I've a kymco zing 125cc I've had it a few weeks now and it used to drive at 70mph then over the week and lastnight it dropped again to 60mph and the bike coughs and spulters once it hits 60mph

Most 125cc scooters are restricted to between 60 and 70 mph often because of certain laws in each country they are sold in. These 'Twist and Go' automatics alway sound like they want to go faster but the transmission needs to rev that high to work properly.
It is possible to de-restrict your scooter but the top speed you would get would not be a lot higher, maybe 10 to 15 mph more.
Scooters also use much smaller wheels compared to a motorbike which makes the scooter less stable at higher speeds. Mine starts to shake and vibrate when it hits it's top speed of 70mph. To get much higher speeds you would need to spend a lot of money changing gearing ratios roller weights or even swapping complete transmissions. One option if you want to keep at the 125cc size is to change to a bike with a manual transmission like the bigger motorbikes. Some of these bikes can get closer to 100mph but most 125cc bikes and scooters all tend to max out at 70mph. At higher speeds the manuals don't sound like they are working as hard as a scooter does.

Its a single cylinder engine. Are you sure its not a faulty spark plug, or maybe there's no power going to the spark plug?

I found an owner's instruction manual on the net in Spanish that says:
0,05 mm - 0,1 mm

This means that the valve clearance is 0.05 to 0.1 mm. A slightly tight adjustment using a 0.1 feeler gage should be ok and the real clearance would then probably be ca. 0.08 mm.

check your fuel injector one or two may not be sending gas out to carberator..clean the carb and see what happens

There are many sites you can take a look at like
http://www.bitzforbikes.co.uk/Kymco/Zing_125/98_-_03 and http://www.wemoto.com/bikes/Kymco/Zing_125_Custom_RF25/97-06/

Not always necessary to change the regulator/rectifier. Check all connections to make sure they are clean, use a good contact cleaner and finish off with dielectric grease on your electrical connections.
When some bikes get a few years on them the connections get dirty. Check to see if there are any burned or discolored wires anywhere around the regulator. The FIX on some bikes is to cut the connector completely out and solder the wires together and either seal them with heat shrink of a good electrical tape. Loose or bad connections get hot. As long as the regulator and rectifier are working there is no need to replace them if the connections are good.
Ride Safely, Ed

your best try is to contact your dealer, and get them to order you the short red wire that runs from the + terminal on the battery, to the starter soleniod ( also has a thin red wire that runs to fuse holder) i had exactly the same problem and this solved it. i have however re done all the electrical connections on my bike using 7amp wire and bullet connectors, as i have had many faults with the electrics before.
My bike is a Kymco Cruiser 125 (almost exactly the same bike, i use kymco zing parts when replacing) i paid approx £16 GBP for the wire.

the problem may be that there is not anuf power in the battery or the starter motor is on its way out