[TR] carb fuel 'mist' confusion

Randall tr3driver at ca.rr.com
Fri Sep 21 22:35:00 MDT 2007


> Seemingly, if the
> carb's needle is not centered then, at the jet, it still has 
> the same surface
> area of gasoline exposed to the suction, right? 

That's actually right, Paul.   It's important that the needles be centered
for a different reason : if the needle rubs against the side of the jet, the
resulting friction will affect the smooth motion of the piston.  This in
turn causes the venturi size to not be accurately controlled, which means
the velocity through the venturi will also be wrong, and thus the depression
that sucks fuel out of the jet will be wrong.  So, any rubbing will upset
mixture not because the opening is off to one side or the other, but because
both the opening and the depression/vacuum operating on the opening will be
upset.

There is another reason that needle centering is important, which is that if
the needle rubs against the jet at all, it will wear both the needle and
jet.  (They are both brass and fairly soft, plus brass on brass does not
make a nearly as good a bearing as brass on steel.)  The wear will also
upset the mixture in a particularly bad way, by making it richer at idle.
Then when you adjust the idle mixture by the book, the cruise mixture will
be too lean.  I believe this is the source of at least some persistent
overheating problems; at least it was the problem that my Dad's TR3A had
(and which eventually led to the engine swallowing a valve, IMO).

My friend's 59 MGA jets were so badly worn that he could not get the mixture
lean enough at idle even with the mixture nuts all the way up.

Randall


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