yeah ok then...
so what carby u reccomend.? and cam? thats all im thinkn of doing... dont wanna spend much on it...

If ya gonna change the carbs your better to have somthing that has potential for the future though.

If a carb(s) is too big it can be jetted down to make it run right, If a carb is too small it will limit the whole engine.

Twin dellortoes/webbers are probably the easiest carbs to find and if the bug hits you you can still use them if you decide to upgrade more.

Case in point, a guy I know has a terrapin (mini based single seater) when he first built it he was proud of his 1 1/8th twin SU's and no one could tell him different. Now hes running a sidedraught webber, I cant see him going back somehow.

It all depends on what you will actually be using the car for, & how much you wan't to spend, but if you want to be at least a little bit competitive, then your friendly cam grinder will be the best place for good advice there.
If you can come up with an affordable GX head & carbs, then that will be a good start. Failing that, try looking at A14 heads. These had bigger ports & valves than A12's & the GX type oval ports & manifold bolt pattern. Make sure that the head that you look at does not have a noticeably bigger combustion chamber than an A12.
If you use the A14 head, simply overhaul it, & i'm sure that many here can tell you how much to shave it to restore compression. You would also need to check to see if someone has a "pre loved" [used] manifold & carb for sale. A pair of SU's or a stock GX setup in good condition should be satisfactory, but a single side draught will also be good.

A GX exhaust manifold is also good, but a set of extractors may well be easier to find.

There you have it. Relatively cheap & cheerfull, but it should give a good performance. Add electronic ignition too if you can.

One of my friends ran a Holden 202 engine on ONE throat of a Webber carb on his panel van both on the street & on the show circuit back in the late 70's / early 80's. This thing was turbo'd with 9lb's of boost & ONE throat was all it needed to light up the tyres without problems.
For those old enough to remember, it was a green HQ called "Midsummer Dream"

I'm sure that there will be others who will have their own recomendations, but Feral will be a good one to take heed of.
Chris
P.S.
Bullit
Rejetting will NOT cure the symptoms of OVERCARBURETION. This is a situation where the gas velocity at wide open throttle drops below the critical point where it can not draw fuel through the fuel delivery nozzle.

By way of illustration, imagine a 4bbl carb on a lawnmower engine. Now thats GROSS overcarburation & you would be lucky to get it to run at all, but HEY, it's a big carb, so it should mow the yard it 100mph.

In Webbers, the usual fix is to fit smaller venturi's, which in effect is re-sizing the carb. For the well heeled, this is a good way to go as this method, at great cost, effectively converts two carbs to equal one standard Webber carb, while allowing scope to go bigger when the need or situation arrises, but my suggestion is for a more budget oriented engine.

Anyway, what does everyone else think?

Venturis, Jets its all the same to me

Im the first to admit I dont know as much as I would like to.

Still the concept was correct

Venturi's are the restriction, or smallest part of the carb throat that the air passes through. Jets are the metering restrictions that the fuel flows through. You need a certain amount of vacuum, or manifold depression, to draw the fuel into the airstream & too much carb will allow this "suction" at wide open throttle, & at low rpm to drop to the point where it will not draw fuel. Thats what "bog" is when you stab your foot in it off the line. All air, no fuel. Plain air is hard to light.
Chris

OK you blokes ! Here`s a little secret that I bet most people don`t know.
The first question you should ask yourselves is how much carby you can use ?
Most people don`t know the answer,they usually copy an existing set-up or take a ghess(like most on this forum).
EVERY engine has a maximum gas flow rate and an optimum carby size.There is a simple formula to calculate this !

Cubic capacity(in inches) multiplied by Max Revs,divided by 2.
This gives you the cubic inch of gas per minute it can breathe. Divide this by 1728 for cubic feet per minute (CFM).

For example: Lets say this comes out to 650cfm.If you fit a 750cfm carby,you won`t gain more power because the motor can`t breathe in any more ! But you WILL lose gas velocity & as a result droplets of fuel will form and tend to drown spark at lower revs.

Much better to err on the smaller side as, unless you get down to ridiculious levels, it is almost impossible to undercarb an engine.
You will lose top end power, but that`s about all.

FM1200, you get my YAY for the day. A voice of sanity in the great carb debate.
You are ABSOLUTELY right.

The only thing that i would care to add is that when one calculates the max air consumption, one must remember that this is a theoretical maximum based on a volumetric efficiency at max revs, of 100% which few engines of our type can approach. I think that The Chrysler hemi's come reasonably close, but the're legendary, & i'm biased, so we won't count them.

To get the true volume of air, the formula must be adjusted by figuring in the percentage of volumetric efficiency.
For stockers this figure can be about 80% with souped up examples on the street running at something like 85% & the real hotties looking at about 90% AT FULL SPEED.
Lets see, a 1200 is 71.5 cubic inches. Now my formuls is diferent, but gives the same result. It is CFM = RPM x CID/3456 divided by VE

So our example is a 1200 at 6000rpm Thats 6000 x 71.5 = 429000, divided by 3456 =124cfm. Now divide this by,... lets say 85% [VE] for a street engine like my GX, & we get about 105cfm. That's a realistic carb size, or maximum airflow for THIS EXAMPLE.
Now we can round this number up or down a little to fit in with a carb[s] capacity that we can buy & feel confidant that we are not short changing ourselves & not reducing performance due to overcarbureting. I could cope with an increase of 5% to 10% on this number, but it will not really yield any noticeable improvements.

Lets be realistic blokes, how many RPM does YOUR street engine see for 95% of it's running life. Mine is red lined at 5,500rpm, i run stock GX carbs, & i still have the same engine 23 years later.

FM1200,.. you the man.
Chris

um ok you worked out that someone needs 105cfm carby, but how the bloody hell do you know which carby that is?

my GX setup revs to 6,400 in a straight line so you must be doing something wrong , (7500-8000 round corners)

just kidding, it gets bloody loud and i wouldnt do it often but it is definately possible, i also have a bigger cam and overbore (40 thou)

i had twin 42 webbers previously on the same motor and the SU's dont make the top end that the webbers did, but my venturis were too big in the webbers and it ran like crap under 3,500 so i took em off for the mean time

1200RC

Good question, & if i come up with an answer, i will let everyone know, but there must be a way of "sizing" carbs. I wonder if some of the carb manufacturers have websites with the means to answer these questions?

My GX is strictly street & it is currently about .140" oversize [76.5mm] but it's now worn out again, so i will hit it at 77mm.
I keep the revs down a bit so that it will live long enough for me to be able to drive it home & i'm too old to be a highway hero anymore. My cam is of an unremembered grind, but it was described as a "light sports profile" & my engine idles quite ok at about 900rpm. It pulls smoothly from down low & this is also a benefit of the variable venturi carb design [SU's]
I also got more than 50mpg one day when i drove for economy when i went to Holbrook for the Ultralight Fly In weekend.

I also managed an AVERAGE speed of just over 70mph while turning 42mpg [screw metrics] late one night between Albury & Bendigo. I figured that i had to be happy with that.
If i were to go seriously racing i would do some things diferently, but i can't afford it, so i tailor everything for maximum street pleasure & that doesn't impress the wankers. Not my problem.
Chris

hehe you talk about wankers a lot, i do what i like to my car, dont worry bout anyone else, but i did make an effort to prove this

as all my family totally despise neons, so i put a neon in my car that flashes with the music, its a red one so not too bad and im not really gonna turn it on much, maybe at cruises and stuff

hope this doesnt divert the thread too much, sorry