Valve Adjust & Throttle Sync

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How to Perform a Valve Adjustment & Throttle Body Sync

Ken Sexton (of Roadstar fame) has put together a document detailing a valve adjustment and throttle body sync of the 'liners.

Note: This Valve Adj. PDF link also contains A GREAT TB Sync Procedure, with photos (just scroll down past the valve adjustment section) at following link:

Thanks Ken!

And here a link to a handy 2-Bottle Manometer (sync tool) that is not jerky and can't let liquid get sucked into the engine:

2-Bottle SyncTool

(just scroll down the page,once at the link, until you see the Bottles)

It is recommended to read the two following threads (or until you are bored with the second one!) before starting the Throttle Body sync for the 1st time.

TB Sync Forum Discussion Thread >

Here's Another older lengthy one>

How to Synchronize the Throttle Bodies

As has been noted earlier, one thing to consider when balancing the throttle plates is that adjusting the central single throttle plate screw affects both the cruise and idle throttle body balance settings, and in the end, compromise settings for each may be required.

The reason for that is the throttle plates in each throttle body are tied together on a common shaft that's rotated by the twist grip. Adjusting that common shaft screw determines how different (more open or closed) they are relative to each another. That's all it does, and when the air pressure we read with a manometer downstream of the throttle plates is close to equal, the engine in theory should be the smoothest for whatever rpms we choose do the adjustment. Like at cruise rpms of 2500, or whatever we choose to spend the most riding time at. Idle smoothness is secondary to me at least, as long as it doesn't quit at a stoplight. The auto idle air system should take care of that.

Please see FXstein's blog for an explanation and great adjustment cure for stalling at idle if you ever encounter that problem.

If one throttle plate is opened a bit more during the cruise adjustment described below, then at idle it would leak more air. If it leaks more air at idle, the manometer reading for that cylinder will fall (less vacuum), and then the brass air bleed screw would need to be turned in to reduce it's throttle plate bypass air flow to equalize the throttle bodies at idle. Therefore, it's possible to run out of adjustment with the idle air screw, and have it all the way in and still not be in balance at idle.

I've not done mine yet on my new Liner. When I do I'll note how far each brass throttle body air bleed screw is out from bottom. On the last liner I vaguely recall the #1 (left side and feeds rear cylinder) was about 3/4 of a turn out from bottom. It's the factory preferred reference setting to which the right #2 is then tuned to match it in the normal idle speed throttle body synch.

Ideally, after all is done, both idle bypass screws should be out a bit from their closed setting (about 1/2-1 turn), and the manometer values within 1 cmHg at both idle and cruise. If that's not possible, then compromise by setting the cruise throttle shaft screw to whatever balance setting that will prevent one of the idle screws from being turned all the way in (usually it's #2).

The general procedure I use is:

1. Set up the manometer as called for in the Service manual, and put a cooling air fan in front of the engine. Warm up the engine a bit if not already done. Open the door if in a closed garage. Use safety around a hot engine and exhaust, etc. Pay attention.

2. Set the #2 (right side) throttle body idle air bypass screw to match #1 on the manometer readings within 1 cmHg. Stop the engine and carefully turn both idle screws clockwise until they lightly bottom, then return them counterclockwise to what you just set them at. Write down the setting for each. This is important so you can return them to factory normal, or figure out how to compromise the procedure later.

3. Start the engine and slowly increase the rpms to desired normal cruise setting and note the difference in manometer readings. They should be the same as at idle and within 1 cmHg. If they are, you're done. The readings will bounce around a bit, so just take an eyeball average of what you're seeing, and note the difference and which if any is higher.

4. If one is more than 1 cmHG higher, note which is higher and shut off the engine. Write down the readings now. Twist the throttle until the central balance shaft screw is visible. Note the paint mark so you can return it to the factory setting if desired. Turn the screw 1/8 turn towards whichever throttle body gave the LOWER manometer reading noted above.

5. Restart the engine and note any changes. As Arkyliner noted in his post, if it got better and both are more equal you turned the screw the correct way. If it got worse, then go back the initial 1/8 turn, plus another 1/8 turn in the opposite direction and repeat as required. Turning towards the lower throttle body worked for me 7/19/08.

6. Recheck the idle settings after each cruise adjustment and adjust with the idle bypass screws to equalize within 1 cm if at all possible.

7. If one idle screw bottoms out before equalizing is reached at idle, turn it out 1/4 turn minimum, work the throttle plate adjustment around that setting, or turn the opposite air screw out more to even out the readings. You may also have to lessen the cruise adjustment done above to provide a compromise for both idle and cruise as I noted.

If you write down everything you do, the idle screw settings from bottom, and note the paint mark on the shaft screw, you can always return to factory specs and pretend it all never happened! It normally works out for the better at cruise, so don't get excited and take your time. Repeat annually for peak performance and smoothness as the engine and throttle bodies wear in and tolerances change.

Reference Discussion: