In addition to the above, I will add (based on your image) that you painted the yellow over the blue.
If possible, paint the yellow first (after surface prep again) and mask for the darker color, blue in this case.
Usually, you should try to paint from light to dark.
Next, it looks like the yellow is/has "orange peeled" meaning the yellow paint did not go on completely wet. The paint has already started drying from the time it left your airbrush to the time it landed on the surface of the engine. Try painting at a lower PSI or slower passes or thinner paint.
Get yourself some plastic scrap and some masking tape and practice different combinations of the above settings and see what happens.
Hope this helps.
Charlie
