Paul... here is a basic guide for most ped engines.
GENERAL MOPED TROUBLESHOOTING and TUNE-UP PROCEDURES
BASIC TROUBLESHOOTING (follow these steps to get it to run)
The 3 most common reasons why mopeds don't run are....
1...dirty carb (inside)
2...dirty, worn, or mis-adjusted ignition points.
3...fouled spark plug.
The first thing to check on a moped that won't run is to see if you have spark at the spark plug.
Pull the spark plug out and hold the metal part of the plug firmly against the cylinder head while you kick or pedal the moped rapidly with the key and switch on.... it might help to do this at night or in a dark garage to make it easier to see the spark.... it might also help if you have 2 people... 1 to pedal/kick, and 1 to hold the plug firmly against the cyl. head... you are looking for a blue spark to jump the gap on the plug.
Make sure you have a good spark plug to start with, a black or gunky or wet one will not spark.
It is smart to just buy a new plug to start with.. you can always save it for later if the old one turns out to be good.
(working for hours only to find out it was a bad plug is frustrating)
If there is no spark.... clean the ignition points (like it says below)
If there is spark... squirt a little bit of gas (like a spoonful) into the spark plug hole and try to start it.....if it starts and runs for 5 seconds and then dies, then check for fuel flow (like it says below)
If you have fuel flow to the carb and it still won't run.. then clean the carburetor (like it says below)
FUEL FLOW to the CARBURETOR
Remove the fuel line going into the carb.. turn the gas on.. does fuel flow freely out the gas line?.. No?.. You may have a vacuum operated petcock (if you do there will be another rubber line going from the engine to the petcock).. take this second line off the engine or carb and suck on it and watch for fuel flow.. If no flow, you must take the petcock off and disassemble and clean it..
CLEANING the CARBURETOR
Clogged or dirty carburetors are the most common reason for poor performance.
The parts of the carb that are dirty or clogged are the small holes inside the carb (air and fuel passages).. not the outside.
The carburetor must be removed... then you take off the float bowl (on the bottom)... then you remove the brass "main jet" in the middle of the carb... hold it up to the light and look thru it... it must be clean and clear.... if it is not clear you must poke a piece of fine wire thru it.... a wire plucked from a wire brush works good.... or some soft multistrand copper wire like from speaker wire or lampcord wire... don't use a drill bit !!.
Next you should screw the idle mixture screw in.... (( Make sure you count how many turns out it was set at !))..... then remove it and all other screws..be careful.. don't forget where everything goes.. and don't lose anything!
Now you must clean out all passages in the carb with aerosol carb cleaner and compressed air... (like 100 psi)... with a blow nozzle... squirt the cleaner in ALL orifices one by one followed by a blast of compressed air.... while you are blowing air thru the holes feel with your fingers to feel where the air is coming out of and blow the other way too.... take your time.... then reassemble all the parts.
Remember to turn the idle mixture screw out to its original setting... usually between 1 and 2 full turns out from all the way in) and reinstall the carb.
Older engines have "breaker points" ignition.. which can get dirty, wear, and need adjusting occasionally.
You will find them by looking through holes in the flywheel.. (usually on the left side of the motor)
Newer motors don't have points.. they use an electronic ignition called a CDI.. and there is nothing to clean and set.. but you can still check the timing with a strobe light.
CLEANING the POINTS
You can clean them through the holes in the flywheel..(you don't need to remove the flywheel)
To clean them you need some sand paper (use 400 grit wet and dry sandpaper), a piece of clean paper, some scissors, and some aerosol brake cleaner or carb cleaner and some compressed air with a blow nozzle.
Remove the ignition cover and look for the points in one of the holes in the flywheel.... lay the bike over on its side and sit on a milk crate or something to get comfortable.
Then cut some thin strips of sandpaper (like 1/4" wide, 3" long).... pry the points open with a small screwdriver and stick the piece of sandpaper in between the points and let them close.... then pull the sandpaper out.... do this several times to each side.... now pry the points open and blow them off with compressed air... then spray them with the cleaner.... then cut a strip of the clean paper and pry the points open again and drag the paper thru a few times (the paper should be clean and should drag through smoothly).... blow them off again with air while open.... now they should be good.
SETTING the TIMING
You set the timing with the flywheel ON
Correct ignition timing means the spark plug is firing at the correct moment in the engines rotation.. a little before TDC (top dead center.. when the piston is closest to the spark plug).
The spark plug fires the instant the points "break" open.
You want the points to open when the "F" mark on the flywheel lines up with the mark on the engine case.
Clean the points like it says above FIRST.
Then look on the outside of the flywheel for some lines and letters.. there should be a T mark next to a line... and an F mark next to a line... There should also be a mark on the engine case .. the T mark will line up with the mark on the engine case when the piston is at TDC.. you can remove the spark plug and stick a screwdriver in the hole against the piston and turn the flywheel.. when the piston pushes the screwdriver all the way out, the T mark will be lining up with the mark on the engine case.. the points will have already opened BEFORE that.. they should just START to open when the F (Fire) mark lines up with the mark on the engine case.
To check this accurately you would need to have special tools.
But you can get close with the "paper test"
For this you need a very thin piece of paper (like cigarette rolling paper..or a cigarette pack piece of cellophane)
All you do here is put the thin strip of paper between the points... and keep light tension on the paper (like as if you are gently trying to pull it out) while watching the F mark and the case mark as you slowly rotate the flywheel with your other hand.
Remember you are rotating the engine in the direction it normally travels (CCW as viewed from the left side of the bike..CW as viewed from the right)..the paper should slip out as the points start to open when the F mark lines up with the case mark.
So it's...one hand on the flywheel.. other hand on the paper...eyes on the F mark.
If the paper pulls out too late.. you will have to move the points set for a larger gap.
Too early.. move the points for a smaller gap.
GAS and OIL
Most mopeds have 2 stroke engines.... 2 strokes consume oil for lubrication.
Newer mopeds might have an auto-lube oil pump.. so that you don't have to pre-mix the gas and oil.... For older mopeds you will need to "pre-mix" the oil with the gas.
With modern 2-stroke oils you should probably pre-mix between 3 oz.(43 to 1) or 4 oz.(32 to 1) of oil per gallon of gas.
Make sure you buy 2-stroke oil.
Some people say synthetic works better... But I have found regular 2 stroke oil to be just as good as the much more expensive synthetic oil..
Mopeds are low performance engines designed to run on the lowest octane of gas you can buy (87 oct.) because they are afraid that somebody who doesn't know better will put the cheap stuff in..... and higher octane will not make it run better or faster or make more power.
But higher octane will not hurt your engine either.
I also like to put a little gas treatment in the gas every once in a while.. because moped carb jets are so tiny that they get clogged easily...the gas treatment helps dissolve stuff in the gas that might clog these jets.
I use one capfull of gas treatment per moped tankful (you buy the gas treatment at a gas station or Auto parts store)