If you've checked everything else:

-Valve clearances
-Balanced the throttle bodies
-HT leads
-Sparkplugs
-Splits in the crankcase breather tube
-Splits in the plenum chamber inlet tubes
-Splits in the inlet spigots
-Dirty air filter
-split/disconnected filter box to plenum chamber hose
-Misbehaving AFM
-Split/missing throttlebody takeoff rubbers
-Dodgy hall effect sensors
-(insert anything else here because I have to have missed something)

then I've got another one for you. Your inlet spigots could be warped and allowing air in underneath i.e. between the spigot and the crankcase. If a bit of WD40 or similar sprayed very carefully at the base of the spigot results in an increase in revs then this could be your problem.

There's a molded sealing ring in the base of the spigot which I assume compresses when the spigots are bolted down. When I carefully lifted the spigots from LFB it was obvious that all four were warped and the molded ring wasn't doing its job as there were very visible traces of fuel and WD40 all across the crankcase inlet faces. A straightedge didn't reveal any daylight, so I assumed the warpage was in the order of tenths of a mm. I cleaned the spigots and crankcases with acetone, applied a thin smear of Threebond to the underside of the spigots and bolted everything back up.

I swear LFB's engine has never sounded so good.