Without the prop, you will not easily start the engine. Likewise, if the belt is slipping somewhat, the prop's rotational inertia is not available to carry the piston through the initial compression stroke after firing. The Solo 210, like the DK's engines and most other paramotor engines, have static timing i.e. same firing at idle and full power. The result is that it fires a tad too early at low rpm. The result is a Mule-like kick, feels like ripping your arm out its socket.
Wooden props make starting easier.
Some manufacturers are removing the flywheel to reduce the weight of the paramotor by 800 grams, relying purely on the crankshaft counterweights and the prop to provide sufficient inertia. It cheats on the spec-sheets by showing lighter than realistic paramotor weights, which seems to be a major selling point even though the weight is only apparent during a few moments of ground handling and is NOT an issue in flight. Replace your starter rope with a longer length, such that the reel is completely full when retracted. Proper pulling technique is essential. Here is how it is done - take up the slack, then pull slowly as the prop turns, until you are up against the compression stroke.