How to test a moped automatic choke unit
Why do we need a choke?
When a scooter engine is cold it needs a richer mixture (extra petrol) to start and run. In the briefest of terms this is because petrol in liquid form does not actually burn, it's the vapour it gives off that burns. Petrol vaporises much more slowly when it's cold, hence you need more of it to create the same amount of vapour. The choke circuit provides a method of increasing the fuel supply until the moped has warmed up and then shuts off, either by a manual choke which you switch on and off yourself or an automatic choke which does this for you.
Either way the choke is just a simple plunger which opens or closes (blocks off) a small channel inside the carb (the choke circuit) which allows (or doesn't allow!) extra fuel into the engine.
If you look at this manual choke here...click here... you can clearly see the plunger which is simply lifted up or down manually by the black lever to open or close the moped choke circuit as the rider wishes. You can see the same plunger in the photo of the automatic choke at the top of the page.
You'll notice that the automatic choke has a needle on the end of the plunger whereas the manual one doesn't. The needle allows the automatic choke to gradually allow less and less fuel to flow through the choke circuit until it finally closes it fully after the engine has been running for a few minutes. In contrast the manual choke is simply on or off, nothing inbetween !
How does an automatic choke work?
Inside the automatic choke there is a small wax capsule with a heater coil wrapped around it. When the engine is started a voltage is applied to the heater and the wax heats up. As the wax begins to heat it expands and pushes the plunger out slowly. After approx 4 or 5 minutes the plunger is fully extended closing off the choke ciruit and no longer allowing additional fuel to flow. As soon as the engine is switched off the wax capsule begins to cool and the plunger is slowly retracted again after about 15 minutes.
How do we test it?
The easiest and surest way to test the choke unit is to test it on the bike.
- Start and run the bike (fast tickover is fine) for at least 5 minutes. (This should ensure that the choke plunger is fully extended)
- Switch off the engine and *quickly* remove the moped choke unit. You need to do this ideally within 30-60 seconds max of switching off the engine, so make sure you've already removed any panels beforehand which are in the way. To remove it just under the 2 screws holding in the horseshoe 'U' clip and pull it out. Measure the distance A as shown as in the picture below
- Now leave the choke for at least 20 mins to cool down and measure the distance again- distance B in the photo below
You'll be able to get a good idea if your choke is working correctly by comparing to the pictures and whether or not the plunger has retracted, but for reference distance A should be at least 2mm greater than distance A
It's not working!
There are 2 possible reasons for the unit not to work. Either the choke unit itself is faulty, or there is a problem with the power supply to it. If you have a multimeter you can test the power supply by disconnecting the automatic choke and touching the meter leads onto the wires which the choke was connected to whilst running the engine at a fast tickover - (make sure not to touch them together!)
The wires should give you a reading of 12-14volts AC. Note that the choke normally runs off the lighting circuit which is usually AC on mopeds, so make sure you test with the meter set to AC as well as DC! The choke itself will work off either AC or DC so as long as you have 12-14 volts there in either DC or AC the power supply is fine.
If you don't have a multimeter you can test the choke by removing it from the bike and connecting it directly to a fully charged moped battery using suitable wires (be careful not to short the wires together!). It doesn't matter which way round the wires are connected to the battery.
After 5 minutes of being connected it should have fully extended as in the 'hot' photo above and subsequently retract as in the 'cold' photo after the battery has been disconnected for 20 minutes.
If the plunger still doesn't move the choke unit is faulty and will need replacing.
You can either replace with an identical automatic choke unit or you can do away with it and instead fit a manual choke over which you have full control !!