How to get your 2-stroke engine to start.

Two stroke engines are fantastic power plants. They are small, have few moving parts, and have an amazing power-to-weight ratio. Rebuilding and modifying them is easy (with proper guidance), and you can literally double their power output with the right parts. 

However, these engines also have a reputation for quirks, or 'problems' for the pessimist, that can prevent them from starting & staying running. In the hands of a mechanic (or an enthusiast) familiar with 2 cycle engines, most of these problems can be diagnosed and fixed in a few minutes. However, if this is your first time working on a pocket bike, I hope this article will give you some quick pointers to get your bike or scooter to start.

What your engine needs to run

Proper gas premix:

Pocket bike and scooter engines usually have a separate gas tank. The gas you use has to pre-mixed to a specific ratio because, unlike 4-strokes, the entire engine is lubricated by the oil added to the gasoline. Usually, the mixture should contain more oil during break-in, and less (a leaner mixture) once the engine is broken in. A good rule of thumb for most engines is to mix 4 fl oz of synthetic, premium two-cycle oil to 1 gallon of premium (91+ octane) gas. For break-in, manufacturers typically recommend 5 fl oz to 1 gallon of gas (25:1).

Correct fuel & air mixture:

When you start your engine, the inside mechanical movements create suction, forcing air through the carburetor — which is responsible for mixing the fuel with the oxygen-rich outside air. This mixture is funneled into the cylinder, where ignition occurs.

Most starting issues arise at this stage. You see, a cold engine needs a different ratio of air and fuel to start, compared to one that is already hot. The carburetor needs to be in proper working order, not clogged up, and the choke on it must be used properly, otherwise the engine will not start no matter how hard you try.

Successful ignition:

The piston compresses the fuel/air mixture inside the cylinder, reaches peak compression, and the spark plug fires, exploding the fuel. The piston shoots down. Inertia makes it come back around and compress the fuel again for the next ignition cycle.

You need thousands of volts electricity to create a spark. If there is no spark, or if the spark is not sufficiently strong, the fuel will not combust. There are four things than can cause bad spark. They are:

1. Damaged spark plug.
   Over time, spark plugs accumulate carbon. It could build up the point that the spark is too small, or the plug is completely fouled up and will not fire. 
2. Bad magneto.
   The magneto (coil) is the little brick that the spark plug is attached to. It is located next to the flywheel, which has magnets that create a current in the coil as they pass by it. If the coil, or its wire/connector, is damaged, it might not generate enough current, if any, for the spark.
3. Damaged flywheel.
   The flywheel is the round disc (with cooling fins) that contains the magnets. Those magnets themselves may be damaged.
4. Short circuit.
   Many bikes have electrical add-ons, such as keys and kill switches, that usually connect to a second (thinner) wire coming from the magneto. This wire, if it touches the metal parts on the bike or engine, creates a short circuit that stops the spark plug from firing. You may have an unintentional short-circuit preventing your engine from starting, such as a stripped wire, broken kill switch, or (at worst) a mis-wiring. When all else fails, unplug everything from the magneto except the spark plug. Your engine will start like magic.

Good compression:

In order for ignition to happen properly, the cylinder where ignition occurs needs to create a tight seal with the piston so that no gasses can escape. Good compression also provides good suction for the fresh fuel mixture. Piston rings create the seal between the piston and the cylinder it travels in. If these rings are worn out or damaged, bad compression could well be the reason why your engine won't run. Piston rings do wear out over time, and eventually need to be replaced. This is referred to as a head rebuild. They can also fail if the engine is not broken in correctly (abuse), the wrong gas mixture is used (abuse), or overheating.

Unrestricted rotation:

The piston in the engine is attached a crankshaft, which rotates the flywheel on one side of the engine, and the clutch on the other. There should not be anything in the way of a free engine rotation on either side. If you try pull-starting the engine and it is jammed, it could be one of the following:

1. Broken pull start: common, especially if you repeatedly yank it in hopes that the engine will miraculously start instead of reading this article. It is the the easiest part on the engine to replace.
2. Shattered flywheel: this could happen if the bike crashes and sustains damage to the motor. Unless there is more damage to other parts in the engine, a replacement flywheel will fix the problem. You will need a flywheel puller to perform this repair (around $25).
3. Broken clutch or clutch springs: this will usually cause a rattling sound instead of a complete restriction of rotation. Luckily, this is among the simplest of fixes.
4. Seized piston: this will require a head rebuild and happens when the engine is abused.
5. Broken crankshaft: this is very bad and should not happen under regular engine use.
6. Cracked crankcase: this is also very bad and happens if you crash, drop, or throw your bike with a lot of force.

Troubleshooting Directions

If you would like to be guided through this instead, see the troubleshooter here. Also, be sure to download and thoroughly read & understand our Quick Start Guide. It is suitable for most two stroke engines, not just our bikes.

I. Check your gas and oil mixture

1. What gas mixture are you using? You should be mixing 4 ounces of synthetic, premium two-cycle oil to 1 gallon of premium (91+ octane) gas.
2. When you first get the bike, you should use 5 oz to 1 gal and switch to 4 oz after you use up the gallon.
3. It is not recommended that you use any included mixture bottles because they are always confusing, don't mix enough fuel, and more hazardous than using a standard, state-approved gasoline container.

II. Make sure the fuel supply is on

1. Make sure that you are not low on gas. If you are low, gas flow may be inconsistent.
2. Ensure that the fuel line is connected from the gas tank to the carburetor and has no kinks in it.
3. Make sure fuel is passing through the fuel filter, if there is one. If the filter is translucent, you should be able to see the fuel in it. If you cannot see it, disconnecting the fuel line from the carburetor after the fuel filter is a good way to test that the fuel filter is passing fuel. Careful though: be prepared to stop a gas leak in progress!
4. Make sure the fuel shutoff valve (white tab on carburetor) is in the OPEN (up and down) position.

III. Follow proper starting procedure

1. The choke lever on the carburetor needs to be in the UP (closed) position when starting the bike if it has not been running.
2. Pull start the bike by pulling the cord toward your chest in a single deliberate motion. DO NOT let the cord recoil into the pull start assembly and return it there slowly.
3. ONLY twist the throttle slightly when starting the bike. If you apply too much throttle, you will flood the cylinder which will prevent the engine from starting.
4. If you think you have flooded the cylinder, either let the bike sit for 5-10 minutes or (a) close the fuel line, (b) pull start the bike 3-5 times, (c) re-open the fuel line and start back at #1.
5. One the bike has started, slowly move the choke DOWN (open) while opening up the throttle wider. 6. It is recommended to lift up the back wheel and rev up the engine a few times, then let the bike warm up for 1-2 minutes before riding. DO NOT allow a 2-stroke air-cooled engine to idle for extended periods of time in hot weather without riding it. This may cause the engine to overheat as it relies on air movement to stay cool.

IV. Check electrical systems

1. Ensure that the key is in the "on" position if your bike has a key.
2. Wiggle the spark plug lead to make sure it is securely attached to the plug.
3. Follow the wire from the kill switch to the magneto and check for damage to it which could cause it to short.
4. Make sure the kill switch is not jammed.
5. Some models have a green ground wire coming from the key switch. It should be connected to an exposed metal frame of the bike. Usually, it will be sandwitched between the engine and pull start. Make sure the metal lead on the wire is exposed and it is securely touching the metal of the engine.

V. Perform a spark plug test

If a bike does not start with the basic troubleshooting techniques there might be an electrical problem. We need to test if there is a spark. If there is no spark when the engine turns, the gas does not get ignited, hence the engine will not start. This is a common troubleshooting test that will pretty much tell you whether the problem is electrical or carburetor related.

1. Unplug the spark plug wire from the plug.
2. Remove the spark plug from the engine using the included spark plug removal tool or your own tools.
3. Re-connect the plug to the wire, but do not reinsert the plug into the engine.
4. Have someone pull start (or kick start, depending on model) the motor WHILE HOLDING THE METAL OUTER SHELL OF THE SPARK PLUG against the exposed metal engine cylinder. Make sure the engine part is NOT PAINTED. Do the above procedure in a dark location and look for a spark in the spark plug. If there is no spark, that means the problem is electrical. In most cases, it is because of a defective kill switch which creates a ground, preventing the spark plug from working. We recommend following the kill switch wire and unplugging it from the engine.

VI. Spray carb and choke cleaner into air filter

If the headings 1-5 revealed no problems (you mixed the gas correctly, are starting the engine correctly, and tested for a spark), try the following. It has been known to work like magic.

1. Purchase a spray bottle of carb and choke cleaner from a local auto store.
2. Use the included straw to spray 3-4 bursts of carb and choke cleaner into the air filter.
3. For stock air filters (black rectangular box), point straw into the holes on the bottom of the air filter box.
4. For after-market paper filters, you can spray directly on the filter paper / mesh.

VII. Carburetor fuel/air adjustments

Due to nuances that require experience in working on engines, we don't recommend tuning the carburetor unless you know what you are doing. Typically, two stroke carburetors will have two adjustments - the fuel/air mixture screw (which controls how lean or rich the carburetor runs) and an idle screw. The default fuel/air mixture setting on the OEM carburetors for Cagllari, MX3, MTX, and RSR is 1 1/2 turns open from screw fully tightened. Under normal circumstances, the default carburetor setting should not be adjusted.

VIII. Adjust idle screw

If the bike dies when you let go of the throttle after it has warmed up for 1-2 minutes (and proper fuel mixture and starting procedure were used), turn the idle screw on the carburetor clockwise until it can idle. It is recommended that you inspect the throttle cable for any kinks prior to adjusting the idle screw as the default setting will usually suffice. The engine is idling too fast if the drive wheel spins when you are not holding the throttle, or the bike moves forward on its own. A LITTLE BIT of wheel drift is normal when the bike is idling, but if the bike exhibits any force forward when idling, the idle screw must be turned down. Otherwise, damage to the clutch will occur.

I hope these instructions help you in getting your pocket bike engine started. Any comments on what works best for you are appreciated. Be sure to indicate what bike you have.