>Bill Eastman writes:
>
>> is that they do
>> not lean out enough at cruise. They respond to the velocity of the air
>> across the jet bridge such that a given velocity of air gets a given amount
>> of fuel.
Etc., etc.
The SU carb is called a constant depression or constant velocity type
carburettor for a reason. Unlike a Holley or Weber, there is always the
SAME airflow velocity going across the jet bridge. As the piston rises, the
VOLUME of air increases, but since the aperture is larger, the airspeed
stays the same. And as the piston rises the amount of fuel entering the
chamber increases due to the smaller of the portion of the needle blocking
the jet.
It took me about six months of half-heartedly studying these carbs to
realize how they work and then it hit me how idiot-proof they really are.
As long as they are properly tuned, you can't flood them and barring wear,
they don't change. IOW, if your car suddenly starts acting funny, IT'S NOT
THE CARBS. Check everything else thouroughly, but but give the carbs a
quick check and leave them until last. I say give them a quick check
because I once had a B-A-A-D idle problem develope on a trip from Nashville
to Oxford, Mississippi and I couldn't figure out what the hell it was until
I finally pulled the air cleaners off and discovered that when I'd finished
the spring checkup right before the trip and put the air cleaners back on,
I didn't make sure the distance tube was on the mounting bolt and
discovered it lodged under the piston. But the car ran fine at cruise...
You might, if you really want to check out another carburettor, look at the
HS4. I've used both the HIF4 and HS4 on the 69, and find that the HS series
has a wider adjustment range before resorting to swapping needles. HS4's
will allow manifold porting and are about as simple as you can get. The
HIF's do however have the advantage (IMO) of the integral float bowl and,
if you're picky, automatic temperature adjustment, so you don't have to
retune in the spring and fall.
Damn. It IS that time, isn't it?
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No one in Tuna knows what I do for a livivng.
Glenn Schnittke I'm cured. Leave me alone.
Nashville TN 615-385-2800
schnittke@mindspring.com
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