Re: Manual choke

[jlandry@domain.elided (John A. Landry)]

The aliens commander decided laura busch <lauramarie_b@domain.elided> would
make a perfect specimen for dissection, and he yelled...

>I was wondering if anyone could let me know some of the finer points
>of driving a manual choke.  And heck, while at it, what are some of
>the basics?

Laura, 

My experience has been that each carburetor equipped with a manual has it's
own certain "way" that works best when starting it.  Each one will be a
little different, and you'll sort of learn through trial and error the best
way to use it.  But here are some of my pointers that may help you get
started...

For starters, if you don't understand how the choke works, I'll briefly
touch on that.  When you pull the choke knob out, you cause a small "plate"
to close over the opening of the carb.  This has the effect of restricting
the air entering the carb, which makes the air / fuel mixture richer (more
fuel).  A cold engine needs a richer air / fuel mixture to start and run.
Also, when the choke plate is closed, a cam in the carburetor's throttle
linkage acts to open the throttle plates slightly so the engine idle speed
will be increased.  So as the more choke is applied, the engine rpm will
increase in unison.

Tips:

1. Obviously, the colder the engine temperature, the more choke application
and resulting richer air / fuel mixture will be needed (pull the knob
farther out).  A warm engine restart usually requires no choke at all.  A
cold engine on a warm day will likely need just a little choke, but not for
very long.

2. Depending on how it's adjusted, pulling the choke knob out too far will
likely prevent the engine from starting altogether, as this prevents the
engine from getting enough air.

3. Once the engine has started, as it warms the engine rpm will continue to
increase.  The choke is pushed in in little increments to offset and reduce
the idle rpm until it's no longer needed.

4. The usual goal is to get the choke all the way as soon as practical to
avoid wasting gas, creating excess pollution and causing possibly
accelerated cylinder wall wear from the effect of the extra fuel washing
the oil away.  As soon as the engine is running smooth enough to idle
normally, the chokes needs to be off.  The choke is not a throttle lock and
really shouldn't be use to increase the and hold the engine rpm once the
engine doesn't need the richer air / fuel mixture.

In my 500cfm Edelbrock equipped 345, I have found the following choke
technique works best for starting cold:  I initially push the throttle all
the way to the floor to get a good squirt of fuel into the intake from the
carburetor's accelerator pump.  While momentarily holding the throttle to
the floor, I pull the choke knob all the way out.  It's easier to pull the
choke out while the throttle is held down because it unloads the fast idle
cam on the carburetor linkage.  I release the throttle completely and push
it back down about halfway.  I then turn the starter over while at the same
time immediately pushing the choke in about to the halfway point.  The
engine will immediately start and I adjust the choke knob in small
increments to get a good fast idle, depending on the outside air
temperature.  As I mentioned earlier, my engine will not start if the choke
is held all the way out.  By pulling it all the way out initially and then
quickly pushing it back to the halfway point, it seems to help the engine
get an initial burst of rich air / fuel mixture which aids starting.

It all seems complicated in writing, but it only takes a second or two and
comes completely natural to me.  I have a motorcycle which likes the choke
to be applied in a completely different manner.  So you'll just have to
experiment until you happen upon a procedure which seems to work best.  Let
me know if you have any other questions and I'll do my best to help.

Have fun,

John L.
-------------------------------------------------------------------------- 
jlandry AT halcyon DOT com      | 
Conservative Libertarian        |  "The road to  tyranny, we must never
Life Member of the NRA          |   forget, begins with the destruction
WA Arms Collectors              |   of the truth."
Commercial Helicopter - Inst.   |                     William J. Clinton
http://www.halcyon.com/jlandry/ |    10-15-95, speech at the Univ. of CT