Thu Oct 17 12:05:36 1996 - Message No. 96
From: Pierre DEBRAS

Subject: Choke or not choke


Hy guys,

I thought a little bit theory would help clarify the mistery of the
choke (joke?).

The purpose of the carb is to mix the fuel with the air going into
the cylinders. The engine sucks the air, which causes a depression 
inside the carb, which in turn causes the fuel to be "pumped" thru
the jets. What's came out the carb is a mix of air and very fine
bubbles of fuel. This mix is burned inside the cylinder ... and
that why your bike runs (in short).

The problem with a cold engine is that the fine bubbles of fuel will
condensate when they meet the cold walls of the pipes and the engine. This
reduces the amount of fuel in the mix air-fuel ("lean" mix), and that's
why the engine won't start when it's cold ... if you don't 
pull the choke.

What happens when you pull the choke: the entry (toward the air
filter side) is partially masked. This reduces the amount of air
entering into the carb and increases the depression. This results in
a "richer" air-fuel mixture which will compensate for the condensation of
fuel in the cold pipes.

This was for the theory. Practically, you have to define yourself
which combination of choke/throttle position is best. My AT starts
immediately (when cold) with the choke level at mid-range and a
very short opening for the throttle (turn right hand a few 
millimeters!!).

Usefull tip:
if the cold engine don't start and you smell the fuel 
(caused by non-burned fuel coming from the exhaust), that means that 
the mix is too rich => decrease choke and increase throttle.

If you experience such a smell with a hot engine which does not want to
start: open the throttle at max. at let the starter run for 20~30 seconds.
NEVER USE CHOKE
WITH HOT ENGINE.

To summarize, here are the causes for non starting cold engine:
- bad procedure (see above);
- poor battery condition;
- poor engine condition (carbs, sparks, air filter, ignition ...).
- humidity which causes loss of current in the high voltage circuit.
- cold oil (thicker => engine harder to run).

Hope this help,
Pierre



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