Motorcycle Repair: Yamaha XS400 Carburettor, venturi effect, aftermarket filters

Question
I have a Yamaha XS400 Special (82 model I think). The previous owner took out the original airfilters and replaced them with small ones connecting to carbs directly. This resulted in too much air getting into the carb. I took out the main jets and drilled them bigger (from 1.5mm to 2mm) Now the bike has lots of power at high revs, but struggles at low revs. I've played a bit with the float levels, but without much success. Is there anything else I am missing? Any info that could help me?
Thanks

Answer
Vos,
 There are a lot of variables you are dealing w/ here.  Float level has to remain at the stock setting regaurdless of jetting, this is the proper level for the valve to create the venturi effect.  You are now running aftermarket filters thus eliminating the effect the stock air box had and drilled the jets 1/2 mm over stock.  These alone can create many variables.  Sounds like you are running rich at low RPM due to the new jet size and there is not much you can do.  It sounds like you did not purchase a new jet kit w/ a complimentary needle.  This may be part of your problem.  Also, running aftermarket filters can drastically increase your air intake, lean your mixture and effect the venturi action at the main jet.  You may have to play w/ the fuel air ratio screw but keep in mind, these adjustments change things across the board.  You'll be getting the desired effect at low RPM as at high RPM and this may not be good.  Sounds like you might want to call Dynojet and let them know what you are running (fiters an all) and have them suggest a jet kit for your carb.

Sorry I cant be of more concise help,
Mike