The goal with this adjustment is to bring all of the throttle plates or slide heights into sync with each other to help improve the bikes idle so it runs smoothly. This carb sync procedure can only be done on TWO or more carburetors. This adjustment should be made with the engine at operating temperature AFTER all pilot screw adjustments have been made and the bike has the ability to run at idle.
If the sync is out and one carb is running faster or at a higher RPM then the others, it can cause unnecessary vibrations throughout the motor. Most commonly you can hear primary chain rattle, cam chain noise, as well as spits or sputters from inside the carburetor body caused by the uneven pulse vibrations. Otherwise, carburetors should be in the ballpark even after cleaning to at least allow the bike to run.
Carburetor Synchronization: Adjustment Styles The video below is the full procedure. Be sure to subscribe and Like the YouTube channel if it helped! Step 6: Set up a fuel supply You can rig up a small container to feed the carbs, but I put the tank onto a workbench, then ran a long fuel pipe to the carbs.
Step 8: Set the engine to just above idle Get the engine back up to temperature then, by adjusting the idle speed screw, let it sit at just above normal idling speed.
Step 8: Synchronise the carbs When adjusting the carbs, you must be aware that as you press on the screw to turn it, it will throw the reading off. Before and after carb balancing. Recommended articles. Bike maintenance and kit care. How to adjust the valve clearances on your motorcycle. Used bike buying checklist: How to buy a second hand motorcycle. Adjusting the throttle valves varies depending on the carburetor configuration.
If the individual carburetors are far apart, as on a boxer twin, the adjuster will be at the throttle cable going to each carb. Parallel-twins and inline-fours have individual carburetors lined up side by side, with screw-type adjusters connecting the throttle valves between adjoining carburetors.
Some older slide carburetors have adjusters atop each slide. On a twin, there will be one adjusting screw; on a four cylinder, there will be three. On a bank of four carburetors, each outer screw adjusts the throttle valves of the adjoining outer carburetors, and the centre screw adjusts the throttles of the left- and right-paired carburetors to each other.
Twist that until the fluttering calms, but does not stop completely. Gun the engine in between adjusting, until all the needles on the clocks are fluttering somewhat calmly at idle. Open her up, and see those needles rise Your next step will be to make them rise in unison.
Loosen the countering nuts on the butterfly valve adjusting screw of each carb. Depending on your bike, you might have one carb that cannot be adjusted: that is the one to which the others have to be adjusted to. Loosen or tighten those screws, bit by bit, opening up the throttle in between, until all the needles on the gauges rise and fall simultaneously, in perfect harmony.
This does take its time, and can be a bit frustrating, not to mention noisy. Patience and perseverance does it! Once you have acquired that, and have also tightened the countering nuts, and the needles are still in perfect synchronicity, voila! You have successfully synchronized your carbs! However, if your machine, and her carbs, is old and worn, you might never get those needles to behave like a well trained dance troupe.
For all those that just want to keep the old beater running, preferably at a tolerable pace, there is a second way of adjusting it. Method 2. Use this method if you can't do it the "real" way. Maybe you don't have vacuum measuring gauges, are too poor to buy some, or the old horse is just too worn out for them to work correctly? Stuck in the middle of some godforsaken place without any mechanics, workshops, anything? Fear not, my friend, here's the bush method for synching your carbs. All you need is a few little metal balls from an old ball bearing, of about 5mm in diameter, one for each carb, easily found in any scrap yard, and added to your emergency tool kit.
Check carbs for obvious leaks in the bodies or manifolds, by spraying brake cleaner or lighter fluid on them - carefully! If the engine revs higher when you spray a particular spot, that's where an air leak is. Close it with some liquid gasket or chewing gum.
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