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The original SU's don't hold the motor back until you've done a decent amount of work increasing the airflow. They are pretty good at being adaptable with some different needles, springs, etc. But really, at some point, they start to be come the main limiting factor on the motor.
About 4 years ago or so I replaced the SU's on the PV's motor (2130cc, R-sport head, roller rockers, Isky VV81 cam, headers & 2.25" exh., etc, etc) with a pair of DCOE's. Probably a size too small (DOCE 40's with 36mm chokes) but the price was right - $357 shipped with a Volvo manifold and linkage, on fl-eBay. After a bit of tinkering (had to disassemble them completely to clean out all the little fuel passages) - WOW! The old motor just never, ever seemed to work as hard with the SU's as it did with the DCOE. Power is now instantaneous when you put the pedal down. And it doesn't taper off at higher RPM's either, although that is a combination of the head, cam, and exhaust system the SU's just couldn't keep up at high rpm.
I stuck a wideband oxygen sensor on the car for a few months to help tweak them. Tuning them raises one important issue - there are practically no adjustments on the carbs. I think Dellorto's are similar in this respect. There is an idle speed adjustment (cracks open the throttle valves) and an idle mixture adjustment (which does nothing unless at idle or *very* near idle). That's pretty much it, although you can mess around very slightly with float bowl levels. Anything else involves buying expensive little machined jets and venturies from places that don't accept returns. And each change involves a hunch about the way the carb will react, which may or may not be true. The great strenghth of these carbs is that everything is changeable, and that they can adapt to anything from a Harley to a Merlin airplane motor. But that's also their weakness too, if you don't know what you are doing you can really wander into the woods and get lost. Probably well worth taking it to a shop that has experience in them and probably has a treasure chest full of jets, air correctors, pumps, and venturis and doesn't have to pay for each experiment. And has experience in doing it too so they go up fewer dead ends.
In my case I was pretty lucky, the carbs came jetted for a B18, but the original jets seemed to work really well on my motor once I changed the venturi's from 32 to 36, and lowered the fuel levels slightly. But you have to approach changes to the carbs in a certain sequence, some changes (like the venturi selection) can change the effects of almost all the other variables, so you have to settle on that first before changing anything else.
In my case it's made the PV pretty thirsty. I probably average just under 20 mpg for mostly city driving. But I usually drive it only short distances and tend to drive it pretty hard (it's really fun). If you have more money and time it's probably possible to tune them to have good economy - leaner at cruise, smaller 'idle' (really idle and up to mid-throttle) jets. But still have good WOT performance - larger main jets. Although in my case they seem to get a bit cranky if the mixture gets anywhere leaner than about 13 - 13.5:1. They seem really happy at around 12 - 12.5:1. And the wideband shows them to be a bit erratic and random at times at doling out the fuel. Bumps and left hand corners seem to make them spike rich. I never had the wideband on with the SU's so I can't compare and contrast.
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'63 PV544 rat rod, '93 Classic 245 + turbo
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