My Civic has D15B3 engine which is recently serviced and tuned. The problem I am experiencing is that the RPM fluctuate (500 to 1000) when engine is on Idle and AC is running. The other issue is that the engine takes a while to start on cold ( sometimes couple of long selfs). My mechanic seems to don't know what causing this. The car is completely on CNG since last six months.

Some one told me that there is an Air control valve in the carburetor which might be clogged but I think this valve is not present in D15B3 engine. Could that be a carb problem?