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A new carburettor synchronizing theory

Subject: A new carburettor synchronizing theory
From: F.vanDalen@LR.TUDelft.NL (F. van Dalen)
Date: Wed, 4 Oct 1995 11:17:17 +0100
After having put oversize spindles in my SU carbs (allow me expand a little: my
local MG mechanic showed me how to use HIS reamer on one carb, let me do the 
other, gave me coffee, checked whether I hadn't mounted the carbs upside down,
noted that my choke linkages were assembled incorrectly, got me another carb to
serve as example, warned me I was there to work instead of talk to the other
customers, let me use his workshop facilities and tools, got me more coffee and
all for the price of two carburettor spindles. What a guy! Off course, that is
after a history of handing over all my savings to him in exchange for spare 
parts - where was I?) anyway, with the new spindles in, I've decided to put in
new jets and needles this weekend. That, as we all know, involves synchronizing
the carburettors once again.

For this purpose I keep a length of fuel pipe in my tool kit. It seems to me 
though that the noise you get from the tube is more sensitive to where you hold
it exactly w.r.t. the carb mouth, and to how hard you poke it in your ear, than
to the airflow through the carb. My idea is however, that the carbs should be
perfectly synchronized when both carb pistons are raised by an equal amount, as 
long as compression on # 1 and 2 matches # 3 and 4. All pistons move at the same
speed, sucking in the same amount of air (neglecting differences in blow-by), 
and when the throat cross-section is the same on both carbs, the air flow speed
and therefore the static pressure has to be the same and synchronization has
been achieved. This would imply that the carbs can be synchronized visually, 
which I would find a lot easier. Anyone care to comment on this theory? If 
visual synchronization works, why does everybody recommend this silly tube? Is 
there some practical joker on the Haynes payroll?

Thanking you for your time,

Frank van Dalen '69 B

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