My Peugeot Speedfight 2 is having trouble starting when cold.

I took down the carburetor and cleaned it. The spark plug is also pretty new.

I've read that automatic choke can be faulty but I don't know how would I test that. It is a choke that has an extending needle. I've read up on how choke operates but found only examples of air restricting butterfly valve chokes. So I'm not sure what the needle does in automatic choke.

From my understanding the needle extends and closes the hole when the engine is warm. So am I right in that I can remove the choke, try to start the scooter without it and it should start cold?

  • great now i understand ,with auto choke you come to bike choke is on ,as if you had pulled out the choke on a manual one,then as your bike starts up the auto choke starts to slowly extend closing off the choke as if you were pushing the choke back in on a manual choke. easy when someone explains thanks . Nov 15 '18 at 12:12

How the choke works

The choke of a scooter consists of a dedicated choke nozzle in the fuel reservoir and two narrow drillings leading to the venturi (the pipe where the air goes through):

enter image description here

When the motor cranks and the choke is active, the slight underpressure in the venturi sucks in extra fuel through this drillings. When the choke is inactive, a piston (golden) moves down and closes the drillings.

In reality, the choke looks like this, and you can easily recognize the piston, which is the brass barrel with the needle:

enter image description here

Inside the black choke body, there is a small cavity filled with a liquid like oil, and an electric heating element. When power is applied, the oil heats up and expands slowly, pushing down the piston. When power is removed, the oil slowly cools down and retracts the piston.

The choke is directly connected to the alternator, i.e. it only gets power when the motor is running. It takes several minutes for the piston to move from the "fully retracted" to the "fully extended" position, and the difference is really small, may be 5mm (1/5").

How to test it

This should be clear now. Simply check if the piston moves out within maybe 10min when power is applied. But since it takes so long and the piston moves a little only, you should definitely measure the position with a caliper or similar. And also check if the piston is retracted after an hour of cool-down or so. Ah, and as first test, check if the choke gets warm.

To apply power, either take a ride or connect the choke directly to the battery. But note that the choke draws about 1A, which can drain a small 7 not fully charged / weak scooter battery.

The choke typically fails in two modes: Either the piston does not retract (choke always inactive) or it does not extend (choke always active).
And of course, it's also possible it doesn't get power...
Further more, it is rarely possible that the piston does move, but not enough. Then you would need to know the positions for a working choke.

Driving without choke

  • If the choke works fine, disconnecting the cable will make it "always active". The motor will start fine, but consume more gas and build up more soot.
  • If the choke is simply removed, the motor will draw additional air from above instead of additional fuel, making the mixture leaner. This makes it even more difficult to start and may cause damage due to the lean combustion. And the additional air didn't go through the filter. Instead, it comes from the dirty, muddy and wet place where the carburetor typically sits...
    However, if the choke failed in the "inactive"-position, you can remove it, put your finger on the opening, (try to) start, and then quickly plug the choke in again.
  • When the scooter should be driven without the choke, not only the opening should be sealed, but also the entire bore in which the piston moves. This disables the choke and makes it harder to start the motor. But the mixture is fine when the motor is warm.
  • Thanks, I will remove the choke, seal the hole with something and then try o start it. How does the open hole helps adding more fuel/less air? With butterfly valve it is easy to understand this.
    – ragezor
    Mar 5 '16 at 14:10
  • I've added a sketch. usually, the carburetor sucks gas only though the choke system, but if you remove the choke, air will flow in from above.
    – sweber
    Mar 5 '16 at 15:06

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