What causes a moped two slow down and die?

The spark plug gap for a 2010 Suzuki 49cc moped should be 0.6-0.7mm or 0.024-0.028 inches.
It's important to consult your owner's manual or service manual for the specific gap recommended by the manufacturer, as different models and years may have different requirements. It's also a good idea to check the gap of your spark plug when you install it as they can sometimes be out of spec.

How old and how many miles are on this moped? Have you had any work or repairs done on it recently?
Does it 'Pop' when you let go of the throttle as you are slowing down? 'Popping' often means your fuel to air mixture is wrong. Your either running too rich, (too much fuel in the mixture), or too lean, (not enough fuel in the mixture). You need to work out which one.
Is the scooter running better or worse? Is the exhaust gas a different colour, possibly black? Are you using more fuel than normal? Can you smell fuel from the exhaust? Does it run rough when warm? Does it idle smoothly? Running better, black exhaust fumes, Using more fuel, a smell of fuel at the exhaust and running rough when the engine has warmed up are signs it is running rich. If it seems to have less power and the engine seems to suddenly surge, (sudden little bursts of revs from the engine), the engine could be running lean. Clean or replace the filter before using the scooter again. Always fit an air filter as this protects the engine from the dust and dirt in the air.
When was the last time the moped was serviced? An old spark plug, dirty air/fuel filter or bad quality fuel can all affect how an engine runs. Service the moped anyway and that could solve the problem.
If yours uses a carburetor get it serviced by a specialist as the internal parts can wear or get dirty enough to affect how it works. parts like the float needle. Use the Comment button below with more details of EXACTLY what it does and the answers for the checks above and I'll see what other help I can give you.

Their is a vacumn pipe from carb to the valve on the bottom of the fuel tank ,,,check it out is pipe split is there a vacumn ? has the valve jammed .

Try Carl Salter website & scroll down to Jawa section, might be something suitable. http://www.carlsalter.com/classic-bike-service-manuals.asp

You probably need an ignition module . you can get them for $47 on ebay

Your IDLING JET IS CLOGGED or the settings on the SLOW speed on the idling screw is not correct. Sometimes dirt can cause issues and so it might need an overhauling of the carburater.

HI,,empty your gas tank,,and drain the carb put some new fuel in the tank and dont use additives again,,some do more harm than good (especialy in 2-strokes)

well it is 2 stroke so if its rev'in slow thats not dunna help . my sister had one and its the kind of thing that u don't let it idle just keep it going. thats what i found best