I pretty much started exactly how Stealth describes. The bike is all put back together, so a video wouldn't show much, but I can write up some pointers:
Step 1 - Think again if you really need to do this. The biggest thing I learned is that at low throttle settings and idle - where the sync matters the most - the computer is controlling a lot with the idle air bypass solenoid, so there's probably not much benefit to be had.
Unfortunately I did not get a vacuum reading prior to tearing the bike down. This was all done during my 22.5k service and I hadn't planned to sync the throttles when I started. I got a little ahead of myself and figured I didn't need to set a baseline on the workbench, thinking I could just dial everything in with the bike running using the vacuum gauge. Wrong. Some screws don't do what you expect them to (details below). So I tore it all down and started over. Here are my lessons learned...
Pointers:
- Make a good mark/indication/note of where the throttle stop screws and linkage rod are set from the factory so you can go back to those settings if needed.
- There is a throttle stop screw on each throttle body. They are small set screw that takes a 2.5mm hex key. Buy an extra, good quality 2.5mm hex key so you can shorten it to a custom stub length. I was surprised to find on my bike that the vertical cylinder stop screw was set so that it wasn't actually serving a purpose - there was a gap even with the throttle closed. That turned out to make sense for 2 reasons: 1) you can't access the vertical cylinder stop screw when the airbox is on the bike, and 2) it's redundant - the horizontal cylinder stop screw can serve as the main throttle stop for both throttle bodies and the sync is adjusted with the linkage rod
- This proved to be a fantastic tool:
The Digi Sync Top Page. No affiliation on my part. Works great. The hoses are the right size for the vacuum barbs underneath the throttle bodies. This is the way I measured the sync
- I began like Stealth said. I backed off the throttle stop screw to let the throttle bodies fully close and set the idle adjustment screws fully closed (they were both 1/2 turn open from fully closed from the factory). Since I don't have a way to flow-test the throttle bodies on the workbench, I used a 0.03mm feeler gauge (smallest I had) to set the baseline sync. To do this, just see if/where you can slip the feeler gauge around the throttle butterfly when it's closed. You're looking for a very slight friction fit. You will find that you will have to settle on "close enough" as the throttle butterflies do not have identical clearance in the throttle body bores between the two throttle bodies. For example, on my bike, I was able to slip the feeler about halfway around the butterfly with very slight resistance on the horizontal cylinder. On the vertical cylinder, it would only slip about 1/4 of the way around, but felt looser. We're getting into thousandths to ten-thousandths of an inch here. Visually, they looked identical, and fortunately, this proved to be spot on. I didn't even need to adjust the linkage rod when I re-installed everything on the bike. The vacuum gauge read pretty much identical vacuum on both cylinders at idle and within +/- 10 units through some part throttle settings at 4000-6000 rpm. That's about as good as you're probably going to get. Remember to make sure the vertical cylinder throttle stop screw is recessed so it is not doing anything, otherwise you will not be able to make good adjustments. You will use the horizontal throttle stop screw to serve both throttle bodies, just like the factory did (on my bike, at least).
- Don't forget to reset the TPS, otherwise it may try to idle at 5000 rpm!!! Not that I would have made that mistake...
- Once you have everything back on the bike with the throttles fully closed and the idle adjustment screws fully closed. Start the bike and be amazed that it will probably idle very well. Note your readings on the vacuum meter. You'll use the linkage rod to sync them at this point, but if yours is like mine and you did a good job with the feeler gauge, they are probably spot on and no adjustment is necessary.
- Now take a flat tip screw driver and go nuts on the idle adjustment screws and be amazed that they have almost no effect on the idle speed or quality. This is where I realized the ECU is doing almost everything via the idle air solenoid. This is somewhat frustrating as it appears the idle maps in the ECU assume a certain amount of flow through the throttle bodies. Note that you can screw up the sync with these, so make sure you're still looking at the vacuum gauge.
- I found the bike too difficult to start and idle with the throttle bodies set at the fully closed 0.03mm feeler gauge setting. The throttle was also a little sticky off idle as the throttle butterflies were more closed than they were from the factory and rubbing on the walls. I went a full turn more open on the throttle stop set screw. The bike started a little easier, but would idle at around 2300 rpm when fully warmed up after a test ride - no good. The horizontal cylinder throttle stop screw is only accessible with a stubby 2.5mm hex key. I had to grind one down to make it extra stubby. It's still tough to get the hex key on the screw. You will only be able to turn it 1/6 of a turn at a time.
- For testing purposes, I set the idle screws at 4 turns more open than they were from the factory, thinking this would ease cold starting while not affecting the idle (since the ECU was controlling idle). Although it was easier to start when warm, and engine braking was slightly reduced (very slightly), the next morning I had the hardest time starting the bike that I've had in a long time - so that test was a failure.
I ended up with the throttle stop screw set about 1/2 turn more open from my baseline setting. The idle adjustment screws are at 1 full turn from fully closed. As I said above, it starts, runs, and idles pretty much just like it did before all this, maybe 50-100 rpm higher.
So I ended up with a bike that runs great, but probably not appreciably better than it did before. I don't think it's worth doing all this, unless you like doing this stuff for fun, which I do.
