I posted this alternative TB Sync Procedure about a year ago. Here it is again, Have fun. Tracy...
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Alternative TB Sync Procedure
Adjusting the TB air screws alone does not provide the most effective TB sync because they only compensate for air flow at idle. Most of your riding takes place at higher RPMs. The throttle valves (butterflies) of each throttle body need to be in sync with each other at high (off-idle) RPM.
This procedure is not covered in the factory manual. However this adjustment is common and dealers that know what they are doing (and are willing to take the time) perform this adjustment on many multi-cylinder bikes. Having the TB’s in sync at cruising speeds (not just at idle) makes a huge difference in engine smoothness and vibration.
You should not have to adjust the individual throttle stop screws to achieve this. This assumes that the throttle stop screws have not been grossly misadjusted at the factory (guy on the flow bench had one too many bottles of Sapporo the night before) or the lock nuts have come loose.
To get the engine’s intake tracks in sync the four throttle body linkages are adjusted. TB’s #1 & #2 are one pair and #3 & #4 are the other pair—finally the two pairs are adjusted together (#1 & #2 and #3 & #4). This is a simple procedure but does take patience. Here is what I did to get my FJR in sync.
1) Get the engine hot by going for a short ride. When you are at 4200 RPM, squeeze you heals against the frame to feel the level of vibration. This highly precise measurement will be used later to determine how good a sync job you did.
2) Raise Tank and connect sync tool.
3) Turn all four air adjusting screws in until they are lightly seated.
4) Back each screw out 1 turn
5) You may want to place a large fan in front of the radiator to keep things cool (including you).
6) Start engine.
7) The TBs should be out of sync to some extent but not more than 1 to 4 cm of Hg. The actual vacuum level does not matter. If they are out more than this keep going through these steps—we’ll fix this later in the procedure.
8) REVING THE ENGINE Slowly rev the engine up to around 4000 RPM while watching your sync tool. What you are looking for is even vacuum levels as engine speed increases. If you try to do sync the TBs with the engine speed steady (say at 4000 RPM) you will find that the vacuum levels between TB will never be in sync no matter how many adjustments you make. It is more important for vacuum levels to be even during slow, smooth, engine acceleration and not at a steady throttle setting.
9) Using the butterfly sync screw between TBs #3 and #4 adjust the vac level so #3 and #4 are the same when you accelerate the engine as described in step 8, “REVING THE ENGINE above. This will take several tries. You may not get them exactly even either, but get them as close as you can (usually within 1 cm of Hg.).
10) Now perform the same procedure on TBs #1 and #2. You may have to rev the engine half a dozen times to get this close.
11) With vacuum levels for TB pairs #1/#2 and #3/#4 matched it’s time to sync the two pairs of TBs together.
12) Rev the engine as described in step 8, REVING THE ENGINE while watching the two pairs of TBs. If they are off by more than 1 cm or Hg., an adjustment is needed.
13) The adjustment screw can only be accessed with the engine off. The screw is located between the two pairs of TBs. (The throttle has to be opened about ¾ of a turn to get a screw driver on the screw). Turn the screw about 1/8 (either way) of a turn and restart the engine.
14) Check the balance (vacuum level) between the two pairs of TBs. Shut the engine off and readjust as needed. (By now you and your FJR’s cooling system will appreciate the large fan you are using, step 5.)
15) When the two pairs of TBs are even (when you smoothly rev the engine) you are finished with this step.
16) Now let the engine idle. Adjust the idle speed to 1000. The tach on the FJR is very accurate. I checked it against a $500 electronic sync tool that counts the vacuum pulses during the intake stroke of cyl #1 and then generates an RPM signal on the tool.
17) Now it’s time (finally) to adjust the air screws on each TB. Use cyl #3 as reference and adjust the other cylinders to the same vacuum level as #3. The actual level does not matter (I know, everyone seems to be hung up on how may cm of HG., but it really doesn’t matter as long as idle speed is kept at 1000 RPM.)
18) Make a final idle adjustment (1000 RPM) and you done. If after syncing the TBs with the air screws you can’t get the idle speed to come down to 1000 rpm, DON’T KEEP BACKING THE IDLE ADJUSTING SCREW OUT. The TB air screws all need to be turned in ½ turn. Now the idle adjusting screw will allow you to adjust the idle speed to 1000 rpm.
19) Go for a ride and check engine vibration at 4200 RPM, it should be better than when you started. It it’s worse, you screwed up the adjustment. The amount of improvement really depends on how far out of the sync the TBs were before you started. When done correctly most of the vibration you normally feel at 4100/4200 RPM should be gone—at least it was on my bike. Good Luck!
