Hi Mike, you don’t say if this is your first experience with an MZ but if it is, I can tell you that most MZ ‘rides out’ are happily not as mechanically exciting as you have described.
I am not sure whether you should be looking for a mechanical reason or should be phoning a priest to perform an excorsism!
Anyway a few thoughts:
– Contact the seller – may be able to give you a clue – take a couple of burly mates and suggest having your money back!
– Re the gear change problem, there can be many reasons for this which involve grubbing about with the gearbox internals but firstly check that the detent ball and spring are still in place in the plug under the engine.
If the level screw was missing and oil was leaking over the motor then it may indicate that the oil has been recently changed and its easy to remove the ‘wrong’ plug and lose the ball and spring in the old oil.
I have had 2 MZs with the ball and spring missing.
Re the screaming, remove the plug and check that the ignition switch actually turns the spark on and off.
If it carries on running even without the plug cap on then there must be a hot spot within the combustion chamber – is it the plug electrode overheating because the mixture is very lean or there’s a build up of carbon on the head or even a home made gasket with raggy edges.
Is the carb slide very worn so air is leaking past to cause lean running?
Check the throttle and choke cable are not badly routed, trapped, crimped or just too short when you turn the bars from lock to lock.
If the motor is screaming, it’s obviously getting fuel and air so look at the carb end of the choke mechanism to check the rubber seal is in place and not leaking fuel through, check the carb is a tight fit in the manifold and the manifold is properly fitted to the barrel.
Check the carb throttle slide return spring is doing its job.
Does the carb look new – is it a cheap Chinese copy, many of which work fine but some don’t?
Re the loose exhaust, that might indicate that it was not put together tightly or screwed in without the sealing gasket ring.
Hopefully it’s not because the exhaust port threads are sha**ed!
Apart from that I’m fresh out of ideas but I’m new-ish to MZs so I imagine our more experienced MZ-ers will be able to put you right.
If all fails, I have a nice crisp £10 note waiting and I’ll take the pain away – just down the valley in the N Pennines!!