Carburetors – SU vs. Weber

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.