Warm up first?

Originally Posted by embee

...it's contaminating the oil film on the cylinder bores with fuel and condensation, which reduces the service life of the oil and increases wear rates on the rings and bores....

Can someone explain this to me please along with the enrichment thing? What is the function of the choke/FI alternative?

chokes to spit a bit more fuel in aint it fi just does itself.

I was told to leave mine for about 5 minutes! Cant be arsed with that, I usually wait till its about 30-40

Originally Posted by zsv650

chokes to spit a bit more fuel in aint it fi just does itself.

Yeah but why does it need to do it? What's the purpose of it?

Originally Posted by teriyakimonkey

Yeah but why does it need to do it? What's the purpose of it?

Otherwise it'll just cut out if it's cold and the revs are taken away

[SVGrandad]

Originally Posted by teriyakimonkey

Yeah but why does it need to do it? What's the purpose of it?

A fuel/air mixture that is right for a hot engine will not fire in a cold engine hence the need for more fuel in the air for a cold motor

I know but WHY?!!! Why does the engine need more fuel in the first place? If it's because it's cold then what difference does it make?

EDIT: Sorry Grandad, I was still writing when you piped up!

Originally Posted by teriyakimonkey

Yeah but why does it need to do it? What's the purpose of it?

Its purpose is to restrict the flow of air thereby enriching the fuel/air mixture while starting the engine.

Choke is required because small droplets of petrol don't evaporate well in a cold engine. By restricting the flow of air into the carburetor, the choke valve raises the level of vacuum inside the throat of the carb, which causes a proportionally greater amount of fuel to be sucked out of the main jet and into the combustion chamber during cold-running operation. Once the engine is warm, opening the choke restores the carburetor to normal operation, supplying fuel and air in the correct ratio for clean, efficient combustion.
Fuel injection bikes alter the amount of fuel / air ratio automatically depending on running conditions.

Oil is deposited in the honing marks of the bore as the piston goes up and down by splash lubrication - these act like tiny ledges to hold the oil which provides a lubricant film between the bore and the piston skirt, and a seal between the ring and the bore. Fuel can wash this oil away so you loose the seal allowing blowby into the sump which deteriorates the oil - and you get less protection of the bore by the oil as a lubricant which can wear away the honing marks and polish the bore - worse case scenario is no honing marks = no ledges for the oil = no seal for the rings = looses compression and burns loads of oil as it comes past the rings.

Also, if the top of the engine is very hot from combustion, but the bottom end is cold because it's not moving and warming up - this can cause condensation - the condensation mixes with the by-products of combustion and creates powerful acids that can damage the internal components. Also for every gallon of fuel you burn, you produce a gallon of water - only when your engine gets hot is this water burnt off ( that's why you get water out of a cold bike exhaust - which is why they tend to rot from the inside out )

I think thats right ...

Spokey

[Sid Squid]

It's needed as when the engine is cold the fuel in the intake charge falls out of suspension and recondenses onto the cold surfaces of the engine meaning that the mixture that reaches the cylinder will be severly weakened. You don't actually need a richer mixture to burn when cold - it's just a mechanism to ensure enough fuel actually makes it to the cylinder to ensure reliable burning when cold.
The 'choke' nowadays is rarely that, that term specifically refers to a plate upstream of the carb's jets, (that is; the air filter end of the carb), which when closed over restricted the air intake, such a restriction means a greater proportion of the resultant vacuum would act upon the jets, thereby increasing the fuel flow and richening the mixture. Sometimes the plate would be spring loaded, so that the vacuum couldn't rise too high - a high vacuum would pull open the plate against the spring, sometimes the plate simply had a small hole in it, close over for starting, open for running. Both are from the sledgehammer school of engineering.
Nowadays because of such history we have inherited the word choke, as in choking the intake, but it never is, it's a separate set of jets that flow fuel in a rather neater and more controlled way rather than the intake strangler it originally was.

That's what I'm talking about! Thanks clever people.