why do engines run sluggish on start up?

[Nevada Sixx]

why do engines run sluggish when you first crank them from a cold start? I know the cylinder needs to be warmed up so the hot expanding pistion wont scar the cylinder wall, but why wont it just run crisp right off the start up? is it because the engine isnt hot enough to ignite all the fuel yet?

 

[TRAIL]

I have heard of some bikes running BETTER when you first fire them up for the day. I am guessing it has something to do with jetting?

 

[MikeyG]

My guess would be that the piston hasn't expanded fully yet causing the bike to lose compression?

 

[WFO]

You optimumly jet your bike to run its best when warm. When the bike is cold, the vaporized gas is condensing on the cylinder wall, creating a lean condition. The choke compensates for this by allowing more gas in (richer) per volume air. In other words, its getting you started; its not intended to be an ideal setting for a bike whose temperature is rapidly changing. As the bike warms up, it becomes overly rich until you shut the choke off.
I'm sure tolerances between engine parts that have not expanded to their normal operating size play some part in it too.

 

[Mr. Clean]

I am assuming you are talking about the rough running until it is cleaned out right? If so, it runs badly because when you kill the bike, the hot oil in the engine runs down into the crankcase and makes a puddle there. I have seen many people foul their plugs after a cold start because they rev the engine too fast too soon. To avoid this, I always short shift my bike and clean it out slowly to avoid throwing the surplus oil in the transfer ports all at once. You can check it out for yourself. Next time your cylinder is off rotate the flywheel and usually it has oil on the bottom of the weights where the puddle was.

 

[langston7]

just put it like this.....

What do you feel like and act like when youve first woke up, compared to 10 minutes later, then 1 hour then when its like 12 at nite and your nackered...

just a little thery of mine!