect. But that’s just me.

Now, adjust the idle speed to 1400 RPM if it has changed (it doesn’t always). The adjustment is that black plastic knurled knob at the end of a cable on the left side of the bike just below the tank and above the foot peg. It may be hidden under the frame member. Dig around a bit and you’ll find it. CW increases the RPM. CCW decreases it.

Now, button everything up and GO Ride!