Hi Roler (I've been looking for an opportunity to say that), welcome to Outdoorking.
The primer bulb sounds the most likely cause of high fuel consumption. If it isn't that, you can check whether your decompressor is misbehaving and causing higher fuel consumption just by temporarily removing it, plugging the rubber hose that runs to it, and installing a surplus spark plug in the hole the decompressor comes out of (same thread). If the fuel economy improves without the decompressor, the decompressor is malfunctioning, perhaps by continuing to decompress even when the engine is running.
Yeah I read about that 'trick'. So, do I understand correctly, the decompressor should only be active when the engine is being pulled to start? And therefore, the engine will run fine without it, once started (which might be a bit harder than with decomp)?
