Hi Andrew
You say the surging started after a throttle body synchronization. Was it DIRECTLY after the sync or did it come on gradually?
Note: ALWAYS do an Adapation Reset after a sync.
There are a few things you can look at, and no, the stripped valve cover screw will not introduce air into the system or have any effect whatsoever, beside possibly leaking a little oil.
1. Did you tamper with the idle stop screws at all? Those are the small screws under the plastic caps.
2. Are the idle actuators correctly plugged in?
3. The freeplay on the throttle cables. No, it is not "just the gap between the 2 throttle cable nuts on both sides". It is the distance that you can move the cable casing up and down without the cable core moving the butterfly.
4. Do you possibly have a crack in the rubber intake manifold?
5. This statement concerns me: "The RPM in all gears will rev up to 6,000 or so before "catching up" it feels with the fuel delivery." The way that is worded makes it sound as though the clutch is slipping, which is an entirely different issue.
6. Lambda voltage 2 is not correct. It would not appear that the sensor itself is faulty, just that the mixture control is having a problem of some kind and cannot get the mixture right.
So, here is what I would do.
1. Check the battery terminals and make sure they are clean and firmly connected.
2. Check Sparkplug condition as well as all 4 stick coils. If you have a mate with a similar bike, borrow the good stick coils in order to eliminate those.
3. Run and Autoscan and ensure that there are no errors.
4. Correct the freeplay in the cables. I would in fact screw the cables in all the way so that there is no way that they can interfere at this point. In other words, you will have a LOT of slack in the cables. We will correct this later.
5. Connect the Twinmax, start the motor and see what the readings are. Remember that on the R1200, there is nothing you can adjust at idle! That is what the idle actuators, working in conjunction with the Lambda sensors, are for. If the idle is rough and won't stabilize, and the lambda sensors are not bouncing up and down, stop and go back to the basics.
6. While the motor is running, spray some Quick Start or similar product (even spray deodorant will work) around the intake manifold/throttle body joint. If there is a significant change in engine pitch, idle speed or roughness, you have an air leak. Do not even attempt anything further until that is sorted.
7. Check valve clearances. Make sure they are all exactly correct. Also make sure there is not an old feeler gauge stuck in there. May sound strange but I have found those on 2 occasions where the previous technician forgot to remove them after the adjustment.
8. Remove and thoroughly clean the idle actuators and the bores they fit into. Use some Carb Cleaner for this. Just remove the rubber O-ring first as the Carb Cleaner will chew it up. Do not try to activate them outside of the throttle body and they will fall apart.
9. Refit and start the motor and get readings from Twinmax. If the readings are still way out, and idle still rough, do a compression check and a leakdown test. Leakdown test will tell you if you have a leaking valve or simlar problem.
10. If all is well and idle is smooth, you can now sync the throttle bodies. First adjust the LHS cable casing to have about 2mm slack. Fire up the GS911 software and use the Syncronize function. Then using the throttle, bring the revs up to about 3000 rpm and check the twinmax. (It help to have a healthy fan on the motor at this point to create some airflow) Adjust the RHS cable to give you a balanced reading on the Twinmax.
11. Once done with the sync, once again reset adaptation and all should be well.
Hope this helps.