From MopedWiki

Contents 1 Engine Does Not Start / Running Engine Stops 1.1 Version One 1.2 Version Two 2 Engine Runs Unevenly or Misfires 3 Engine Power Drops / Poor Performance 3.1 Version One 3.2 Version Two 4 Other Engine Troubles

Here is a typed-out version of a troubleshooting guide from a moped manual. Fred's Guide is also an excellent resource for troubleshooting moped problems and explaining how to fix them.

Engine Does Not Start / Running Engine Stops

Version One

The problem is listed with the solution in parentheses.

1. Fuel valve is closed. (Open [petcock]] or switch fuel valve to "reserve.")
2. Fuel tank is empty. (Switch fuel valve to "reserve" or fill with fuel mixture.)
3. Spark plug is contaminated. (Clean spark plug.)
4. Spark plug is defective. (Replace spark plug.)
5. Ignition cable has worked loose or come off. (Properly plug spark connector.)
6. Too much or too little gas. (Open throttle about 1/3.)
7. Vehicle put out of operation with fuel valve open. (Start with throttle wide open.)
8. Choke operated with warm engine. (Remedy as above.)
9. Petcock is clogged. (Have it cleaned by a workshop.)
10. Main jet is clogged. (Clean main jet.)
11. Float needle not fixed in its notch. (Remove float needle and engage it.)

Version Two

The engine will not start because...

There is no ignition spark because

1. Spark plug is oiled, wet, or damaged.
2. Spark plug is wet (outside).
3. Ignition cable is loose or broken.
4. Short-circuit switch jams or leads from engine are damaged.
5. Contact breaker points are oiled up, wet, or fused.
6. Ignition armature or condenser is damaged.

Engine is getting no fuel because

1. No fuel in tank.
2. Fuel valve/petcock closed.
3. Strainer in the fuel valve/petcock clogged with dirt.
4. Fuel line/hose jammed.
5. Jets clogged.

There is no ignitable mixture because

1. Fuel/air mixture too rich due to excessive priming.
2. Water in carburetor.
3. Wrong fuel/air mixture due to loose carburetor.

Lack of compression because

1. Piston rings broken.
2. Piston and cylinder excessively worn.
3. Cylinder head or spark plug loose.
4. Decompression Valve dirty or stuck.

Engine Runs Unevenly or Misfires

The problem is listed with the solution is in parentheses.

1. There is not enough fuel in the tank. (Open fuel valve to "reserve" and fill with gasoline mixture.)
2. Float leaks. (Replace float.)
3. Ignition cable is not properly connected. (Properly plug spark connector.)
4. Spark plug is defective. (Replace spark plug.)
5. Jet needle is loose. (Clamp needle in its notch.)

Engine Power Drops / Poor Performance

Version One

The problem is listed with the solution is in parentheses.

1. Choke is working all the time. (Push back choke – full throttle.)
2. Exhaust pipe is clogged. (Cleaning the muffler.)
3. Spark plug is defective. (Replace spark plug.)
4. Exhaust port is clogged. (Clean exhaust port.)
5. Float leaks, float needle deformed/jams. (Check all parts of the float chamber and replace if necessary.)
6. Float needle is loose. (Clamp needle in its notch.)
7. Air filter is clogged. (Clean air filter.)
8. Spark plug gap is set incorrectly. (Gap spark plug correctly.)

Version Two

Engine power drops...

Due to dirt because

1. Air filter clogged.
2. Exhaust port and transfer ports in cylinder clogged with carbon.
3. Thick carbon deposits in cylinder head.
4. Exhaust pipe and muffler are dirty.
5. Vent on fuel tank cap clogged.
6. Spark plug vitrified.

Due to reduced compression because

See "Lack of compression" under Engine does not start

Other Engine Troubles

Irregular running because

1. Ignition cables loose or damaged.
2. Engine gets too hot, causing fuel vapor bubbles to interrupt fuel supply.
3. Contact breaker points oiled or burnt.
4. Supporting pin for contact breaker arm worn.
5. Spark plug or connector or ignition armature defective.

Engine four-strokes and does not accelerate because

1. Carburetor flooded due to dirty or worn needle.
2. Float is leaking.
3. Exhaust port is clogged with carbon.
4. Carburetor not correctly adjusted or jet loose.
5. Air filter dirty.

Engine knocks while running at full throttle under load because

1. Too much engine advance.
2. Excessive carbon deposit in the combustion chamber.
3. Cooling fins dirty.

Engine backfires or spits into carburetor because

1. Engine does not get sufficient fuel.
2. Spark plug glowing due to wrong heat value.
3. Spark plug bridged, dirty, or vitrified.
4. Engine getting wrong fuel/air mixture.
5. Glow ignitions.

Engine does not idle because

1. Idle screw incorrectly set.
2. Control cable for throttle valve incorrectly set.
3. Ignition armature damaged.

Engine consumes too much fuel because

1. Gas tank, fuel valve/petcock, or carburetor leaking.
2. Jet needle or needle jet too worn.
3. Jets too large.

Engine does not accelerate smoothly because

1. Low engine power (see above somewhere).
2. Clutch plates worn.
3. Clutch plates glazed due to overheating by two people riding moped, too high idle speed, insufficient or wrong type of Transmission fluid used.

[Taken from this discussion on the forums.]