Sachs 505 and 504 motors only have one clutch. It serves double duty- both starting and driving. The two problems they have is that the plates will wear out, and need to be replaced, or the gear shaped holder that they all ride on slips on the main shaft. The only way to keep the clutch together is to make sure that the nut on the end is really tight, and not pinching any of the washers at the end of the clutch stack.
If you can't get the nut to stay tight, you can either take the crank to a machine shop to have a keyway cut in it, or do it the home-brew method by drilling a hole on the joint between the clutch carrier, and the shaft. use something like a nail or set screw to keep the holder from rotating on the shaft. That's what I did when I couldn't get mine to stay together. It worked too well, I damaged parts when I had to take it back apart.
Sachs will make a weird noise that sounds and feels like a shift when it finally gets up to speed. It's the weights fully expanding and putting pressure on the clutch.
If it's actually dying when you are at speed, check your kill switch wire to make sure it's not grounding on anything.
The horn on mine used to make it backfire whe I was at full throttle. Depends on what flywheel and coil you have.