Also found this for the B18U engine (don't have this in mine so it was new to me
), might be worth having a look at:
You mentioned that it's alright when cold but as it warms up the revs rise. Could be that the flap is always closed and is sucking in cold air for a longer amount of time (would also mean that the engine might not warm up fast enough). Just a thought...
2_23_B18_35.JPG