Synchronizing the Carburetors

The goal of synchronizing the carbs is to make sure both sides of the engine are performing the same. This adjustment has to be made at 2 places - idling, and throttle partially open, because 2 different mechanisms control the openings.

First you will need some kind of device to measure how hard the carburetors are working. Most use a device attached to the vacuum nipple and measure the vacuum side. You can also get devices that fit over the carb mouth and actually measure the air flowing through them. Since Ural carbs since 98 1/2 have the vacuum nipples installed this is definitely the easiest way.

There are many gadgets for measuring the vacuum. Fancy isn't required - you don't need an absolute measurement; you just need to be able to compare the left and right sides. If you review this board you will find home made setups, mercury sticks, rotary gauges, and electronic ones. All will do a great job.

Now, how to use them to balance your carbs!

Have the engine at operating temperature. It's a good idea to have a box fan nearby to blow on the engine your first time as it will take longer and you don't want to overheat things. After you have some experience you can do it quickly enough to skip the fan.

Attach your measuring device(s) and start the bike, allow it to settle down to an idle. Look at the devices and compare the cylinders. If you are using the vacuum nipples, the higher reading means that side is working LESS (it's further closed, (less gas), which makes more vacuum). If you use a flow-measuring device the higher reading is the one working harder.

If the bike is idling too slowly, pick the side with the highest reading and tighten the phillips screw with the spring on it that controls the idle. If it starts idling too fast loosen the same screw on the other carb. Tweak them until you have them equalized.

Blip the throttle a bit and let it settle down to check your work. Readjust if needed.

This completes the idle portion of balancing the carbs. Next you have to balance them at a partially open throttle.

When you open the throttle, the idle screws no longer control the carburetors - the throttle cables do. If one side is tighter than the other one, that side will work harder.

You will need an 8mm wrench and a 10mm wrench to adjust the cables.

Open the throttle some with the engine running and compare the sides - just go easy, enough to clear the idle screws but you don't want to wind the engine way up with no load - aim for about 2000 rpm or so for a bit. Note the levels and release the throttle.

In order to keep slack in the throttle I tend to pick the one with the lower reading (with vacuum) and loosen the cable on that carb by slightly unlocking the lock nut and then screwing the ferrule down (clockwise). You can do the other one, the main thing is to pick 1 carb and stay with it for all your adjustments.

You'll have to use a little trial and error to find out how much to turn the ferrule; try a small adjustment at first and resnug the locknut or you'll get a change caused by the looseness of the ferrule. Open the throttle again and recheck. Repeat this until they are as close as you can get them (you may not get 100% equal, differences in cable drag, valve gap, etc can cause some variations).

Essentially you are done; it's not a bad idea to swap the measurement devices from cylinder to cylinder just as a double check; some devices may not always read the same.

Shut the bike off, put the plugs back on the vacuum taps, clean up and ride!

 

Vance  Blosser

 

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