Are two carburetors better than one?
I have a 1977 MGB with a single ZenithStromberg 175 CD-2 carburetor. It runs well with that setup, but I am considering replacing the single carb with a double carb setup I got cheap from a dismantled Jaguar of similar vintage. These carbs are also Stromberg 175 CD-2s, complete with linkage. I would like to know if this new carb arrangement would result in an appreciable increase in engine power and performance, and if there are any major hurdles of which I should be aware.
Those Zenith Stromberg CD-2 carbs come in various throat sizes from ¾”, 1 1/4”, 1 1/2” to 1 ¾, though from my literature it appears that the stock Jags imported to the U.S. for 1977 used two Zenith Stromberg 175 CDSE units. At any rate, because your MGB’s engine is less than two liters in displacement, and the Jag XJ6 is 3.8 to 4.2 liters, you will be over-carbureting your MG by adding two bigger CD-2 or 175 CDSE carbs from a Jag.
To begin with, the stock CD-2 that is on the car is entirely adequate for all but racing or performance applications if nothing else is done to the engine. The only time your engine would run out of breathing capacity as it was equipped from the factory would be at high rpm…beyond where most people would drive the car in normal circumstances.
An engine of a given displacement can only consume so much fuel/air mixture, and to go very far beyond the stock carburetion would cause the carbs to run too lean at low rpm and have a mushy stumbling mid-range response. Also, I doubt that the Jag throttle linkage will work without adaptation. The XJ6 is an entirely different engine.
Going to two SUs the way the earlier MG engines were set up does have some advantages in that the carbs have a more direct and shorter inlet path into the engine and will give you better performance in the high rpm range. The slight disadvantage being that you may have to tinker with the needle jets and balance the twin carbs to work together.
However, in states with strict smog laws such as California, your altered MG wouldn’t pass smog inspection. The MGs of the era imported to the U.S. were equipped with single Zenith Strombergs to meet American pollution standards. That is also why the Jags went to two of them, and fuel injection soon thereafter.
All of that being said though, if you port the head and match the intake and exhaust manifolds to the head for maximum flow, then add optimum size valves and do a three-way cut for the valve seats in addition to adding the extra carb, you will see a major difference in performance. A freeflowing exhaust header and hotter cam will make a big difference at higher rpm too. However just adding an extra carb alone— even of the optimal size—would entail going to a lot of trouble for not much gain.