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You don't want to be using a knife on the throttle plate. You don't want to nick or scrape it. I think what Paul Grimshaw is talking about is called "knifing" the throttle plate where you bevel the leading edge of the plate. It's supposed to help reduce the vortex of air behind the leading edge as the throttle plate begins to open. You can also bevel the trailing edge (with the bevel on the opposite side, visible from the front), but the effect is much less noticeable. This trick goes back decades into the carb era where the venturi effect was used to deliver gas and the sooner a smooth flow of air could get over the venturi the quicker the engine would start to pick up revs. The improvement was in the order of 1%, but when added to a few other tricks you could maybe get an extra 5% of pick up off the start line. Many manufacturers already "knife" their throttle plates and I believe Volvo has on certain engines. On modern fuel injected engines the improvement is probably negligible, but I do it nonetheless.
The method is to file a bevel on the *back side only* of the leading (lower) edge of the throttle plate to a knife edge of about 30 degrees. You want the bevel to come close to the edge, but not close to the point where the roundness of the throttle plate is affected nor where the edge could easily be bent and damaged. Polish it smooth. On re-assembly, you need to make sure the plate is precisely centred in the throtttle shaft so that it can fully close without binding and so that the throttle shaft doesn't stick too far into the TPS (throttle position sensor switch) so as to cause the switch to bind. After you've got the plate installed and the tails of the brass screws have been spread to lock them in place, you can file the exposed ends of the tails off to remove their miniscule obstruction to air flow. You should probably use a drop of weak thread locker on the brass screws when doing this, but I don't bother. About the only trick is to use the appropriate sized slot screwdriver so you don't chew the heads of the brass screws.
As an aside, but related to the original question, I use something benign like a toothbrush as suggested by others for surface cleaning. Carb cleaner is best, but paint solvent like Varsol will work in a pinch. I'll often finish off by polishing with extra fine steel wool (rock wool #00) or even a light duty Scotch pad (the white one?) soaked in cleaner -just make sure there are absolutely no fibres left in the orifices. Use a slightly undersized drill bit to clean inside the nipples. Use a pin to clean the tiny hole on the bottom of the throat. Thoroughly flush or blow out the holes. Put throttle body and flametrap/PCV cleaning on your cars routine maintenance list -like every other oil change.
For an extremely dirty throttle body, also do the following. Remove the brass nipples and clean the base of the holes (use a tiny wrap of teflon tape on the threads when reassembling if you want to, just be carefull not to block the holes). Clean the idle air bypass section. Remove the thumbscrew (or open the assembly if you don't have a thumbscrew) to clean and flush the area. Replace the thumbscrew o-ring if it becomes worn or swollen and loose, otherwise use a very thin coat of siliconized grease or a little siliconized spray (like Armorall) on the o-ring to keep it moist. Check the main air supply tube attached to the throttle body and wipe out any oil accumulation. Remove and flush the flametrap hoses. Rod out any carbon build-up in the small vacuum hose from the flametrap.
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Dave -not to be confused with a real expert, just goofing around at this
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