I'm not Brian P, but I'll take a stab at it.
Carburetor synchronization (C.S.) is all about vacuum and having Fun! (note heavy sarcasm). C.S. will not cure a stumbling, shoddy running engine. It will however provide the smoothest performance, easiest acceleration that your engine can develop.
A motorcycle equipped with multiple carburetors and an equal number of cylinders could easily be considered to have multiple "Engines". Each cylinder has its' own fuel supply, air supply, compression and spark. However, each of these 'engines' will fight each other (common crankshaft) if their performance is not balanced.The intent of performing carburetor synchronization is to balance the performance of each cylinder or "engine". Balancing the vacuum that each cylinder creates will provide the smoothest performance, easiest acceleration.
Commonly C.S. is not done with a load on the engine (dynomometer), and therefor it is only possible to set vacuum accurately at idle / just above engine speed. 1/4 - full throttle range isn't metered.
The first step to C.S. is checking intake and exhaust valve clearance. If your I/E valves are out of adjustment or the adjustment of the exhaust and intake valves is not as near to even as possible your carb synchronization will actually result in an overall LOSS of power. Think about it:
I/E Valves that are too tight will result in a loss of compression. Less compression = less vacuum.
If one or more cylinders' (but not all) valves aren't opening to their peak (loose valve clearance), then you won't be able to create as much vacuum as the remaining (unable to draw in fuel/air mixture in equal amounts or evacuate exhaust gases in equal amounts). Either way; you will either tune all of the cylinders to perform as per the failing cylinder or, find it impossible to reach balance. If your valves are tight or loose you MUST adjust them prior to performing the CS or you WILL LOSE POWER! If the valves are within spec and yet you have a few that are near opposing limits, you can still do the CS, but you are 'leaving a little bit on the table'.
Step 2, perform compression test. Here we're looking to see that each cylinder provides a balanced reading with the other cylinders. Be sure the battery is in a good state of charge and remains so throughout the compression test. Compression test should be done with the engine cold (don't pull spark plugs from an aluminum head while it is hot!).
Shut off fuel. Remove the spark plug wires from each spark plug. Engage engine 'kill' switch. Remove one spark plug and install compression tester. Hold throttle wide open. Crank engine until gauge stops climbing. Document reading, remove compression gauge and install spark plug. Move to next cylinder and repeat.
If you find that one or more of the cylinders has low compression (15% difference or more) than the others; STOP. You've got issues beyond what a carb sync can eleviate.
Step 3 Remove carburetors and clean if required (Fun!). Pay special attention to the pilot and main jets. You can skip this step if you are confident your carbs are clean. If you are a weekend warrior and run ethanol contaminated fuel, they are likely not. Have a look at your diaphragms as well (CV carbs) and make sure they are intact.
Re-install carburetors (Fun!)
Step 4. Mechanically synchronize the carburetors. This process insures that all of the carburetors' throttle valves are at equal depth while the throttle is released. Pull the air box (Fun!) and locate the 'master' carburetor. It's the one that doesn't have an adjustment for throttle valve height (usually #2 or #3); open the throttle, set a 4" piece of 14 gauge single conductor bare copper wire (ground wire for your house) under the lip of the throttle valve in the 'master' carburetor as near as possible to the center of the carburetor's throat (you'll have to slide the wire in beside the needle, and deep enough that it reaches beyond the outer edge of the throttle valve) and close the throttle, pinching the wire in place. Now lower the remaining throttle valves to match, using another piece of 14 gauge wire as a feeler gauge. Set each carb to an equal amount of 'pull' as the first. Remove all copper wire.
Now we can start the actual process of performing a carburetor synchronization.
The air box should be in place as it greatly affects the flow of air to the carburetors. Some models might be trickier than others to access the air/fuel mixture screw (Fun!). Suck it up butter cup, long bladed screwdrivers are a must. Attach your manometer to the bike (Fun!). If you're this far into the process and haven't already started looking for an easier answer (Gunson's colortune, 1 per cylinder), odds are you have a manometer that's capable of measuring multiple carbs. Commonly there is a vacuum port that is capped by the factory on the intake manifold of each carburetor. Connect your manometer hoses here. Ensure good quality connections as they will greatly effect the overall efficacy of your C.S.
Starting from scratch:
The engine must be brought up to operating temperature. After performing the mechanical synchronization of the carbs, you may notice that your idle speed is higher or lower than normal (assuming adjustments were made). Set the engine idle speed as per your manual (operator or service). Place a fan in front of the bike to keep some air flow over the engine; your not looking to cool the engine but just keep air flow over it.
Set yourself up so that the vacuum line for each manometer is attached to each sequential carb.
Shut the engine down. Slowly turn in all of the air/fuel (also known as pilot) screws until it is seated SOFTLY (DON'T CRUSH it, you may damage your carburetor!), monitor how many rotations it takes to get each screw seated softly (usually 1 3/4 to 1 1/4) and document. Back out each screw 2 full turns.
Re start the bike.
Identify which of the cylinders has the least vacuum (pulls least fluid on manometer). Adjust that cylinder's air/fuel (pilot) screw slowly 1/4 turn out, then start to slowly turn it in. Locate the point at which it's vacuum is highest and move to the next lowest cylinder. Repeat, Repeat.
Identify which of the cylinders has the least vacuuum and which has the maximum. If there is a massive descrepancy, STOP! Check for intake manifold leaks or unseated air box boots etc. Double check the quality of your connection to the intake manifold for your manometer!
Adjust the each of the remaining cylinder's air/fuel (pilot) screw until they match the lowest. Repeat, Repeat, repeat (Fun!)
With any luck, your manometer gauges will balance, each of the air/fuel (pilot) screws will be somewhere between 1 - 3/4 to 1 - 1/4 turns out from softly seated.
If You've done something foolish like add a performance exhaust or air filter / pods and not made the appropriate jetting changes (SHAME! Note, manometer readings will balance, but you'll likely be 2 or more turns out on the A/F - pilot screw)
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