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.
