Firstly my idle was well low at 500rpm. This turned out to be blocked bleed valves around the throttle plates on each throttle body. So start by cleaning the needle valves out as mine were in a bad way. (How many miles? - mine had done 152k miles)

Also make sure that the throttle plates are fully closed. If not adjust to make then fully closed.

I removed the needle screws and observed the number of revolutions required to take them out. Not an exact sceince I know but at least they all went back to a similar position to where they were removed from.

Next stage is to run engine and record vacuum on each throttle body at idle. This was done with an ordinary vacuum guage fitted to the vacuum pipe on each body. The guage will flicker quite badly but should deflect into the idle region of the guage when that particular cylinder inducts. If not adjust the screw so that it is in the idle range. Do all six and you should end up with a constant idle speed approaching 900-1000rpm. If it is low then adjust each screw say a quarter of turn to get more air flow past the plates hence raising engine speed.

I also checked C0 emission levels which can be altered for idle via adjustment of the bleed screw on the air flow meter. Mine runs at about 2% C0 at idle (no cat or O2 sensor on my M635) which is fine for the UK MOT.

I also looked at vacuum at say 3000rpm. The vacuum in the throttle body should be representative of the vacuum in the air box. Mine showed some flickereing so I think I may have some leaking valves.

Engine performance especially gearchanges have inproved drastically since removing the crap from the throttle bleed vlaves. Well worth doing.

Ian

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