Copyright 2004 Arlen Bloom
The Unauthorized Throttle Body Synchronization Procedure
There has been a lot of interest in the Throttle Body Synchronization Procedure lately, with a new generation of Waiters finally taking delivery of their 2005 FJR’s, so I thought it timely to discuss the procedure recommended in the service manual as well as another I used to synchronize the bike at highway speeds as well as at idle.
My motivation for seeking an alternative procedure came when I found all four throttle bodies synced at idle, and all but one synced at 4200 rpm. Many FJR owners complain of excessive vibration or buzz in the handlebars between 4000 and 4500 rpm and I thought synchronization at an engine speed we spend more run time at might eliminate a good portion of that. The service manual only illustrates syncing at idle, but in theory, everything should also work in unison at 4200 rpm. To confuse things more, the manual also says to NOT adjust the butterfly adjusting screws and does not include any information on how the throttle body assembly actually works. In fact, if you need to replace certain throttle body parts, the parts diagrams indicate you would have to order a complete throttle body assembly! I decided it was time to unravel some of the mystery about throttle body adjustments.
So How Does the Throttle Body Assembly Work?
The purpose of the throttle body assembly is to mix fuel and air and introduce that mixture to the combustion chambers via the cylinder head. It is important for the airflow in all throttle bodies to be equal so that each cylinder carries its share of the load and does not create an engine that runs rough, hot, or too rich or lean. Equalizing the airflow is accomplished by performing the synchronization procedure.
Carburetors mix airflow with fuel, but throttle bodies only control airflow. While carburetors use slides to change airflow above idle, throttle bodies use butterfly valves. At idle, airflow is to be adjusted with air screws.
The FJR uses its #3 throttle body as a baseline to which the other throttle bodies are synced. In addition to the air screws, synchronization is affected by:
the initial vacuum setting of the #3 throttle body
the mechanical adjustments between throttle bodies
idle speed
interference of idle speed by throttle stop screw adjustment or throttle cable adjustment
any vacuum leaks between the air box and the throttle bodies, or between the throttle bodies and the cylinder head
Throttle stop screws are adjusted to set the minimum idle speed that can be achieved by turning the idle adjustment screw. Butterfly sync screws adjust the butterfly openings of adjacent throttle bodies so they have the same amounts of airflow at a given engine speed above idle. Air screws bleed off a small amount of vacuum to adjust the amount of airflow to equal that of throttle body #3 at idle.
Synchronization By-the-Book
Let’s start with the synchronization procedure that everyone has been doing up to now. The steps are modified a bit to tie it all in with the Unauthorized Synchronization covered later. You will need a long, thin-bladed screwdriver to adjust the air screws, a vacuum gauge, and a carburetor synchronization tool. I use a TwinMax electronic meter, but a mercury stick or Morgan Carbtune will allow you to look at the vacuum readings of all four throttle bodies at once and eliminate a lot of hose-switching between adjustments.
After warming your engine up to at least two bars on your bike’s graphic display (while avoiding exposure to carbon monoxide) and raising the gas tank to access the throttle body area, follow these steps:
Adjust the idle speed to 1000 rpm using the idle adjusting screw found on the right side of your bike. This would be just inside your right knee if you were sitting on your bike.
Find the appropriate vacuum port for throttle body #3 and remove the little rubber cap. Connect the vacuum gauge hose to the vacuum port and place the gauge where you can easily read it while making adjustments to the #3 air screw.
Adjust the #3 air screw until you read 9.8” Hg (+ 1” Hg per 1000 ft. above sea level). You may need to restrict the vacuum hose to keep the needle from jumping around. Check your idle speed again and readjust until both adjustments are attained simultaneously.
Remove the vacuum gauge hose and attach the carburetor synchronization tool to vacuum port #3 and one of the other vacuum ports. (All three if you are using a 4-hose tool.) Note which hose is connected to #3.
Turn the air screw(s) to adjust the vacuum at each throttle body to match that of #3. All four throttle bodies should be within .4” Hg. Check your idle speed again after adjusting each air screw and readjust vacuum and idle adjustments until both are attained simultaneously.
With the hoses still attached, use your throttle to increase engine speed to 4200 rpm and see if your gauge still indicates synchronization. If they all look good, you do not need to go any further. Otherwise, you may want to perform the Unauthorized Synchronization Procedure.
Synchronization Without Authorization
I have given this procedure a rather ominous name because I want to impress upon you that this is serious business. As it is not covered by the Yamaha service manual, this is uncharted territory (until now anyway) and if you are all thumbs when it comes to mechanics, you should put down your tools and step away from your bike. This method of synchronization is one that must be done meticulously and from the first step through the last. Having a brain fart or nervous breakdown in the middle of it is not acceptable! If you were to screw up the idle speed synchronization in the previous section, it is no big deal. Your bike will still start and run and you may not even realize you screwed it up. But if you screw THIS up, you WILL know it. Chances are real good your idle speed will not adjust to lower than 1300 rpm. And it goes down hill from there! If you do this procedure, it is YOUR decision and YOU suffer the consequences! I take NO responsibility for YOUR actions!
You still with me? Good. I just have to make it sound scary to chase off those who shouldn’t even be reading this, let alone jumping in with a screwdriver. Be aware, however, that we will be messing with painted screws and it is uncertain whether they were painted to keep them from turning or to indicate when some idiot messes with them. My logic is that Yamaha doesn’t plan on doing anything warranty-related with throttle body adjustments, but if they aren’t in sync at the speed you normally ride, it is required that something be adjusted. So come on lemmings! The cliff is this way!
In addition to the tools you used for the idle speed synchronization you will need a cross-tip screwdriver, 8mm combination wrench, and a 2.5mm hex wrench. Make sure your idle speed sync has been done with reasonable success, and your gas tank is raised for access to the throttle body area. This may take some time, so consider pointing a fan at the front of your bike to help keep it from melting. And don’t expose yourself to a carbon monoxide poisoning situation! Now follow these steps:
Adjust your throttle cable at the grip so there is a small amount of slack. We don’t want the cable to interfere with our idle speed when we are making critical adjustments.
Back off the idle adjustment screw until idle speed quits dropping. This will show us the minimum idle speed and it should be about 750 rpm. Don’t get carried away, if you go too far the screw will fall out and you will be on your own.
Look at the painted throttle stop adjustments. These are the pairs of small hex socket set screws with lock nuts located between throttle bodies #1 & #2 and #3 & #4. The levers connected to the butterfly valves bump up against these stops at their minimum opening. All four levers should touch their stops at the same time and the idle speed should be 750 rpm. Close any gaps by adjusting each stop screw until it just touches the lever. If idle speed is above 750 rpm, you will probably see a gap between one or more of the levers and their stop indicating a throttle body is out of sync. Back off each of the four stop screws by small amounts until idle speed drops to 750 rpm. If idle speed is below 750 rpm, adjust the stop screws to increase the idle. Adjust them by loosening their lock nuts and using the hex wrench to turn the screws then tighten the lock nuts. To gain enough clearance for my combination wrench, I removed the two screws for the fuel rail and took out the aluminum standoffs. Grinding a flat spot on each standoff allowed me to get my wrench over the lock nuts. Next time, this won’t be a problem. Be careful not to cut the little injector O-rings when reinstalling the fuel rail.
Boost the idle speed back up to 1000 rpm using the idle adjustment screw. This idle speed is very important and should be maintained after any other adjustment is made. Make an adjustment, readjust your idle speed, then check the new adjustment again until the adjustment and the idle speed are both where they should be. This applies to every step throughout the entire procedure from this point on.
Adjust the vacuum on throttle body #3 for 9.8” Hg (+ 1” Hg per 1000 ft. above sea level) using the #3 air screw. This screw should not be touched again! Throttle body #3 is our reference point and we have just set the base line. Leave it alone from this point on! Use the idle adjust screw to make sure your idle speed is 1000 rpm throughout this adjustment.
With your carb sync tool attached and throttle speed increased to 4200 rpm, adjust the butterfly sync screw between throttle bodies #3 & #4 until the vacuum on #4 equals that of #3. These painted screws have cross-tip heads and springs on the shafts. Use the idle adjust screw to make sure your idle speed is 1000 rpm throughout this adjustment. Also make sure the throttle stops do not touch the butterfly valve levers.
Sync throttle body #4 to throttle body #3 at idle by adjusting the #4 air screw. Use the idle adjust screw to make sure your idle speed is 1000 rpm throughout this adjustment.
Compare the vacuum of throttle body #1 to the vacuum of throttle body #2 at 4200 rpm and adjust the butterfly sync screw between them to make the vacuums equal. Use the idle adjust screw to make sure your idle speed is 1000 rpm throughout this adjustment and make sure the throttle stops do not touch the butterfly valve levers.
Sync throttle body #1 to throttle body #3 at idle by adjusting the #1 air screw. Use the idle adjust screw to make sure your idle speed is 1000 rpm throughout this adjustment.
Sync throttle body #2 to throttle body #3 at idle by adjusting the #2 air screw. Use the idle adjust screw to make sure your idle speed is 1000 rpm throughout this adjustment.
Sync throttle bodies #1 & #2 to throttle bodies #3 & #4 at 4200 rpm by adjusting the butterfly sync screw between throttle bodies #2 and #3. To make this adjustment, you will have to shut off the motor and twist the throttle grip until the butterfly sync screw is accessible with your screwdriver. It will probably take you about three repetitions to make the adjustment. Use the idle adjust screw to make sure your idle speed is 1000 rpm throughout this adjustment and make sure the throttle stops do not touch the butterfly valve levers.
Sync throttle bodies #1, #2, & #4 to throttle body #3 at idle by adjusting the respective air screws. Make adjustments to the idle adjust screw after each air screw adjustment to ensure that idle speed remains at 1000 rpm.
Compare the vacuum readings of all four throttle bodies at 4200 rpm to see if they are within .4” Hg of each other. If they are, your throttle bodies are now completely synchronized and you are finished. If not, go back to the beginning of this section and perform the procedure again. It is not uncommon to have to go through the procedure one or two more times as the synchronization of the throttle bodies is very, very dependent upon idle speed and every adjustment you make along the way affects the idle speed. If you do one step with the idle speed at 1000 rpm and two steps later you make an adjustment when the idle speed has varied by as little as 50 rpm, this method cannot be completed successfully. But the second time you go through the procedure, all your adjustments will be much smaller than the first time and the idle speed won’t be affected as much. This will make your subsequent adjustments more precise and throttle body nirvana will be achieved. While it is possible to sync all throttle bodies within .4” Hg at both 4200 rpm and 1000 rpm, it is ultimately up to you to decide what level of accuracy you want to achieve. In the end, however, I think you will find that this method is effective at reducing at least some of the vibration formerly experienced between 4000-4500 rpm. Keep in mind, though, that there are other things that could also cause that same vibration; this just deals with one of those causes.
