This for instructions on how to clean the carburettor on these engines:
https://www.gyroscope.com/CombustionEngines/
Each engine has one Carburettor per cylinder. It is a T shaped part on the top of the cylinder. A brass gas pipe connects in a the side of the T-shape part. The opposite side of the T is a rod that goes up/down; this rod has a fatter section that engages a valve. This valve controls the timing of when the gas is allowed out.
In the centre of the T-shape part is a brass hex nut. This has a tiny hole in it. It is carefully sized, to limit the amount of gas flow. It is important part of the design and sometimes can be clogged with debris from the gas cannisters.
When the gas passes through the hole it then goes in to the section of the carburettor that has a number of holes. The gas mixes with the air and the combined mixture lingers brief until the inlet valve of the engine opens and the gas/air mix is drawn in.
If the hole is clogged then the cylinder won't fire or does not fire very well. It is fairly common for this part to get clogged at some point. You can take apart the Carburettor. First remove the gas pipe (undo the gland) and carefully bend the gas pipe out of the way. On the other side you will need to remove the rod (undo grub screw lower down the engine). Make sure you you know its position for when you refit it.
You can now unscrew the top of the T shaped part. This will show the brass nut. Unscrew the brass nut. Hold the nut up to a strong light source. You should see the light pass through. If the light can't be seen or if the light is a strange shape it will be blocked. If it is blocked find some electrical wire with a fine strand. Usually mains wire is a bit too thick. With the single strand of wire push it though the hole to clear the debris. If you have an air compressor a quick blast of air usually clears the blockage.
Check it is cleared. Then reassemble everything.