Why do engines run rough when cold?

[DaveInPortland]

This is purely out of curiosity. I'm looking for an accurate technical answer, not a guess. I've been guessing at it for years!

Why do engines, especially engines of older, carbureted vehicles like VWs and Lammbrettas, run roughly when they are cold/just started up?

[tony]

Actually you've almost answered yourself. Carburated engines are set up (or should be) to be at the correct mix when an engine is warm. The choke mechanism is a crude enrichening device to get the thing to fire whilst all its components are cold drawing off heat. Cold fuel doesnt vaporise very quickly so by restricting the air (or increasing the fuel) we adjust the ratio helping the ignition process.(basically allowing more fuel to air to be drawn to get some vaporisation going)also choking the motor means exactly that tbh.. it helps a non running motor draw as there is no vacuum to start with as its not running obviuosly.Now there is another reason that even once running they take a while to 'clear' and become happy , more so on a tuned motor than a standard one. But thats getting into another area.

[lifes a drag]

thats just what i was gonna say tony

[cezeta]

Now there is another reason that even once running they take a while to 'clear' and become happy , more so on a tuned motor than a standard one. But thats getting into another area.[/quote]

prey tell brother Tony?

[J1MS]

hydrocarbons burn according to pressures and temperatures, as well as A/F ratios, so there will be a change in the running of an engine as revs rise, exhausts resonate, and temperatures change...

[tony]

[DaveInPortland]

Thanks for the explanation. You got at what I really wanted to ask but wasn't aware it was the correct question: Why does it take awhile for an engine to become clear and happy? I am going to guess it has to do with operating temperatures of the metal.

[tony]

Temps have got to climb to get the pipe to work.