My K75 afterfires on decel as well so I researched the reasons and possible rectification. Here's what I've collected:
1) check that the throttle position switch on the rear of the throttle bodies is working - when the throttle is close you should have continuity on one set of the contacts .. this cuts the fuel supply to the motor if the revs are over say 2,000 rpm. The throttle switch position is adjustable.
2) there is an air by-pass that adjusts the fuel mixture at low air speeds .. under the fuel tank inside the air cleaner bits.. but check no (1) first. This air mixture is normally set with a CO2 meter.
3) Another thing that can cause the afterfiring on deceleration is a leak anywhere in the leading 20 (or so) inches of the exhaust system. There is a gasket between the header and muffler that may be deteriorated. The BMW replacement exhaust gasket is 18121457142, but at $16.83 it's possible that Permatex Ultra-Copper high temp RTV silicone gasket maker #101BR may work also. Good to 700˚F intermittent. Available in auto parts stores.
4) Could be the fuel pressure regulator, measure fuel pressure...should be 2,5 bar. Next, check the water temp sensor.... measure the resistance between contact 10 and 13 at the FI ECU connector. You should measure ~2,5 Kohm at 20 degree Celsius.
If all of these areas of the bike are in spec on your K75, then the advice above about throttle control should be followed.
