hello.. I've had some problems with my streight 6 engines.. Well not the engines but maybe the carbs. Distributor caps were cleaned. One carb was cleaned.. New batteries.. . The port engine starts to backfire and loose rpms at around 1500.. I messed with the high speed intahe valve and the low idle. It starts and runs great but getting up over 1500 it starts to sputter slight backfire. Had em running good when I first put it in the water. I could run both engines at 2500 rpms with no problem. In neutral I can get the rpms up just fine but now when it's in gear. I have changed the fuel filters three times.. any thoughts? Sugestions? or better yet anyone know a good mechanic in the chicago area I can get to take a look at them.. ?? thanks!