150cc gas motor bikes

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1. Scooter Carburetor Adjustment

   If there’s any one component that’s more likely to go out of adjustment on your scooter, it’s the carburetor. If it does go out of adjustment lots of things can happen. It can make the scooter hard to start, it can make the scooter slow to accelerate, it can cause the scooter to idle badly and stall or it can make the scooter idle fast so that it’s trying to go all the time and needs the brake to hold it stationary. None of these things are good.

   What the carburetor does is regulate the amount of fuel and air which are supplied to the engine. To go faster you need more fuel and more air and they must be in the right proportion. The fuel also has to be atomized as much as possible (i.e. be supplied in tiny droplets). The carburetor controls all these functions and generally does it quite well. Without removing and disassembling the carburetor, there are only two adjustments you can make, but these should take care of most minor problems. The first is the adjustment of the idle speed.

   Above is a picture of a typical GY6 engine carburetor and the view is that from the right side of most engine configurations on 150cc Chinese scooters. There’s a small spring loaded screw which adjusts the idle speed. It’s right next to the throttle control which is connected to the twist grip on the right handlebar. On most 150cc scooters, idle speed should be around 1500rpm when the engine is fully warmed up. Don’t make adjustments when the engine is cold. Take a 5 or 10 minute ride first. When the engine is cold the automatic choke is in operation and that can change the idle speed. If you have a tachometer, setting the speed is easy. If you don’t, you want to set the speed to a low idle. Fast enough so that the engine doesn’t stumble or stall, but slow enough that the clutch doesn’t engage and try to move the scooter forward. On most scooters from 50cc to 250cc, this will be somewhere in the 1500-2000rpm region.

   The other adjustment you can make is to the low speed fuel/air (“mixture”) ratio. This is done on most scooters using a screw adjustment on the other side of the carburetor as shown below.

   Comment by master — 05/12/2010 @ 3:15 pm

2. What is a Scooter?

   Well, here’s what Wikipedia has to say:

   A scooter is a style of two-wheeled motor vehicle traditionally defined by characteristics such as a step-through frame, wheels less than 16″ in diameter, and an engine located below the rider and to the rear.

   That pretty much sums it up. Motorcycles have an engine mounted in the middle of the frame with a gas tank above it. The rider sits astride the engine with the gas tank right in front of them. Motorcycle wheels are also typically larger (over 16″ diameter) than those on a scooter. Another major difference is that the engine of a motorcycle is attached to the frame, while on a scooter it is usually part of the rear suspension. So when a motorcycle rear wheel goes up and down, the engine remains stationary, while when the rear wheel of a motor scooter goes up and down, the whole engine and transmission moves up and down with it.

   These days, 99% of scooters have a “twist and go” automatic (CVT) transmission, which means you don’t have a clutch to control and you don’t have to change gears, while 99% or motorcycles have a manual clutch and you have to shift gears manually (with your left foot). This also means that on 99% of motor scooters you apply the rear brake with your left hand, while on 99% of motorcycles you apply the rear brake with your right foot. While there are still a few manual scooters and there are a few automatic motorcycles, both are very much the exception rather than the rule.

   These differences result in a motorcycle usually having better acceleration high speed handling characteristics, but a motor scooter is often more maneuverable at low speed and is easier to ride, especially for novices.

   Scooters usually have a small engine, from 50cc to 250cc, though there are 400cc and even 800cc scooters, so engine size doesn’t define them. While most are limited in speed, scooters with a larger engine can exceed 100mph and cruise easily at freeway speeds, so speed isn’t a defining characteristic either.

   Comment by master — 05/12/2010 @ 3:15 pm

3. Troubleshooting

   My Scooter Won t Start

   To start a scooter needs three things:

   Gas
   A spark
   Cylinder compression
   If 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?? 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 kick starting 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.

   Comment by master — 05/12/2010 @ 3:15 pm

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