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Tony Hwang

wrote:
On Mon, 18 Jun 2012 15:01:48 -0500, dpb wrote:

On 6/18/2012 2:22 PM, TimR wrote:
On Sunday, June 17, 2012 3:32:36 PM UTC-4, (unknown) wrote:

What I can't understand is what pulling the cord (without starting)
does that the starter motor doesn't ?? The only difference I can see
is that the cord revs the flywheel from the top and the starter motor
revs it from the flywheel teeth below - both in the same clockwise
direction.

Not sure that's the case. When you pull very slowly, there is no
resistance. You are clearly not pulling against compression. I'm not
sure what the mechanism is, but some speed is required to lock up
andspin the engine. Is there a centrifugal clutch, maybe?

I know that doesn't help with the diagnosis, but it might spell the
difference between the rope pull and the key start.

When pull very slowly valves don't close rapidly enough to build much
compression so don't notice it (much). That also gives somewhere for
the vapor to go if it is vapor lock.
I don't THINK you have a real case of vapour lock, since there is no
fuel pump. Fuel boiling out of the carb and "dry flooding" the engine
is a possibility (mixture too rich to light) and turning the engine
through slowly by hand COULD be moving enough air through without
drawing more fuel like it would with a fast enough crank to try to
start the engine.

Pulling the starter rope turns a "sprague clutch" that catches the
flywheel (using different mechanisms on different engine - ball
bearings in a race on some old Briggs engines, steel tabs on some
others - and likely several I've either forgotten or never seen) which
turns the motor. MOST small engines in the last 40 years have some
sort od "compession release" which either holds a valve open or closed
below a certain speed to allow the engine to be spun up to starting
speed easily. When up to speed the compression release goes off,
allowing full compression, which starts the engine.

The ONLY thing I can see hand cranking the engine slowly
acvcomplishing is diluting the charge in the intake and cyl by moving
air through the system.

Hmmm,
If it is too ruch(flooding), could smell it. No?