Synchronizing the throttle body on the Honda VTX 1800

Synchronizing the throttle body on the VTX 1800
Information courtesy of Biglry

Preface
I have been asked about how to sync the throttle body on the 1800 engine so here is another "How I do it" post. What this adjustment does is make the vacuum in the intake manifold or intake runners equal at idle with the throttle closed. The VTX does not have a common intake manifold, but two separate intake runners cast together as one unit with no common plenum chamber. When the idle vacuum is equal the bike will have a smoother idle and it helps cut down on decel popping when slowing down with the throttle completely closed. Comment by Bare - Unless you have a basic understanding of this process I would not undertake this without supervision by someone who does.

Preparation/Supplies

Two 10mm wrenches (sockets or wrenches) to remove the bolt at the rear of the tank so you can raise the tank

Whatever tools are necessary to remove your airbox (OEM requires 5mm allen and #2 Phillips screwdriver)

Short flat screwdriver or pilot screw Wrench - Honda OEM #07908-4730002. I use This one from Carbtune. You can also buy similar offset drivers at most auto supply houses.

Vacuum guages - I use a Carbtune II, but any good quality mercury carb stick will work. You can get a Carbmate or Vacuummate as they are the ultimate in air sync tools IMO, but the Carbtune II has no mercury and has worked well for me. I don't care for liquid filled gauges since they need to be calibrated to be certain they are reading the same.

Process
First ride the bike for a few miles to be sure it is good and warm. Shut the bike off, lift the tank and remove your air box to gain access to the air adjustment screws on each side of the throttle body and the vacuum hookups (B & C in the pic below). Here is a pic of the throttle body and where the air screws are located - I know you can't see the one for the rear cylinder in the pic, but it is just like the front cylinder one.

Carefully turn each air screw all the way in, counting the turns, and then bring it back out the same amount, this is just to see where the screws are set to begin with.
DO NOT TORQUE/TIGHTEN THESE SCREWS CLOSED just use a light touch.
If the air screws are close to being the same (within a half turn) then go on to the next step, if not then adjust the rear cylinder to within a half turn of the front. I have never found the air screws to be very far apart, if I was given a throttle body with them removed I would start each at 1.5 turns.

Below is a picture of a 49 state intake from a VTX1800. You will note I have marked where there is another brass nipple ("D") on the CA intake that is not found on the 49st intake.

Hook your vacuum gauge to port "B" (front cylinder) & port "C" (rear cylinder) on the throttle body (picture above). If your bike is desmogged you will have to remove small rubber vacuum plugs to get to these. If your smog system is still intact there will be hoses hooked here for smog stuff.
Start the engine and let it idle.
DO NOT "blip" the throttle with the vacuum gauges hooked up
Watch your vacuum gauge and note the differences between the 2 cylinders. Using the air adjustment screws adjust the the higher reading cylinder down to the lower cylinder and reset the idle if necessary. If you adjust the idle then repeat the entire process until no changes are necessary.

Tips/tricks

Make sure the air box and filter are off! You can adjust the air screws without removing some aftermarket air boxes, but don't do it.

Don't "blip" the throttle with mercury type vacuum gauges or you will get mercury in the intake!

Check to be sure the butterflies are completely closed.

Reset idle after every air screw adjustment if needed.

I like 900 RPM for the 1800 engine idle, I won't run them any lower even though Honda says 800RPM +/- 100 is OK (this is just a personal preference).

You may need to readjust the idle if the air filter is really dirty when put back on.

Finishing up
With everything set and the idle adjusted you can remove your vacuum gauge, (don't forget to reinstall vacuum plugs or smog hoses) reassemble the air box and secure the tank. Make sure that anything else you may have disconnected is reattached.

That's it - you're done - congratulations!

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