why choke a cold engine

[dkortje]

Can someone tell me why you must choke a cold engine. I understand that the choke changes the air/fuel mixture, but what is it that makes a cold engine need a different ratio than a warm engine when the outside temp is not changing?

 

[350thumper]

Due to the ambient temperature inside the cylinder the fuel air mix needs more fuel to ignite. The choke restricts air and in turn gives more fuel for each unit of air in the mix. This allows a cold enviornment to become more volitile and flammible. When the Temp inside the cylinder is warm or hot, The ammount of fuel needed to make the mixture volitile is less.

hope this helps,
danman

 

[jmics19067]

From what I gather the heat of the engine helps vaporize the fuel so it mixes with the air better.With a cold engine the fuel tends to stay in large droplets and there are going to be "bare " spots of air that won't ignite easily. By adding extra the fuel you help try and cover up these bare spots.

The bikes I know of don't have a "choke " they have a fuel enrichment lever. Open this up and it lets more fuel pass thru the carb into the airstream. Cars that have "chokes" have a little flap over the carberator that closes off the airflow thru the carb,creating more vacuum in the carb, drawing more fuel out of the idle and off idle circuits.

 

[steve.emma]

The reason a cold engine requires a richer fuel air mixture is beacause in a cold engine the droplets of fuel (in the mixture) stick to the cold inlet manifold and intake port,intake valve walls and fall out of suspension of the air mixture making the motopr run lean. When you put the choke on it combats this effect,when the motor and its components heat up the droplets of fuel that try to stick to the inlet port etc are vaporised and don't fall out of the mixture. The reason a choke uses a device to speed up the idle is beacuse of increased drag on engine components when they are cold. Hope this explains things o.k.

 

[WFO]

Just like Steve said. Think of what your breath does when you blow on a cold peice of glass. The moisture in it condenses on the glass. Same thing with the gas vapor condensing on the wall of the cylinder. The air that is left behind is too lean to start properly.