See what Roel wrote:
Some info on my K&N filter:
It cost me 3600 Belgian Franks (about 160 DEM), so it pays itself back quite quickly (the guinine part costs 1300 BF at Belgian Honda dealers!). As the filter was pre-oiled, installation was a peace of cake - just remove the old one and put the new in place. Although my Transalp is from 1991 (model 1992), and not from 1989-90, the filter fits perfectly anyway.
Since this is a replacement filter, there's no need to re-jet the carbs.
The German importer is Racimex in Hamburg. They will tell you of a distributor in the near of you.
K&N part no: HA-6089 for the '89-'90 TA models, but it should work with all others, too. Price in Germany: DEM 92.-
Since '97 also German aftermarket provider Detlev Louis offers K&N air filters for Transalp.
For cleaning and re-oiling you may purchase a service set for DEM 21.-, containing a bottle of cleaning fluid and K&N filter oil spray. You should use this for cleaning and re-oiling your K&N filter. Do not use fuel, diesel or other stuff to clean it, or motor oil, WD-40 or other stuff to oil it. It may damage the cotton inside the filter.
I got my K&N now, and the first impression is, that the engine is "breathing" better. It resulted in a +6 km/h V/max, as my Sigma bike computer told me.
In case an Africa Twin owner reads this: K&N air filters are available also for
Part no. HA 6089 for the '90-'92 models (RD03/RD04) (same as for TA)
Part no. HA 0008 for the '93-> models (RD07)
BTW, if not using K&N, I strongly recommend using genuine Honda air filter. Using third party filters may get you into trouble. Not necessarily, but possibly.