They used to be pretty buzzy and useless. I'm working my way though my 18k service, and as part of it Monday night I adjusted the valves. While I was doing the valve adjustment, I did what it seems none of the dealers ever bother doing - set the rocker-arm endplay.
Quick explaination: The rocker arms do exactly what it sounds like - rock around a shaft, transferring in-out motion from a pushrod going to the camshaft follower to the top of the valve - making it go in-out. Valve adjustment is setting the clearances between the rocker arm and the top of the valve stem. End play is setting the amount of movement the rocker arm is allowed up and down on the shaft it rotates (rocks) on.
#1 is the rocker arm, #4 is the shaft..
The shaft is held in position by:
In particular - the part we're interested in is the piece exploded off to the lower right, with screws 2 & 3 going into it. This is where you adjust the rocker arm endplay.
If there is too much endplay - the rocker arm, besides rocking, moves up and down on the shaft. That does two things.. it makes noise and vibration, and it lessens the valve stroke since some of the in-out motion is now used up moving the rocker up and down. IMPORTANT: This means - if there is excessive end-play - you can adjust the valve clearance to perfection, and you still will not have the correct valve stroke... and the engine will not be running correctly!
The goal of adjusting the endplay is to get as small an amount of free-movement up/down of the rocker while still allowing it to rotate freely on the shaft. BMW specifies a range of something like 0.015-0.0015" (approx numbers, I don't have the DVD with me.) I try to shoot for the minimum clearance since it makes the engine run considerable quieter.
Adjusting it involves slightly loosening 3 torx screws and one head-stud-nut and then tapping the bottom "bridge" up/down to get the correct clearances. I use a small brass hammer to tap the bridge. The torx screws are simple, but the head-stud-nut (#8 & #9 in the lower right side of the image) is something I'd rather do only once, since it's a tighten-to-torque-then-angle to retighten it (AKA angle-torque), and if you do this enough times you stand a chance of pulling the stud out of the crankcase (bad thing..) I want to do this adjustment once and then never do it again. Luckily - once it's set right - I've never seen one change.
Anyway - I made the adjustment. One rocker on one side was particularly out of adjustment. With no pressure on the rockers (at the same point where you adjust the valve clearances) - I could get a loud "clack!" noise if I tried moving the rocker up/down by hand. The others were a bit wider than I'd like them, but not awful. When I was done - they were all close to the 0.0015" clearance and made a soft "tick" noise if I tried moving them up/down, but they still rocked easily.
I then went through and did an anal valve adjustment(tm) - meaning I tried for perfection, not just "good enough"..
When I got it all buttoned up and started it up it sounded very nice. I checked the throttle-body sync - and it's a tiny bit off at idle (have to visit the dealer and get them to tweek the steppers with the GT1) but absoutely perfect above idle, even with max sensitivity on my TwinMax.
I took a very short test ride - and found the engine felt so smooth it was almost like it wasn't running. Took it to work yesterday - and found it's almost totally vibration free - and the rear-view mirrors are now useable from 2,000 RPM-6,000 RPM, with no vibes.
So lessons learned:
- Don't expect the dealers to correctly adjust the rocker end-play. It's supposed to be set at the 600 mile service, but I have never tuned an oilhead (and now a hexhead) where it was correctly set. And I've tuned about 6 different local club members oilheads, all of them had been serviced by the dealers.
- Get the adjustments right.. first the end-play, then the valve clearances. Since the end-play does have an effect on the amount of time the valves are open, it does have an effect on how well balanced the combusion of the cylinders will be. Get this set right once (I've never had to readjust one after making the initial adjustment), and I've found the valves will rarely ever need adjusting again.
- Tweeked to an anal amount(tm) - this is one damn smooth engine. It "feels" a bit less powerful, it's not - that's the smooth non-vibrating feel that makes it feel that way. It still squirts past cars just fine.. sounds sorta like a turbine winding up doing it. Zoooooooooommmmmmmm..
I'm super impressed with this engine, since I learned all the anal-tricks trying to get rid of engine vibration on my old '04 R1150RS (I never could - which is why I bought the R..) and the balance of the R12 engine must just about be perfect.
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I'll bring beer....or whatever you preffer 
....pm sent