The biggest problem I've had with adjusting the carb on my XL175 has been getting the needle and seat to seal. It seems to leak, sending the bowl level too high. An easy way to check this is shut off the fuel petcock while it is idling. If the rpm gradually picks up as the bowl level drops, the correct level is not being maintained. This of course assumes that the mixture screw is adjusted right, about one turn out. Since you seem to have access to emission equipment you could run this test while you check. Another sign is engine starting without the choke on especially if its cold outside. You should need the choke. Someone suggested an inline fuel filter, seems like a good idea since the screen is probably too course to catch everything. I need to try that one myself. Also make sure the vent passage is not plugged. Its the little hole in the small brass tube that sticks out next to the idle speed screw. If this is plugged then pressure can't equalize above the fuel in the bowl. I had this on mine and the engine would start to miss above 7500 rpm. Make sure its open all the way to the bowl area. When everything is right the engine will drop right to the idle speed you set it at and stay there when you release the throttle. If it first hangs at a higher rpm, then drops down, then wants to stall, its too rich because the level in the bowl is going too high. If fuel leaks out of the standpipe (the fitting where the hose is connected on the bottom of the float bowl) with the fuel shut off, the float needle and seat are leaking. Hope some of these hints help.