The best thing to do if you suspect a carburetor problem is replace the carb with new. That way you can rule out the carb as a problem right away. Then, if you want to fix your carb, put it back on and start troubleshooting–at least now you know it’s the carb.
Two carb types are common in paramotors: membrane (Walbro, Tillutson), and float bowl (Bing). Troubleshooting or Tuning each type is completely different. For that matter, there are significant differences in tuning each model and engine.
A carburetor endeavors to deliver the correct fuel/air mixture to the motor through the entire throttle range. In a perfect world, carbs would be made specifically for a motor but that would be cost-prohibitive so engine makers select one that’s best suited to their engine.
A few engine makers do some modifications to their carbs so be careful and putting a stock replacement on the motor will degrade the engine’s performance.
Problems and Solutions
1. Fuel/Air Mixture related problems. Tuning means making whatever adjustments are required to get the mixture correct through the entire throttle range. It also includes replacing membranes, cleaning filters when applicable and insuring the pop-off pressure is correct. Before tuning, make sure any fuel filters or screens are clean and the reed valves are in good shape since problems with the reed valves can appear to be carburetor problems.
2. If there is a bubble in the fuel line right as it enters the carb, first replace the fuel line to eliminate that as the cause. If the bubble is still there, there’s a problem in the carb. Install a rebuild kit whose most important part is the rubber membrane. Small, nearly invisible holes, can allow air where it’s not supposed to be. If there’s still air, replace the carb.