how to Fix It - part 2
Troubleshooting - Scooter won't start
To start a scooter needs three things:
Gas A spark Cylinder compressionIf your scooter won't start it due to one of the above not being present. Before you start troubleshooting, make sure there is gas in the tank by actually looking into it. Don't just trust the gas gauge!
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 (usually near the right grip) has to be in the "on" position The rear (left) brake lever has to be squeezed. The starter button must be pressed.Some scooters may have other interlocks on the front brake or the center stand. Check your owner's manual to make sure that you are doing all you need to to complete the electrical circuit which enables it to start.
If the scooter doesn't turn over when you've done all the required operations, there's 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 voltage to the starter motor terminals. If you are, the starter motor is likely defective. Many scooters have a kick start (see your manual). If the battery is OK and the starter motor doesn't work, try kickstarting it. It's usually not too hard. My scooter usually starts OK on the 2nd or 3rd kick.
If the scooter does turn over but still doesn't start you have to check for a spark and make sure gas is getting to the engine. To check for a spark, remove the spark plug from the cylinder, hold the tip of the plug against the cylinder head and crank the engine. You should see a spark across the gap. If you don't, there's a problem with the ignition. Could be a faulty coil or electronic ignition module which will have to be replaced, or it could just be a lose wire.
If you do have a spark, you need to check the gas. Make sure there is gas in the tank first! If there is you need to check the fuel valve. Most scooters use a gravity fuel feed by locating the fuel tank higher than the carburetor. The fuel usually then flows through filter and then through a vacuum operated valve. The valve has three hoses connected to it. One if from the fuel tank and is for the gas going in, one is connected to the carburetor and is for the gas coming out and one goes to the engine manifold which provides a vacuum. If you disconnect the hose from the carburetor, fuel should run out when you crank the engine (be careful to catch the gas if you do this and don't do it with a hot engine). If you don't see gas the fuel valve may be faulty. You can try applying a vacuum to the control port of the valve and see if the fuel flows. Sucking on the line is one way to do this (but make sure it's the vacuum line, not the gas line if you try this!).
If you are getting a spark and you are getting gas to the carburetor and the engine is turning over on the starter OK, 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 of it as shown on the left. It's the only electrical powered component attached to the carburetor, so it's usually not hard to spot. If you have a voltmeter you can measure the resistance across the leads of the choke. It should be somewhere in the region of around 10 or 20 ohms. If it's an open circuit, it's burned out and will need to be replaced. If it seems to be OK it may be stuck, or the wiring to it may have a problem. Check to see that it's getting voltage.
It's also possible that there is a problem with 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 gas in the carburetor, the gas may have evaporated and left a sticky "gum" behind that will prevent the carburetor from working properly and which must be removed.
If the automatic choke is OK, the starter cranks the engine, there's gas and there's a spark then there may be an engine problem. You need to check the cylinder compression. You need a compression tester to do this. It screws in instead of the spark plug and measures cylinder pressure. When you crank the engine you should see a reading of around 150 psi or more. If it's 100 psi or less there's likely and engine problem (bad valve, bad piston, failed piston rings) which will need the engine taken apart to find.
Probably the most common problems 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.
If your scooter starts but won't rev or run well, there are basically five most probable areas of trouble:
Problems with fuel flow Problems with the spark Air/vacuum leaks Problems with the carburetor Problems with valve clearance Fuel flow problems can be caused by a clogged fuel filter, a clogged fuel line, a faulty fuel supply valve or a faulty fuel pump (on those scooters that use one). One way to bypass all these components is to rig up a temporary gravity fed fuel flow directly to the carburetor. If you connect a fuel supply directly to the carburetor from a container held above the level of the carburetor and the scooter revs freely, you've identified the problem and you need to check (or replace) each component of the fuel supply to make sure it's working OK.If the scooter starts, it must be getting some sort of spark, but the spark may be weak or there may be a problem with the ignition timing. The first thing to do is remove the spark plug and take a look at it. If there are heavy deposits on the tip (black or white), or if it looks worn or damaged or if the gap isn't what it should be, it should be replaced. You could clean it and regap it but plugs are cheap ($5 or less), so you should probably replace it anyway and see if the engine runs better. If there's no difference, at least you now have a spare plug if the one in the engine ever does fail!
Today there are very few scooter engines on which you can alter the ignition timing. All timing functions are done via the CDI (Capacitor Discharge Ignition) module. If it's bad, all you can really do is replace it. They're pretty reliable though, so that might be something to try if all else fails to find a problem.
Leaks in the air or vacuum lines can cause problems too. Usually it's leaks in the vacuum lines which control several functions related to the carburetor operation and fuel supply. First check all the vacuum line connection and the lines themselves for splits and cracks. If they look OK you can spray water on them while the engine is running and see if there is any change in engine speed. If there's a leak, the liquid will temporarily seal it and the engine will run better. Some recommend spraying WD40 or carburetor cleaning fluid on the lines, but if you do there can be a risk of fire, so be careful.
Carburetor problems are the most likely source of poor running. First check the idle speed and air/fuel mixture screws (see carburetor adjustment). If the adjustment is OK, or you can't get the engine to idle well, then comes the next step. Over time (especially if the scooter is stored for long periods of time with gas in the carburetor), "gummy" deposits can build up. These are the residues which are left behind when gas evaporates. They can clog internals valves, jets and passages inside the carburetor which can affect fuel flow. The best way to deal with them is to remove the carburetor from the scooter (see carburetor rejetting), take it apart and soak it in carburetor cleaning fluid, which will dissolve the deposits. Make sure the jets are clear too.
Valve clearance ("valve lash", "tappet clearance") can affect the way the engine runs. This is the clearance between the camshaft and the valve stem. If it is too large, the valve won't open all the way (and you'll get a loud "clattering" noise) and if it is too small (or non-existent), the valve may not properly close. Either way is a problem. You either won't get fuel/air into the cylinder properly or you won't get good compression and/or exhaust and the valve may overheat and suffer damage.
To adjust the valves the engine must be COLD. If it's not cold, the setting will be wrong. Then remove the valve cover. On single cylinder engines there are two valves (one exhaust, one inlet). Then follow the instructions in your manual if you're lucky enough to have one that gives them. If not then you can try this. Put the scooter on the center stand and/or a jack so that the back wheel is off the floor. Put a wrench on the nut holding the center of the fan and rotate the engine until it's at what is known as "Top Dead Center" (TDC), which is when the piston is at it's highest on the compression stroke. This may be marked with a "T" on the flywheel (see image below) At this point both valves should be fully closed and the gap between the rocker arm and valve stem (clearance) should be at its maximum. Both rocker arms will be at minimum height positions on the camshaft lobes. There may be marks on the flywheel to indicate TDC, and/or there may be marks on the outside of the cam sprocket. Check your user manual if you have one.
If you can't find any timing marks for TDC you can rotate the engine clockwise the cam lobe is about 180 degrees away from the rocker arm.
This means that the clearance between the rocker arm and the valve should be at it's maximum and you should be able to wriggle the rocker arm back and forth a bit (that's the clearance). You can also remove the spark plug, put a straw into the cylinder and rotate the engine until you feel the piston is at its maximum height both valves are fully closed (note that there are two position in a 4-stroke engine at which the piston is at it's maximum height in the cylinder.
If you can't find any timing marks for TDC you can rotate the engine clockwise the Be sure you get the right one). Once you're at TDC, loosen the locking nut and adjust the clearance using a set of feeler gauges. When you have it set correctly, tighten down on the lock nut, and re-check it. Now do the same thing again for the other valve. For most 150cc GY6 engines the clearance should be around .004", but check your manual. For 50cc engines I've seen .002" recommended.
If your scooter starts but won't rev or run well, there are basically five most probable areas of trouble:
Problems with fuel flow
Problems with the spark
Air/vacuum leaks
Problems with the carburetor
Problems with valve clearance
Fuel flow problems can be caused by a clogged fuel filter, a clogged fuel line, a faulty fuel supply valve or a faulty fuel pump (on those scooters that use one). One way to bypass all these components is to rig up a temporary gravity fed fuel flow directly to the carburetor. If you connect a fuel supply directly to the carburetor from a container held above the level of the carburetor and the scooter revs freely, you've identified the problem and you need to check (or replace) each component of the fuel supply to make sure it's working OK.
If the scooter starts, it must be getting some sort of spark, but the spark may be weak or there may be a problem with the ignition timing. The first thing to do is remove the spark plug and take a look at it. If there are heavy deposits on the tip (black or white), or if it looks worn or damaged or if the gap isn't what it should be, it should be replaced. You could clean it and regap it but plugs are cheap ($5 or less), so you should probably replace it anyway and see if the engine runs better. If there's no difference, at least you now have a spare plug if the one in the engine ever does fail!
Today there are very few scooter engines on which you can alter the ignition timing. All timing functions are done via the CDI (Capacitor Discharge Ignition) module. If it's bad, all you can really do is replace it. They're pretty reliable though, so that might be something to try if all else fails to find a problem.
Leaks in the air or vacuum lines can cause problems too. Usually it's leaks in the vacuum lines which control several functions related to the carburetor operation and fuel supply. First check all the vacuum line connection and the lines themselves for splits and cracks. If they look OK you can spray water on them while the engine is running and see if there is any change in engine speed. If there's a leak, the liquid will temporarily seal it and the engine will run better. Some recommend spraying WD40 or carburetor cleaning fluid on the lines, but if you do there can be a risk of fire, so be careful.
Carburetor problems are the most likely source of poor running. First check the idle speed and air/fuel mixture screws (see carburetor adjustment). If the adjustment is OK, or you can't get the engine to idle well, then comes the next step. Over time (especially if the scooter is stored for long periods of time with gas in the carburetor), "gummy" deposits can build up. These are the residues which are left behind when gas evaporates. They can clog internals valves, jets and passages inside the carburetor which can affect fuel flow. The best way to deal with them is to remove the carburetor from the scooter (see carburetor rejetting), take it apart and soak it in carburetor cleaning fluid, which will dissolve the deposits. Make sure the jets are clear too.
Valve clearance ("valve lash", "tappet clearance") can affect the way the engine runs. This is the clearance between the camshaft and the valve stem. If it is too large, the valve won't open all the way (and you'll get a loud "clattering" noise) and if it is too small (or non-existent), the valve may not properly close. Either way is a problem. You either won't get fuel/air into the cylinder properly or you won't get good compression and/or exhaust and the valve may overheat and suffer damage.
To adjust the valves the engine must be COLD. If it's not cold, the setting will be wrong. Then remove the valve cover. On single cylinder engines there are two valves (one exhaust, one inlet). Then follow the instructions in your manual if you're lucky enough to have one that gives them. If not then you can try this. Put the scooter on the center stand and/or a jack so that the back wheel is off the floor. Put a wrench on the nut holding the center of the fan and rotate the engine until it's at what is known as "Top Dead Center" (TDC), which is when the piston is at it's highest on the compression stroke. This may be marked with a "T" on the flywheel (see image below) At this point both valves should be fully closed and the gap between the rocker arm and valve stem (clearance) should be at its maximum. Both rocker arms will be at minimum height positions on the camshaft lobes. There may be marks on the flywheel to indicate TDC, and/or there may be marks on the outside of the cam sprocket. Check your user man have one.
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Troubleshooting - Scooter won't run
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