im going to try out these SU's ive got sitting in my shed instead of the webers, my mechanic has convinced me they are just as good is you maintain them, well see

there is a poblem with 42mm webers which i wasnt aware of till today, ill explain it more here later but its too early in the morning so ill do it later, basically they come off a V8 maranello or something.

Zoinks! I didn't know those were 42mm Webers! I'm surprised they worked at all.
I'll bet you'll be much happier with the SUs, until you get the Webers choked and jetted down to a size more suitable for an A-series.
But I could be wrong.

1200rallycar,

i agree, there is not much difference between webers and Su's when they are working flat out. Constant velocity carbs (SU's) love full throttle, however the webers in my opinion are far more responsive and deliver more power through the entire RPM range.

Whats the prob with the 42's. What jets and venturies are you running. Be specific. emolsion tubes, mains, air corrector, pump jets, idle, pump jet with exhaust orriface (located in the bottom of the fuel bowl), main venturi and auxillary venturi

What CC is your engine? What RPM (be realistic) do you want to run the engine at? and finally tuned for power or economy.

Hopefully i can recommend some jets, that will suit your needs, at least a starting point

I have alot of info on jetting webers. I have this great jetting table from weber.

The right jets can make the webers work like a dog or go like a rocket.

Hey there,

Talk to the guys at www.ztherapy.com. they know more than any sane person should about SUs. the website talks about z car stuff, but they love 1200's just as much.

Good Luck

ive got a mechanic which im very happy with, he know a lot and helps me out with prices a lot, he does all the work with the carbs for me so i dont know a lot about jet sizes

i know the chokes at the moment are 34's he says i need 29 or 30's but there is a problem with 42 webers, not exactly sure what it was now, but some tube or something, he was saying that he could drill holes in it to correct flow of air? i think, but it was a bit of a hit and miss thing which may or may not correct the problem

he said i could either use 40's or 45's with no problem and chokes would be easier to get also, the problem is in the body of the 42's so im probably better off selling the 42's and buying 40's or 45's, then he said even a pair of 1.5" SU's would be good, i said i had a set at home and he proceeded to convince me they are a good thing if maintained so i took those off today and will take them down to him to check out their condition tommorow

i had another thought though, if i sold the SU's and webers id be very close to having enough for an injection setup, (saw one for sale on ozdat for a grand a while ago-if i could find something like that again id grab it)

dimlight, whats the reason behind what you said???

edit- forgot a couple of your q's

its standard a-12 cc's
its for rally work and everyday road use-basically got to whale up high and be nice down low aswell, hard but theoretically achievable, especially with injection, my current motor revs to 8000, havent tied reving past that, no more power up there anyway, it would probably rev between 5000-5500 the majority of time when rallying but more on corners, i dont really rev hard on straights as suspensions not sorted and i dont have a cage

doing castor and camber adjustments soon- will start other topic about this

Well, here I go again, quoting "conventional wisdom" without firsthand experience. It is said that with the variable venturi of the SUs, in proper tune of course, the engine will never get more air/fuel mixture than it can use, no matter where the "nut behind the wheel" has the throttle set. For a street car, this is the way to go. For a race car, where the thing is either flat out or idle and rarely in between, the higher flow rate and subsequent top end power gains of a fixed venturi carb are more important. What I was saying is I thought that four 42mm throats on an A-12 would be too much venturi, but you seem to have that sorted with the 34mm chokes. I was not aware of a casting or jetting problem with the 42s, that is interesting. My vote for the best bet would be to join the late 20th century and swap to fuel infection. It really is the best way to go. (When even a shizbox Diahatsu [no offense to Diahatsu lovers {if there are any}] comes with FI, there must be something to it!) One of those setups that use a dual side draft manifold and quad throttle bodies will flow as much air as the dual side drafts and stand alone engine management is almost infinetely tuneable. Carbs are on the way out.

Did I just "bunny" this thread?

Quote:I think they mean it won't get more air than needed. The a/f mixture can be just as incorrect as with a conventional carb, if the jets, needles, etc are not the best ones.

Quote:Yes. The highest-performing engines since the early 1940s have been fuel injected (when the Germans perfected mechanical fuel injection). Top race cars have used injection since at least the early 1960s. Electronic fuel injection has simply made it less expensive as fewer precision parts are needed, and more controllable by electronic circuits. With injection, a venturi is not needed. The venturi in a carb is a restriction that somewhat limits ultimate airflow. In the 1950s and 1960s, the fuelie Corvettes had the most power ... still true.

And, EFI is easier to work on than an emissions-controlled carb, generally speaking from 1st hand experience. I've rebuilt and jetted many carbs successfully, and tuned EFI systems of various sorts just as successfully. The tools and methods differ, but once you learn both, you'll probably agree with me. On the other hand, I hear the newest cars from the mid-90s up are needing specialized equipment. I have no experience with these newest systems, but can tell you that the late 80s EFI systems don't require special tools to work on.

Quote:

I don't want to sound like a smarty pants, but I DID say "in proper tune." If the jets, needles etc aren't correct, it isn't in proper tune. I'm not trying to slam you DD, I'm just defending my original statement.

Ah, you're right. I didn't read it carefully enough. You use the same kind of qualifying phrases I do ... sorry. I've never tuned SU-type carbs ...

OK, let me see if I got it this time. If for great top-end power you fit overly large conventional carbs like dual Ferrari Maranello Weber 42s (or whatever) on your 1200 and open the throttle too soon at low rpms, the engine bogs down. So some engineer invented the vacuum-operated secondary. Datsun instead just put a teeny carb on the engine which responds well no matter what throttle opening is used at any time (but of course limits top-end power).

I too have heard that SU-type carbs can be "bigger" and not have this problem. Is that what you were referring to?

I just re-read the whole thing trying to remember what was going on in this thread. I just realized my first post was a "cranial-rectal inversion" moment. I had my head up my ass about a 42mm Weber being too big. For some reason, I was thinking 42s were the BIG side draft. They are only marginally bigger than 40s (well duh!) and therefore probably not too big after all.
Anyhoo, now that I've confused myself again, I'm going to say: yes, that is what I meant. Bigger SUs can be used because the venturi will open only as much as the engine will draw. That's my story, and I'm sticking with it.