1980 Suzuki GS1000L, 1980 Suzuki GS550E, 1987 BMW K100 RS
This is a 40-year old bike so everything relating to fuel delivery typically has issues. You may have a fuel restriction in the petcock, or it’s the carbs. I would start by replacing the petcock, flushing the tank and replace the fuel line/ vacuum lines. Look for signs of rust in the tank and do an acid flush (a strong citric/water solution for 12 hours) or have it treated professionally if needed. Next, the carburetors need to be rebuilt. I would buy 4 (K&L) carb kits and go with factory specs for now unless you are changing the exhaust. I would consider replacing the air box with clamp style air filters to boost performance. If not, replace the air box filter. It’s a good winter project. That thing will run like a champ. Also, always use non-oxy premium fuel and a good motorcycle grade oil for changes like Lucas, as wet clutches need it. Don’t ever put automotive oil in a motorcycle.