Carburetor Synchronizing seems to be one of the most misunderstood carburetor adjustment topics.
“Does your V-twin or inline four Honda seem to idle sporadically? This may be your fix!
Whether you’re a YouTube-certified mechanic or decided to learn a new trick to do on your own bike, let’s erase everything we thought we knew as well as the bad habits we have created and bring this topic to rest. In this post, we will go through basic information to gain a full understanding of how a carburetor synchronization tune is done and how it effects your bike.
Carburetor Synchronizing: Purpose
Carburetor synchronizing has NOTHING to do with air/fuel mixtures and how they are metered inside the carburetor. Simply put, the purpose of carb synchronizing is to ensure that your bikes carburetors are all in the same RPM range at idle, so that each cylinder is drawing the same amount of vacuum through the carbs…
I know.. MIND BLOWN.
Think of it this way, I’m sure you are aware of your main idle adjustment knob. Most often they are black plastic knobs (sometimes metal) that allow you to raise and lower your total or overall idle RPM of the bike. Carb sync adjustments allow you to do this exact thing but for individual carburetors. 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.
Carburetor Synchronization: Symptoms
Rough and/or erratic idle
Poor fuel mileage
Abnormal engine noises
* This is caused by what is called “power pulses”. 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.
Increased exhaust emissions
Air leaks (vacuum leaks)
* You may notice that no matter how much you adjust a carb’s sync adjuster it doesn’t change the gauge reading. Check your hose connections to the vacuum ports and check each carb’s intake boot at the motor (post carb). One easy trick is to have a can of starting fluid around to lightly spray around the boots or potential leaky mating surfaces, cracked rubber or poor vacuum gauge connections. You will hear a quick fluctuation in RPM once the leak is found. Here is a helpful VIDEO TO WATCH on detecting vacuum leaks from my buddy Matt from www.howtomotorcyclerepair.com/
Engine won’t start
* This is typically caused by a major difference in the vacuum intake at the throttle plates or mechanical slide being too far out of wack. Either the carbs were not put back together correct after a full split and clean, you adjusted something that did not need to be adjusted or worse case scenario you took the throttle plate screws out, didn’t re-install the screws tight and dropped a butterfly into the intake port. (Trust me I’ve seen it). 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.
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CV (constant velocity) style carburetors are the most common style found on bikes where the adjustments you make directly affect the throttle plate openings.
Mechanical slide carburetors are different due to the fact that adjustments are made to the internal slide.
I introduce the “Keyhole” style carb.
*Don’t worry, the goal of the carb sync adjustments on this style are EXACTLY the same. How you adjust is the difference.
“Bottle Top” carbs (Pre 1977 for Honda) are different in their own way. With this style below there is no “base carb” instead you can manually adjust all four carbs together with individual adjustments. Honda does have a spec for where they would like the vacuum to be at on the gauges which are at 20-22cmHG
What the heck does cmHG mean?! For you deep thinkers out there, here you go.. (cmHG meaning)
Carburetor Synchronizing: When Should I?
– If you happen to purchase (links below) or own a carburetor sync set, then you are more than welcome to perform this task as much as your heart desires. Realistically, I tend to do this procedure every 24,000 miles as part of a full-service tune-up. Typically, the carbs are within range (40mm) of each other but sometimes they are out. Not to mention the satisfaction of bringing those sync gauge needles as close as humanly possible can be a fun.
– If you just recently had your carburetors apart for a cleaning or full disassembly, this is when I highly recommend this procedure to be done. This should be the final adjustment you make to your perfectly clean carbs. Your finishing touch.