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Getting Your 2-Stroke to Run Properly
(By Fabio Dougie)


Here's some useful tips if you experience problems starting and/or getting your 2 Stroke scooter to run properly.

Three is the Magic Number...3 Things you must have.
To turn a scooter engine over and to get it to start, the engine needs three things....
- Fuel (The ratio of petrol/air must be correct)
- A spark (Sparking at the right time in the engine's cycle)
- Cylinder compression (Must be up to a sufficiently high level of compression)

If your scooter won't start its very likely to be due to one of the above. Before you start troubleshooting, make sure there is fuel in the tank by actually looking into it. Don't just trust the fuel gauge.

Electric Start
Most scooters have four things that need to be done before they can be started with the electric starter:
- The ignition has to be turned on
- The kill switch has to be in the "on" position (usually near the right handgrip. This usually only applies to most non-UK bikes that are fitted with a 'kill switch')
- One of the brake levers has to be pulled (usually doesn't matter which one)
- The starter button must be pressed.

If the scooter doesn't turn over when you've done all the required operations, there's perhaps an electrical problem with the battery. Make sure the battery is charged. If it is, make sure the fuse hasn't blown. If it hasn't, check to see if you are getting a voltage at the starter motor terminals. If you are, the starter motor is likely defective. You can check the voltage that the battery is giving out and if defective you should consider replacing the battery.

Give it a Kick!
Many scooters have a kick start. If the battery is OK and the starter motor doesn't work, try kickstarting it. It's usually not too hard and your scooter should start ideally on the 1st kick, but 2nd or 3rd kick is OK too.

Engine does turn over but won't start
If the scooter does turn over but still doesn't start you should check for a spark and make sure fuel is getting to the engine. To check for a spark, remove the spark plug from the cylinder and re-attach the spark plug cap/lead back onto the spark plug. Hold the tip of the plug against the cylinder head and crank the engine.

You should see a blue spark across the gap, the spark should be strong! If you don't, there's possibly a problem with the ignition. This could mean that you have a faulty coil, CDI unit or electronic ignition module which will have to be replaced, or it could just be a loose wire. If the spark plug is fouled up/oily/overly blackened then it is very likely that it is the cause of your problems and you should consider replacing it as statistically, the second most likely fault with starting a scooter is a faulty/fouled up spark plug.

It sparks! It sparks!.......but where's the Fuel??
If you do have a strong spark, you next need to check that fuel is being delivered to the carburetor. Make sure there is fuel in the tank first and if so you can move on to checking the fuel valve. Most scooters use either a gravity fuel feed by locating the fuel tank higher than the carburetor, or some use a vacuum-fed petcock (on/off tap) controlled by the vacuum created at the carburetor. The carburetor has two fuel-related pipes connected to it. One is from the fuel tank and is for the fuel supply, the other provides a vacuum to suck the fuel from the fuel valve. If you disconnect the fuel supply pipe from the carburetor, fuel should flow out when you crank the engine (be careful to catch the fuel if you do this and don't do it with a hot engine), if fuel doesn't flow out then the fuel valve may be faulty. You can try applying a vacuum to see if fuel flows by sucking on the vacuum pipe(but make sure it's the vacuum line, not the fuel line if you try this!). Another Tip: If there is a spark but you are unsure about fuel supply... squirt a little bit of fuel (a spoonful) into the spark plug hole and then replace the spark plug and try to start the engine.....if it starts and runs for 5 seconds and then dies then you can check for fuel flow to the carb, or blocked jets.

You must be Choking!!
If you are getting a strong spark, fuel supply to the carburetor is good and the engine is turning over on the starter, it's possible that the electrically operated automatic choke has failed. It's supposed to operate when the engine is cold, but if it's stuck or burned out, the mixture will be weak and the scooter probably won't start. The automatic choke is usually a black cylindrical object attached to the carburetor with a couple of wires coming out from it. It's the only electrical powered component attached to the carburetor, so it's usually not too hard to spot. If you have a meter you can measure the resistance across the leads of the choke, you should see around 10 ohms or less. If it's an open circuit its likely to be burned out and will need to be replaced. If it seems to be OK it may simply be stuck, or the wiring to it may have a problem/loose connection. You can also check to see that it's getting voltage across its terminals. A final check...remove the choke from the carb and plug the wires into a 12V source for about 5-10 minutes. The choke body should get warm and the protruding mechanism should extend slightly. Measure the length of the protruding choke mechanism - it should have increased in length by around 5mm more than when it was cold.

All gummed up
It's also possible that there is a problem with the cleanliness of the carburetor and if so you may need to remove it and clean it out. If the scooter has been sitting for a few months with fuel in the carburetor, the fuel may have evaporated and left behind a sticky "gum" that will prevent the carburetor from working properly and which must be removed. Statistically, the biggest problem for a scooter not starting is a gummed up carburetor/ blocked carb jets.


Blocked Jets
A very common problem is blocked jets in the carburetor. The jets have tiny holes in them that the fuel passes through to supply the carb and these become gummed up with fuel residue. You will find three jets in the carb (Main jet-used for Wide Open Throttle, Idle Jet-feeds the carb at idle/low revs, Choke/Power Jet-gives extra-rich fuel supply when starting from cold) and its easy to remove the jets and clean them out with a spot of Carb Cleaning Fluid and/or with compressed air. A tip is to blow through the jets with your mouth and then hold them up to the light to see if you can see clear daylight through the jet's bore. Whatever you do don't be tempted to prod the hole in the jet with a bit of wire or a needle as this will enlarge the jet's bore and be the cause of tuning problems at a later stage.


Setting the idle mixture screw
You could also be experiencing too rich or too lean fuel/air mixture to your carb. To set this you should screw the idle mixture screw fully in.... (Before you screw it in...look at where the screwdriver slot is at... then count how many turns it takes to go all the way in gently... 1 and 1/2 turns for example, and remember the number of turns for later)..... then unscrew it out by the number of turns that it states in your handbook - for my scooter it was 1 and 1/2 turns out.

Check that Compression
If the automatic choke is problem free, the starter cranks the engine sufficiently, there's fuel at the carb and there's a strong spark then there may be an engine problem and you need to check the cylinder compression. To do this accurately you will need a compression tester. It screws in instead of the spark plug and measures cylinder pressure. When you crank the engine you should see a reading of approaching 125-150 psi. If it's 100 psi or less then its likely that you have an engine problem (probably failed piston rings) and you will need to remove the cylinder head to gain access to the piston rings. A trick to try if you haven't got a Compression Tester is to hold your finger tightly over the spark plug hole with the spark plug removed to see if your finger is forced away strongly when you try to turn the engine over - it may just indicate that there is very little compression if the force is very minor. Another tip is to add a teaspoon of engine oil through the spark plug hole and try to crank the engine again. If you then get a better compression its likely that the piston rings are shot, as the oil you added sealed the rings for a short period of time and hence temporarily gave you better compression.

In Summary
Probably the most common problems to starting a scooters' engine are a dead battery or carburetor problems due to bad adjustment or build up of gummy deposits, carburetor problems usually come on slowly though. If a scooter is OK one day and refuses to start the next day, I'd first suspect an electrical problem and look to check/replace the battery.

.

Dellorto Carb jets - view from underneath
Last Updated Sat Apr 02, 2011 3:05 pm
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