Sounds to me like the needle valve is sticking open (unless you
have Grosse (sp?) Jets). If it was pump pressure you'd probably
have problems with both carbs. Even though it sounds like you've
replaced them, there may be something else wrong with the lever
mechanism).
Wish I could recommend Grosse Jets. I have them in my BJ8
and they have performed flawlessly for many years; however, I've
heard newer ones may have quality issues (fixed?).
On a side note, this brought up an interesting theoretical question:
how much pressure can an SU pump develop? Pumps known as
"positive displacement" will continue to increase output pressure
as long as they run (or until something explodes ;). These are
usually piston-type, like most air compressor pumps. Centrifugal
pumps--like a turbo--can only increase pressure to a point before
they "stall" (generally, turbos can provide more boost than the
engine can handle , so the boost is regulated by allowing the exhaust
gasses to bypass the turbine section).
Thinking about it, the SU pump uses a spring to apply the pressure.
The point (or solid state) mechanism uses a coil to retract the
diaphragm and compress the spring, then cuts the current to the
coil allowing the spring to push the diaphragm and hence the fuel.
Therefore, the pump can only develop as much pressure as the
spring can provide. Even if you had a "runaway" pump--assuming
no leaks in the fuel system--when the pressure in the system
equalled the force of the spring the pump would cease to pump.
That is to say, an SU fuel pump can increase the volume of pumped
fuel by pumping faster, but not the pressure.
I believe a fuel pressure regulator is only required when you
have a pump that is capable of developing greater head pressure
than the system requires and you need to limit this pressure
(the advantage being the pump has "head room" and you should
never suffer from insufficient fuel delivery). You could theoretically
increase the pressure an SU pump supplies by installing a
stiffer diaphragm spring, but I can't think of a practical reason to
do that (you'd probably burn out the pump even faster ... and have
leaks from the float bowls--unless you installed a pressure regulator).
bs
***************************************************************
Bob Spidell San Jose, CA bspidell@comcast.net
'67 Austin-Healey 3000 '56 Austin-Healey 100M
***************************************************************
Subject: tricarbs
>I have been having trouble with float overflow. Carbs were all re-built
> and functioned fine for several months. Vehicle sat for about a month,
> when re-started float overflow on rear carbs. Disassembled rear float
> chambers, cleaned and reduced float drop level, carbs functioned fine in
> garage with car up to op. temp. went for a ride and one of rear floats
> started to overflow again. Walked away some what irritated and left car
> for about a week, re started and no leaks. I have not had a chance to
> take a drive yet. A idea why on the road driving might cause the bowls
> to overflow? Are they that sensitive that a little bumping around will
> cause a problem, or is there something I am missing. Replaced everything
> but the floats as they appeared not to leak, and have never found
> evidence of fuel in the floats. Pump has also been rebuilt, is pump
> pressure a issue Kurt
|