2-Stroke Pocket Bike Starting Troubleshooting
Posted by Scott S. on 15 October 2010 07:28 PM
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.
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:
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.
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:
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
II. Make sure the fuel supply is on
III. Follow proper starting procedure
IV. Check electrical systems
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.
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.
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.