If the carb is in otherwise good condition--no blocked passage ways, etc.--I'd look into the air correction jets--and/or the emulsion tubes located just under those jets. The emulsion tubes get fished (bent paper clip will do it) out of the wells they sit in (the a/c jets hold them in place). The emulsion tubes come in a variety of profiles/hole configurations and if not right can cause the mixture to lean out under steady running. -- Dave