Looking at carb stuff, and the ulterative of twin SU carbs has arisen to me, especially because they are a ####LOAD cheaper! Right?.
The question is how do they compare to webers, both downdraught&
sidedraught??

I come across alot of comments about SU falling out of tune too. How
true's this?

Also could you use datsun manifolds (I'm talking siddraft here) on a
corolla head?? Or even a datsun L motor's manifold on K head?? Any ideas?

What price are a set of SU's?

I've got twin SU's on my 1200 - check my webpage for details. The car is still under restoration and has never been on the road so I can't comment about their performance as yet - but I've got experience with them in other cars.

Mine cost me $250 for a full supply of s/hand carbs recondition/rebuild etc and creation of custom linkages to fit the datsun manifold.

There is a wreckers near me in Hobart that has a manifold and set of 1 3/4" SU's to suit a 1600 for about AU$150 for the lot.

I've ran SU's for years on Triumphs and have no real complaints. They can be tuned for good power and give great fuel economy with a nice compromise of pickup and smoothness. They won't give the out and out power of the Webers or will they sound as blarty but they're fun.

The problem is that it all comes down to having the right needle profile and running the right damper springs and damper oil. To get this right you need a lot of skilled tuning and or an air/fuel meter in your car and the patience to carefully modify your needles in a drill press with sandpaper. Otherwise you're just getting a needle that works average in the average car - not great - not bad.

Finding the right needle by trial and error is like look for a needle in a haystack.

Falling out of tune????? Nargh - but you need to make sure that they are in good order before you use them otherwise poor fitting butterflies etc will lead to lumpy idle etc. Once set you can just do routine monthly checks of the damper oil level and the mixture fine tuning (which is done by lifting a test pin lightly and checking for changes in engine speed) - piece of piss.

The two most biggest differences are:

Weber - twin choke carb
SUs - single choke carb

Weber - traditional butterfly valve carb
SU - sliding venturi carb

So two SUs is like a single Weber. Two webers is like four SUs. Kind of.

had 42mm webers, chokes were too big and it ran like crap under 3500 (on my a12)

then changed to SU's and they gave much better all round driving, the top end power wasnt the same, but i saved shiet loads on fuel

also have a a-15 with a 32/36 downdraught weber, it is very good for round town, but at a guess trying to compare a-12 to a-15 the SU's would be better

i have my carbs done by mechanic, but i do not have a complaint about the SU's, i would say in my experience the 32/36 (on mine and friends cars) requires more attention than SU's as the 32/36 always seem to fall out of tune and need diaphrams replaced and stuff like that even after full rebuilds

i didnt have the twin webers on long and i had them rebuilt b4 i used em so never had any problems with them

Yes the number of 'chokes' is a big difference - but a good manifold will help smooth things out there.

Obvioulsy it's nice to get as many openings direct to the ports as possible. My old 2500 Triumph uses 6 butterflies with 6 individual inlet runners per cylinder - it's sweet. - but an absolute bitch to keep all those throttle synchronised.

Let's not forget that SU's and Webers are a very different design in principle. My understanding is that the Webers are a fixed jet / fixed choke or something like that - whereas SU's are constantly variable choke due to the design of the piston in the carb moving up or down as required to change the opening to the mouth of the carb - etc.

I've heard other people say that Webers are great if properly tuned and setup by and expert - they can almost equal fuel injection in performance (outright H.P figures) but it seems that unless perfectly setup the drivability of a car over the whole rev range can be ####. SU's reliably deliver comfortable and smooth power all over the rev range as by design they seem to work with the requirements of the motor for any given air flow / manifold vacuum etc. I'd go for Su's if I wanted something safe - fairly easy to drive and reliable smooth power. If I wanted all out power and the motor had the ability to breath properly (head work, cam bumpy, good extractors) then I'd go the webers and spend the time setting them up.