On 10 Jun 2007 at 18:27, Roger Elliott wrote:
> I can't get it to run quite right...On the front carb the fuel
> is level - or maybe just below with the top of the jet. On
> the back one the fuel level is a lot lower.
>...
> Maybe I have gone about this wrong and the problem is the rear
> carb - When I lift the air piston of the front carb the engine
> speed stays about the same maybe a small increase in rpm. When
> I lift the rear one the engine slows down and almost dies.
This sounds like some of what I went through trying to dial in the
SUs that a PO mounted on my GT6. I got some advice from this list
and some from lots of experimenting.
First, the fuel levels in the bowls don't have to be the same. In
fact, if the engine slants (can't remember if my Spitfire's does but
the GT6's sure does) one carb float bowl will be higher than the jet
and the other carb's bowl will be lower. You can tweak the float
levels a little if you want but the suction of the air in the venturi
and the jet opening size are so much more important for the mixture
that it doesn't matter.
The real problem is this. That whole lift-the-pin trick doesn't work
until both mixtures are about right *and* the carbs are balanced at
idle. (Balancing them at speed is a different matter.) Most likely
your rear carb is providing more of the engine speed, so when you
take the front carb effectively offline the engine still runs about
the same. Another possibility is that the front carb is too rich or
the rear too lean, or some combination of mixture good or bad and
unbalanced idle. You have a crossfeed tube on the manifold, right?
The best approach is keep turning down the idle screws but try to get
them equally sensitive to a minor tweak. Then iterate in on the
mixtures. It's an art. When you get them close, it will start to do
what it is supposed to.
Of course, another possibility is that one or both throttle shafts
are so worn that they are leaking air into the carb. If so you'll
never be able to get them really right.
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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