I've thought about this myself, given that my new amazon is the second SU carb volvo I've owned, I've decided to figure the things out. The decriptions and maintenace manuals decribe them as very simple (compared to multi-barrel carbs) and as suggested eariler, if you can rebuild your engine, these shouldn't be too hard.

A strike against the weber has been mentioned at either swem or vclassics. I think the later. It amounts to simple geometry. The four pipes on the intake manifold are of different lengths and no matter what you do, you can't compensate for that. The carb will always be pushing air and fuel through different lengths of pipe and the result is an inherently inefficient system. I've found this arguement to be the most compelling. Perhaps it is only a minor issue, but it adds up to loss in efficiency at the least.

The volvo owner who sold me my 122 and having more than 40 years of volvo experience told me that webers are nice if you want to tune it regularly. For the most rugged and reliable stick with your SUs.

Also from what I've read, synching the carbs is a simple matter of getting the "hiss" identical between the two. Failing auditory skills, you can buy the $30.00 carb synch tool from IPD which will give you the exact airflow in each carb.

The SU carbs rebuld kit is a tad pricey, however...