As said before, once the carbies have been set up with the right jets it wont go out of tune that much, I use gunsons color plug to tune my cars and to get it spot on,it actually shows the colour of the combustion inside the combustion chamber, with twin carbies i also use gunsons carbalancer which measures the air the goes into the intakes.








these tools make the job soooooo much easier, and i gets it spot on! just follow the instructions and have an understanding of how carbies work is a bonus.

Quote:Fandatstick is right, particularly the last eight words.
Once the carbs have been correctly jetted, & remember that they are fixed jets, not adjustable, then it's a done deal.

The only things that can possibly go 'off tune' are the float levels, [almost totally unheard of], the idle mixture, usually because some dickhead fiddles with them, & the balance between the carbs, usually due to poor quality linkages, or loose fasteners allowing adjusting parts to vibrate & move around.

Hell, Ferraris, Maserattis, Valiant Chargers & several others used multiple sidedraught Webers as standard & these cars are not known for chronic out-of-tune carb problems.

I also agree with Sidedraught when he says that a couple of 40's will work much better on the street than 45's & in my personal view, a single carb will be a much better proposition from a driveability viewpoint as well as an economic one. At least it eliminates the 'balance between carbs' problem.
Unfortunately a single carb does not have the same WOW factor that duals have when you open the bonnet.

Overall, as long as you have a sound instalation, with quality parts & your fuel is properly filtered [I prefer a big filter at the tank & a small one in the engine bay] then any Weber sidedraught that is in good, sound condition should deliver the same reliability as the original Hitachi.

The colour tune plugs seem to be OK for setting the idle mix in a stock engine, they won't be of much help in an engine that has a lumpy cam as the idle mixture in these can be all over the place.
They are also not much help in 'off idle' mode as you really want to see what the mix is at speed & under load & the law frowns on driving around with a mechanic under the bonnet.

thanks heaps all, i will prob get the twin 40's, as for economy, i have my daily drive for work, this is my jumping between petrol stations car (only has a stock 28ish liter with bigger engine and other upgrades)

thanks again

~n87

Dont confuse the size of the carb directly with economy.

It more to do with regulating the velocity of the fuel mixture at lower speeds. 40's typically accomodate the smaller end of the range of the Aux Venturies [= higher gas speeds]. For a streeter I wouldn't go any larger than a 34mm Aux Vent, [even thats more suited for the track].

well I really can't claim to be an expert and I am yet to get "the twins" as I call em properly tuned on my coupe but I find them fine for street driving it's really only the excessivly large cam I installed in the motor that makes the whole setup a little tiresome to drive arround at sane speeds.once shes above 4000 rpm tho, the quick trip to 7000rpm and the induction noise from "the twins" makes that climb to 4k on the tacho a long forgotten memory.

Since putting them on we've only given her a rough tune but she seams to run fine, i've done a bit of twin SU tuning and once you have the basics you'll be wondering what the fuss was about. as people have mentioned it's the base tune thats the hardest, as you need a good selection of bits, jets emulsion tubes ect.. to get it right the first time.

I considered twins again for the next build but am opting for single sidedraft as the fuel does seem to disapear with twin webbers (mainly cause it's so much fun driving it at full noise) I tend to think that the single properly tuned will on the street give just as much fun and less of a shock at the bowser.

good luck

Economy is restricted to the weight of your boot! Mine used to be lead. Now they are alloy. Soon, on the weekends I will put on the steel cap boots as a happy medium.

thanks again guys, i was wondering if there was anyone north of sydney the can do a first time tune 'properly' with dyno and a box of jets to fiddle with?

~n87

how far north of sydney?

I know of a guy that i can recomend that is up here in newcastle. He is very good and knowledgable, he's known as a local guru in setting up carbies.

There used to be an old guy down on the central coast but i'm not sure if he is still open? he was 'old school' he used to tune my uncles cars, i'll find out if he is still open for business.

I would recommend this guy. He knows his Datsuns and Webers. Northwest Sydney

Stewart Wilkins Motorsport

check this out

If i was running a a15 with mild to wild cam and good compression it dwould be producing max power at about 6000rpm.
Therefore using the table you would need
40mm webers with 30mm chokes.

If you were to go bigger chokes you would encounter a flat spot at about 2500 under load that would be impossible to tune out using jets.

Carefull choke sizing is the key when tuning webers. Always go one smaller if you are stuck choosing between two sizes