[Pigeon]

Since most braking effort comes from the front wheels, it isn't very important whether the rear wheels have discs or drums, and the standard drum setup is perfectly adequate on the rear.

What is worth swapping the rear axle for is a limited slip diff; this was an optional extra for the Amazon, but I think it was a standard on some 240s. Someone who knows the 240 range better than I do will have to provide the useful information about that, though.

Originally Posted by hazaa

hmmm strange, i always thote that single carb is easier to set that double and its more fuel economy ...

Any fixed-choke carb, as a Weber is, is fundamentally a conglomeration of bodges to compensate for the inherent inability of any one fixed-size choke and jet combination to provide a correct mixture over the whole of the operating range. Each bodge needs to be adjusted to give the correct mixture for the engine it's being used on over the particular part of the operating range that it is intended to cover. And the only way to adjust them is by swapping parts. So to perform the adjustment, you need for each bodge a range of different sizes of the relevant parts to experiment with - only one of which will be right, so you end up with a stack of sizes you don't need - access to a dyno to carry out the experiments, and the patience to take the carb apart, swap a part, put it back together, measure the difference it's made, over and over and over again, hoping that you manage to arrive at the correct combination - or at least close enough to correct - out of the several thousand possibilities before your head explodes.

If you happen to have the right combination fitted to the carb as supplied, then you're fine, but since Volvo themselves never fitted a Weber, there is no official guidance as to what is "right", and it's basically pot luck whether you do or not. You might be lucky, like Faust, but given the number of possibilities, the odds are against you...

The SU variable-choke design avoids the need for multiple bodges by automatically adjusting the choke and jet size to suit the airflow at any given moment. There is only one swappable part involved in getting it right - the needle. Again there are hundreds of different needle profiles, but this time it's not a problem, because Volvo have already done the work of selecting the best match.

It's also less critical with an SU. Most of the mixture control is handled by the variable-choke mechanism; the different needle profiles are basically fine adjustments to tweak something that's basically nearly right in the first place. Fiddling-with-engines type people often keep an SU around for testing whether some experimental engine basically runs or not, because you can bolt an SU onto more or less anything without setting it up at all and be confident that if it doesn't run it's not the carb that is causing the problem. (I put an SU HS2 off a Mini on my MZ TS250, still with the original Mini needle in it; the profile isn't exactly right, but it's not wrong enough to worry, and it's still an improvement over the original carb simply because it is designed to adjust itself to always be at its optimum operating point.)

The idea that twin carbs give worse fuel consumption than single carbs mainly comes, I think, from people fitting twin Webers or other fixed-choke carbs in place of a single carb as a performance upgrade. The result is that under the part-throttle light-load conditions which make up the vast majority of ordinary road driving, the fixed chokes are even further away from their optimum operating point than with a single carb, and fuel consumption suffers accordingly. (That any other performance modifications carried out at the same time probably also increase fuel consumption further confuses the matter.) Twin SUs, on the other hand, simply adjust their effective choke size to suit the conditions, and work just as well as they do in a single-carb installation; any increase in fuel consumption is mainly due to the driver exploiting the extra power available, not to deficiencies of the carburation.

As for overdrives - the question of what other makes of car used the same overdrive isn't really relevant. You need the correct gearbox and propshaft to go with the overdrive, and both of these are Volvo-specific parts, so basically what you're looking for is either a scrap car that had overdrive from the factory or the parts off one.

Originally Posted by Faust

I'd image the SUs would be better for highly tuned engines or at prolonged high speeds, but I'm willing to sacrifice a bit of top end for day to day driveability and ease of maintenance.

That comment is... bizarrely backwards! Top end is where SUs lose their advantage, everyday driving is where they have the most... that closed-loop system again. "Webers for track, SUs for road" is the usual maxim.

As for "ease", the idea that a carb (any type) constantly needs fiddling with is a fallacy. If it's in good condition to start with there's no reason why it shouldn't stay that way on its own. Yes, SUs do need the dashpot oil checking, but it's an utterly trivial task which takes about one minute with no tools, and which you do as part of the same routine as checking the engine oil and other fluids. I'm not entirely sure where the fallacy comes from, but I think it's largely because people mis-diagnose things like ignition faults and air leaks as being something wrong with the carb, and get very frustrated trying to fix them because they're not actually fixing the right thing so no wonder it doesn't work.

Difficulties or otherwise in syncing multiple carbs are not a function of the type of carb, but of the throttle linkage. Again, if the linkage is in good condition then once correctly set they ought to stay like that; if it's worn or sloppy, then you'll never get a good result no matter what.

And if you want to tweak the mixture to compensate for changes in altitude or climate (something which engine management systems do automatically, but carbs are generally just set to a compromise setting; a few types do try some automatic compensation, but the mechanism they use is usually so terrible that you're better off without it), HS-series SUs are unbeatable... just pop the bonnet and tweak that hex nut a flat one way or the other, with your fingers. No tools, no swapping parts.