<VV> Carb mystery

[Hugo Miller]

I don't have a particular problem with the carb on the Corvair. They 
are no worse than most others out there. But if you are used to a carb 
with one needle and one jet, and no transitions from one jet to another 
as you open the throttle, and no accelerator pump to compensate for the 
fact that the airflow speeds up more rapidly than the fuels flow, and 
all the other gubbins that most other carbs have to compensate for the 
problems that are inherent in building a functioning carburettor, if you 
are used to a carb which is free of all these things, then they all look 
hideously complicated.
There may be only one removable jet on the Corvair carb (and I couldn't 
get mine out of course) but I found lots of things with holes in them 
that squirt fuel in various directions. I never found out what was wrong 
with mine - I just soaked it in carburettor cleaner overnight and it was 
fine.
And the method of adjusting the accelerator pump, if I recall, is to 
bend the operating rod.
The most egregious thing I found was a hole somewhere near the base of 
the carb that was covered by a rubber bung which was attached to some 
sort of lever. I have no idea what its function was, but it struck me as 
a proper Heath-Robinson arrangement which was just asking to give 
trouble (Heath-Robinson may be a British in-joke, I'm not sure).
But my affection for SU carbs is not 'regional', I assure you. They do 
a better job without all the level of complication that all other 
carburettors deem necessary - with one exception; the SU equalises fuel 
flow and air flow by slowing down the air by damping the piston. Most 
other carbs have accelerator pumps which, instead of slowing down the 
air, speed up the fuel. These will give a snappier throttle response, 
although you'd be hard put to detect it.
An SU has only one size of jet. You alter the mixture throughout the 
range by screwing the jet up and down. If you want to make it relatively 
richer or leaner at a certain point in the range, you swap over the 
needle, for one of the gazillion needles they list for every conceivable 
purpose. But SU specify a needle for every vehicle. Obviously not for 
the Corvair, so when I get round to fitting mine, I shall start with an 
educated guess - probably a BEV needle, as that is what I have in my 
Morris Minor with the same carb. I know that will work, then I'll tweak 
it from there. I even fitted an identical 1.75" SU to my Harley & took 
off the fuel injection, purely because with injection, I am powerless to 
do anything if it breaks down on the road and for no other reason. That 
of course needed a different needle, partly due to the irregular power 
stroke on a V twin.
I believe there was a maxim in aircraft design to "Simplicate and add 
lightness". Well, the SU certainly simplicates the whole business of 
mixing fuel and air and feeding it into an engine. I firmly believe that 
simplicity is a virtue when it comes to cars in general and carburettors 
in particular.

On 2020-03-21 20:32, Smitty via VirtualVairs wrote:
> Smitty says.     As one who has been playing with the Corvair and HV
> carbs for around 50 years, I am at a bit of a loss in understanding 
> Mr
> Millers problem with them.  I can understand his regional pride in
> defending SU carbs even though I do not remember anyone attacking
> them.  Personally I would rather use a carb with the ability to
> compensate for use on various similar engines than to go through
> millions of metering valve rods and jets to tailor a carb for one
> specific use.  I am not going to go out and disassemble an HV just to
> verify my memory, but it seems to me there is only one jet in each
> carb.  The main jet.  Yes there are transition slots and a cluster
> from which fuel can be drawn as needed, but in a clean, properly set
> up carb they work remarkably well.  No multitude of jets.
>
> Hugo says
>> A main jet that is big enough to allow sufficient fuel for
>> full-throttle operation is going to be too big to atomise the fuel 
>> at
>> idle. So we have half a dozen different jets which all do different
>> things. And they all have to transition smoothly.
>> The beauty of the SU is that it completely eliminates both of these
>> problems at a stroke by making both the jet and the venturi variable 
>> in
>> size, so the mixture is stable throughout the range. Such a simple
>> solution to a complex problem.
>>
>> The SU is also the nearest thing to a programmable carburettor. You
>> don't need a lap-top either. They make millions of different needles
>> with all possible variations of taper, so you can tweak the mixture
>> strength.
>
>
>
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