The choice of carburation has really become confusing. Each carb. Type offers its own combination of advantages and disadvantages. To clarify some of the confusion, we will be running a test series using a street driven 1975 MGB as a guinea pig. There won’t be winners and losers, the idea is simply to give you a basis for comparison. We’ll do a mileage test, check 0-60 acceleration, give horsepower readings as measured by a dynomometer, and finish off with a subjective view of how each carb “feels” on the street.
As your mechanic for this adventure hates working on cars during the winter months, results won’t be seen until spring or summer (or whenever I get around to it). In the mean time, this bit attempts to deal with the essential differences between constant velocity and real carburetors.
SU and Weber carbs perform the same function, but go about it differently. The SU is a constant velocity carb. This means that the speed at which air flows over the jet never varies. The SU has one jet which must supply fuel during all stages of engine operation (i.e. idle, acceleration, cruise), despite the fact that the engine demands vastly different amounts of fuel and air for different situations. We’ll assume the SU’s operation is magical for the moment, but bear in mind that everything is served by one jet, and the only possible adjustment (normally) is to the idle mixture. There are alternative jet needles, but for any given car you have the choice of weak, normal, or rich. The normal needle is probably correct for 90% of all cars on the road.
Weber carbs need different jets for each stage of engine operation. Unlike the SU, the jets must be tailored to work individually and as a group. The SU varies airflow and jet diameter on its own to meet engine requirements. The Weber cannot do this, it must be set up in advance to meet expected demands.
These diagrams and descriptions point out important differences between the two types of carbs. For the sake of clarity, they overlook some specific details.
The speed at which air flows over the jet never varies, because the piston rises and falls, changing the area of the opening to match engine requirements (the piston rises as intake vacuum increases). The needle is tapered, so it allows more fuel to escape from the jet as it’s pulled up. The piston rises as air flow increases, fuel flow increases as the needle is pulled out of the jet, air/fuel balance is therefore maintained. Remember, air flows over the jet at roughly the same speed whether you’re at idle, or 120 mph. Good acceleration requires a rich mixture, but SUs have no accelerator pump. To compensate for this, the piston damper slows the piston’s rise when you step on the gas. This temporarily increases air velocity over the jet and draws more fuel, thus enriching the mixture. The effect is only temporary, returning to a leaner condition for cruise. A Colortune (Moss #386-210) can be used to check proper enrichment under acceleration, and leaning at cruise. This allows you to test the effect of various jet needles and different weight oils in the dash pots.
The speed at which air flows over the jets varies because the carb throat diameter is fixed. At idle, the throttle plate is almost closed. Air flow is insufficient to draw fuel from the MAIN JET, instead, fuel is drawn through the IDLE JET via an adjustable aperture. The mixture screw gives some room for adjustment, but the jet itself must be changed until the correct range is found.
As the throttle plate opens, air flow increases. Fuel is drawn from the IDLE JET aperture and a set of 3 PROGRESSION PORTS. Each port becomes active as it is uncovered by the throttle plate. At this point, all fuel is still being drawn through the IDLE JET. The MAIN JET will not become active until roughly 3,000 to 3,500 rpm.
Once air flow is sufficient to draw fuel from the MAIN JET, the IDLE circuit tapers off. From here on, the MAIN circuit is on its own. The trick to tuning these carbs is to get each circuit working well on its own and provide a smooth transition between circuits. To complicate matters, there is an acceleration circuit. The accelerator pump (not illustrated) squirts raw fuel into the ports when the throttle is first depressed. Unfortunately, the accelerator pump often tends to hide incorrect jetting by making things uniformly rich.
I would be lying if I said the Weber carb is easy to tune. There are five pieces which make up the various jets. Each of these is available in dozens of different sizes making for literally hundreds (thousands?) of possible combinations. I’m so confused at this point that I won’t even mention some of the other factors which affect tuning.
A few months ago I decided it was about time to take some of the mystery out of the Webers on my TR4. They ran great, but perhaps it was time to see if they could run great and use a tad less fuel. An hour or two with the manual and I was out screwing things up on my TR. I’m feeling a lot more comfortable around Webers these days.
The tech manual (Moss # 211-430 ) covers SU, Weber and Zenith-Stromberg carbs, and is a simple and inexpensive starter for fledgling wizard mechanics. As with most pursuits, the biggest barrier to overcome on the road to successful Weber tuning is fear of the unknown.
'Carburetors – SU vs. Weber' have 3 comments
March 3, 2014 @ 9:08 am bill miiller
I am considering buying an old e type jag and wondering how they preform at high altitude. we live at 8000′ elev with only 75% the amount of O2 in the air and most that I am seeing are at sea level. it would seem that the first thing to do would be to change the su jets to lean down the mix. are the jets readily available? would going to webers be a better solution. any other problems you forsee? many thanks in advance bill
March 15, 2014 @ 10:39 pm Charles
The term “Old E-Type” it rather open-ended.. How old is “old”?
(I have a 1953 Jaguar XK120 and do NOT consider it “old”!!)
It makes a difference with respect to the engine size and which
carbs are fitted.
Many E-Types were, by U.S. federal regs, fitted with the iconic
“Stranglebergs”.. an emissions-bred carb that are better suited
for trot-line weights. The first option would be fitting a pair of
SU Carbs.. if the “E” has the tri-carb set-up, then three SU H-6’s
will work perfectly well.. If racing, track-days, etc are in the
back of your mind, and fuel economy is not, H-8’s or HD-8’s
(sand-cast H-8’s if money is no object) would be another option.
The wonderful thing about the SU range of carbs is, there
are NO “Main Jets” to replace. The “jets” are fully adjustable
(on the side of the road, if necessary) for any altitude from the
Gulf Coast of Texas to “Wolf Creek Pass, up on the Great Divide”
(12,500…give or take a few feet!) The “Needles” are changeable,
depending on the requirements of the engine!
Other factors, depending on the age of the E-Type would be
if the engine is the 3.8L or the 4.2L (or worse, the Ser III V-12!)
Having messed with Webers (and being a professional mechanic
with TOTAL access to ALL the Jets and Needles required to adjust
these things), I would strongly suggest to avoid “building in” THAT
problem!!
Lastly, as with ANY “old” British car, the condition of the engine
factors hugely into how well it performs, it’s fuel mileage, throttle
response, etc. If the engine is “tired” at sea level, then being at 8000’AMSL
will make it run like a moped with a slipping belt!
In short, and as with ANY breed of vehicle, you need to do your homework
to determine if this Jag, or any vehicle, is suited to your needs and location as
is, or will it require extensive (and expensive) repairs to make it run like you
expect (want?) it to??
March 8, 2024 @ 9:27 am Barry Barnes
Please note that, in my point of view, it’s just fine to to use the term “old” when referring to a Jaguar E-Type. Who am I to condescendingly criticize you for accurately referring to a 50 to 63 year old car as old.