Since there's been a lot of interest here lately comparing the SU's and
the Weber/Solex configurations, I thought it might be worthwhile to
"reprint" a comparison of the two types posted by Marc Sayer. The
"quoted" section is part of the post that prompted Marc's response.
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> well. I give it 1/4 throttle and it revs slowly then when its around
> 3-4K I can gradually mash it , it sputters , then goes like a raped ape.
> If I just mash it , its sputters and pops through the carbs, then slowly
> increases in rrr's then starts to climb- still sputtering, then at 4-5k
> starts to take off. Its like someone's kinking the fuel lines then lets
> it go. I have 155 mains 190 air, F2 emulsion tubes, 35 choke tubes,
> idles are 55f9. Are the mains too big? It ran good with 120 main and 170
> air, however it was way too lean.
Your main problem is that the chokes are way too big. You will want to
run 28 to 32mm chokes. The smaller choke will provide better bottom end
and throttle response (which, by your description, is what you are sorely
lacking at this point). Do not get sucked into this bigger is better BS.
The right size is better, and in fact that is almost always smaller than
you would think, at least for a street car. Start with the smallest size
you can get for the 45's which is probably a set of 28mm's, though they
may be hard to find. You would think that a pair of 45's would flow
better than a pair of 40's, however that is not true, especially at the
smaller choke sizes. It is much harder to get 45's to work on too small
an engine than it is to get 40's to work on too big an engine, at least
for the street.
The 40's will outflow the 45's in all choke sizes up to about 33mm at
which point max flow is about 170cfm. This is when the 45's will equal
the 40's for flow. In larger choke sizes than that, the 45's flow better
than the 40's, as you would expect. However with 30mm chokes the 40's
will flow about 160cfm whereas the 45's only flow about 137cfm. All these
flow figures are for a single carb throat (not for both throats of the
carb). When you compare this to the flow of a 1.5" SU, which is about
142cfm you can see that while the 40's offer s slight flow improvement
over the SU's, the 45's actually flow less than the SU's at these smaller
choke sizes. I have talked about this before on the list and it is a hard
idea for many folks to get their minds around. When talking about the L4
series engine, which has no overlapping intake cycles, you must look at
the size of the carb based on what size a single cylinder is fed by, and
not the total combined carb size.
The reason for this is simple enough, since only one cylinder is drawing
in air at any given time, it is making exclusive use of the carb (or
throat) that is feeding it at that point in time. In an SU setup, this
means that any given cylinder has exclusive use of the SU that feeds its
bank of cylinders, during its intake cycle. However it can not draw from
the SU that feeds the other bank of cylinders. In other words you can
think of the stock dual 1.5" SU set up as if it were a four carb setup,
with each cylinder being fed by its own 1.5" SU. This would not be true
if there were a time when more than one cylinder, fed by the same carb,
were on the intake stroke at the same time. In that case you would need
to divide the carb size by the number of cylinders drawing on it, to find
the effective carb size.
However on the dual Weber setup, where each carb throat feeds a single
cylinder, the effective carb size is in fact the size of a single
throttle bore. You can not combine both throttle bores of one carb, or
combine the four throttle bores of both carbs, as is often done in
American V8 four barrels, because each throttle bore has its own discrete
manifold runner and feeds only one cylinder. That cylinder can not draw
from the other throttle bore or the other carb, so as far as it is
concerned, they do not exist. This would not be true if all the carbs fed
into a large plenum, which then fed to the cylinders, but there is no way
to make such a system work properly. So despite the urge to do as
American 4 barrel carb makers do, and combine the flow of all 4 throats
of a Weber dual set up, you really need to think of the duals in terms of
the size (or flow) of a single throat. And despite the fact that there
are only two SU's and that each one has only one throat, you still need
to think of the SU's in terms of the size of a single throat. This is
because that is what a single cylinder can draw through during its intake
cycle.
In this light you can see that dual 38mm SU's would be about the same as
dual 38mm Webers despite the fact that there are two fewer throats in the
dual SU setup, and dual 44mm SU's would be about the same as dual 44/45mm
Weber or Mikuni's, again despite the fact that the SU's have two fewer
throats. However due to their variable choke size, the SU's actually flow
better when you factor in the small fixed choke size required to give the
Weber/Mikuni's good drivability and throttle response for the street.
This is why, for the street, most folks prefer the SU's and for the track
(where drivability is not a factor and only top end power production is
of concern) folks tend to prefer the Weber/Mikuni carbs.
The advantage of the dual weber/Mikuni setup over the dual SU's is not so
much about increased flow (though for racing that can be a factor) as it
is about better distribution. With the one throat per cylinder
arrangement, each cylinder can be fed the same mixture and each cylinder
can be tuned individually. With the one throat per two cylinders
arrangement of the SU's, any tuning of the carb for one cylinder will
also affect the tuning of that carb for the other cylinder in the bank.
Plus the manifolding required to feed two cylinders from the same carb
will almost certainly introduce an imbalance in the distribution of fuel
between the two cylinders in that bank.
To set up your carbs correctly, you will need the following, 1) good air
horns (not the stock Weber type but some of the good shorty types that
TWM sells with the good radius - length is irrelevant on this, its just
the shape of the radiused entry that matters and how the horns will work
inside the air filters) 2) good air filters (K&N's are all I would
recommend - the British made K&N's are better than the American made
versions - contact APT for the filters) 3)a recurved EI distributor set
to about 20 deg. BTDC at idle and for about 34 deg total at about 3500
rpm 4)a manifold with a balance tube that connects all four runners
together (not for the purposes of creating a plenum as I mentioned above
but to act as a damper on the pressure waves that are created in the
manifold as the intake valve opens and closes. Without these four items
you will spend the rest of your life trying to make the carbs run right
on the street.
FWIW, Ron
________________
Ronnie Day
rday@cyberramp.net
Dallas/Ft. Worth
'71 510 2-dr (Prepared class autocrosser)
'73 510 2-dr (Street Toy)
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