Vacuum leaks, or air leaks occur when two adjoining parts either are not tightened together correctly or when a gasket between two parts is no longer sealing the parts properly. The piston creates a vacuum in the cylinder above it, and this suction sucks the air in through the carburetor. If you have an air leak, the suction sucks air in through the gap between improperly joined parts instead of through the carb, which makes your fuel/air mixture more lean.
Air leaks can make your moped run poorly, or can cause the engine to idle faster than normal -- usually regardless of how much you adjust the idle screw. You want to fix any air leaks because having too lean of a mixture can cause your engine to overheat, resulting in seizure. Air leaks also make it hard to a good plug chop and tune your moped correctly because the extra air adds an unpredictable and unwanted variable to the tuning equation.
Finding Air Leaks
Common places to find air leaks are:
- carburetor body to carburetor top
- carburetor body to intake manifold
- intake manifold to cylinder
- cylinder to cylinder head (see also: Fred's Guide)
- cylinder to bottom end/engine cases
- cylinder to exhaust pipe
There are a few ways to diagnose an air leak.
- Start the moped and let it idle on the stand.
- Spray carb cleaner (Flammable spray of some kind works best, not WD40) anywhere two metal surfaces join -- the places listed above are good places to start. Make sure you spray only one area at a time; don't just bathe the whole engine area in carb cleaner all at once.
- If you hear the idle change or the engine dies, there is an air leak at the place you just sprayed.
- Another good way to check for exhaust leaks (leaks between the cylinder and exhaust pipe) is to clean the area well. Then go for a ride. When you come back from the ride, check the underside of the cylinder. If there is black oily seepage or leakage on the bottom of the cylinder, where the exhaust pipe attaches to the cylinder, or dripping down the exhaust pipe, you most likely have an exhaust leak.
Make a block off plate / gasket for the exhaust. i've used thin plastic or innertube successfully.
use a rubber hose adapter from a bike pump to the intake. do not clamp . only rest your hand on the pump handle to push air, as you don't want too much pressure. not clamping the adapter allows it to blow off if you put too much pressure on. I cannot emphasize how little pressure you want to put on the case. besides damaging seals, you can force air past them, making a false leak.
use soapy solution to look for bubbles. Video of the process 
Rebel Moby's mabecanemobylette page on leak testing 
Fixing Air Leaks
Here are some possible ways to fix an air leak:
- Make sure the parts are tightened together well. Don't go nuts tightening things, though, because stripping a hole, stud, or fastener will ensure that you never get the parts to join tightly again until you fix/replace whatever you stripped.
- Check your existing gaskets. You might need to buy new ones or get a roll of gasket paper and cut some new ones yourself. Note: Gaskets in areas of high temperature or pressure, such as the head gasket or exhaust gasket, may require higher-quality gasket material than the average stuff you usually find in stores. (Usually this is a thin piece of copper or aluminum see Head gasket)
- Some places won't have gaskets (for example, the carb/intake joint on Bing carburetors often lacks a gasket.) If you have a Bing with this issue, you can try using an O-ring that fits inside the carb, shimming it, or aluminum furnace tape. If you use the O-ring, make sure to squish the carb and intake together tightly before you tighten the carb back down, or else you won't get as good of a seal. A variation on the O-ring is to find a flat rubber washer that has the same outer diameter (OD) as the carb's intake manifold hole, and then cut the inside diameter (ID) of the washer so that it doesn't block the path of fuel and air flow through the carb. Gasket sealer (liquid gasket) can also work if applied in the correct manner.