You have a conversion kit made by Cannon Manifolds (Pierce is another common system, same idea). These conversions have been popular for years to replace SU, Hirachi-SU, and Zenith-Stromberg carbs on Z-cars as well as most popular british engines. I personally have a pretty low opinion of them, but oters may feel differently. Note that I have a high opinion of the Weber 32/36 DGV carb.
Here's my take. The kits first were marketed to the Triumph TR-6 crowd as a "replacement" for the often maligned Zenith Stromberg carbs which many saw as "smog control" mostly because the rest of the world got mechanical fuel injection. The injected engines made about 50 more horsepower. Why? The US spec carm engines had 7:1 compression, the home market had 10:1, a far hotter cam, and a MUCH more aggressive timing curve. The injection system itself was maybe worth 10 HP and easily cost 5 mpg. The Hitatchi made SU's on early Z cars were looked on negatively for most of the same reasons. Now, by the early 80's these factory carbs and distributors were beginning to wear out (or had been monkeyed with) and the cars performance/economy suffered. So Cannon, mostly through slick marketing, came up with a solution: "Dual Webers". Not the performance DCOE, IDA, or IDF carbs that Alfa, Porsche, Fiat and many race cars had been running since the 50's. But a simple off-the-shelf universal carb. Fine, fair enough. You can probably thank EMPI for starting the DGV craze in the 60's on aircooled VW's (to replace 1 bbl Solex carbs, which WAS an upgrade). Back then, most "car guys" had some experience tweaking VW's, so if it made their VW run better, why not their Triumph/MG/240Z/Datsun 510/whatever? It typically made them run "like new again". Not because the DGV had any more performance potential than an SU, but because it wasnt worn out or hopelessly screwed up. A new Mallory or Bosch distributor helped a lot too (probably more than any carb could). Was the car faster? Maybe. Also, SU and Zenith-Stromberg carbs parts were getting hard to come by.
On my last TR-6 I had a similar setup for a while. Ran OK. Put a set of rebuilt Zenith-Strombergs on it and it ran as good, or better. Put a 3rd Stromberg on it and it ran far better (by this time I had a far warmer cam, 10.5 compression, headers, aluminum flywheel, and Mallory dist). Sold the DGV's to a guy who had worn out Strombergs (and didn't know sh*t about cars) and he still runs them today.
I guess what I am trying to say is, the DGV's are really no better or worse performance wist on a stock engine. Those adaptor manifolds WILL hurt you eventually at higher RPM, if you start modifying the engine. Like a 1 3/4" SU, they will support abour 50-75 HP per carb. If it were mine, and the DGV's were in good shape I'd just run them. I'd even clean/kit them if need be. If you are planning to really increse the power of this car, save up fior a set of triple DCOE's! Check around the Z-car forums, probably get a similar response.
