Grüetzi Kurt and welcome on our forum! I'm really glad to see that there are also Amigos in Switzerland, the last time I have been there, I only noticed one PC50
First some advices:The gearFirst of all, the gear you are talking about is meant to switch between engine power or pedal power and it's highly unrecommended to switch from "off" (pedal power) to "on" (engine power) while the engine is idling. When the engine configuration is correct, you should be able to start it with a simple push with the gear set to on.
Motor oilUse simple mineral oil, the synthetic oil may be a tad to slippery and cleans your engine too good. This may sound weird, but with an old-timer, you want to have some residue in corners and slits because it actually assists in keeping the oil inside
FuelUse normal Octane 95 fuel with a lead additive, the lead is good for the cylinder wall and valves. Also use a filter, these old fuel tanks can rust on the inside.
First check:First we are going to do a "dirty" check: When you try to start the engine or when it idles, do the breaker-points spark? (See below to find out where) If so, your capacitor is defect and needs replacements. If this is the case, let me know
Now we are going to set the engine to the manufacturers default settings:Carburettor (A detailed disassembly manuals is to be found
here)
Throttle needleThe throttle needle has three notches on the op and the ring that is clamped on there, should be on the 2nd position. This is the middle position.
Air ScrewYou have to turn this screw clockwise until you cannot go any further. Then, you turn it 1 1/4 turn counter-clockwise.
Idle ScrewThis has no default position, make sure it's just enough turned inwards so the engine keeps idling, but not so far that it will drive off on it's own. I'm sure there is a law against vehicles driving without a driver behind the wheel
Fuel JetNo particular setting there, it just needs to be clean.
IgnitionYou said you have a spark, so I'm not going into complicated wiring, we're now only trying to make it work properly.
SparkThe spark should be (preferably) blue, but yellow can do too, as long as it's a proper spark.
Spark plugNever save on a new spark plug. They cost almost nothing and they make a great difference in performance. I strongly recommend a NGK C7HSA, which is the same for PC50. More info
here.
Spark plug contact clearanceThe clearance at the contacts should be 0.6mm
So that was the spark plug side, now we're going to the breaker points on the left side of the engine, behind the shiny metal plate, and inside there, behind the flywheel.
On the bottom of the shiny plate is a small notch which you can use to carefully pry it off. Behind there, you will see the flywheel and that has two markings on it. On the top/side you will see a
T and a
F.
Breaker pointsFirst, inspect the breaker-points for wear. This is a bit hard, since your view is mostly obstructed by the flywheel, but when you push the breaker-point open, it should not look pitted or worn out.
Now, you should know that T stands for TOP and F stands for FIRE. T means that the piston is in it's top most position and this is also the widest opening point for the breaker points. F is the moment where the breaker points open. On that moment the spark plug will fire a spark.
It's not the easiest thing to do if you are a novice, but you have to visually inspect if the points open at the F moment. You can also test it with a multimeter if you have a third hand

With this it is a bit of trial and error, once it works, you can play around with that setting to make it perform better.
The screw which loosens up the breaker point, in case you need to set them differently, is shown
here with number 9.
ValvesWe are now assuming that the valves are still in good order, we can address that later.
The valve clearing need to be set as follows: IN = 0.05 and OUT = 0.08
You probably do not have a 0.08 gauge, but just set it to 0.05 and loosen it up a tiiiiiny bit. This valve needs a bit more clearance due to heat expansion.
This concludes the resetting to manufacturers default.
Please let us know how things are going and if you encounter problems, we're always happy to help! 