Bearzah

iTrader: (6)

I'm looking for build suggestions with a 48mm Weber IDA on a ported 12A.

It's on a heavily street-ported 12A with free-flowing exhaust. The car is a road-race car regularly seeing 7000-8000rpm.

Currently, the carb is in great shape. Freshly rebuilt, evidence of ported chokes (guessing ~43mm) and polished bores.

Current setup:
Main jet - 230
Emulsion tube - F11
Air corrector jet - 170
Idling jet - F10/70
Idle jet holder - 120
Pump jet - 60

At this point, the car FALLS on its face at part throttle and under crisp throttle opening. The spark plugs show lean, but, car still pulls strong at full throttle. Although, on long 4th gear pulls, the car seems to be running out of "steam".

What I'm looking to do is install a more realistic choke tube, high-flow 300 needle and seat and put together a better-fit carb setup.

Looking for pointers from experienced IDA guys.

Thanks... and BRAP on!!!

bumpstart

assuming you have already have the 3rd progression hole?

230 on 43 at sea level should be returning a 12 - 13 AFR and so is ideal for that choke

the issue at part throttle is having both circuits running,, the "idle" circuit should support the engine to 3000 rpm and the main circuit takes over

to juggle the turn on point for the main circuit later you have to play the emulsion tube .. or make the air corrector smaller or drop the fuel bowl height a little
( or combination of all three )

firstly .. if its a mild 12a ,, then you should be running the 65 fuel jet on your idle jet holder,, having a 70 will work.. but makes that point where it runs out at higher rpm

( usually with the f11 your battling to end the idle circuit earlier, AND bring on the main circuit later )

from here ,, set up the bowl level to the prescibed 25.5-26.5 mm from the top
find the 65 fuel jet for your F10 idle jet holder
try a smaller air corrector,, down as low as 120 to make it ( main circuit ) turn on later

-- the air jet corrector works like the finger over the top end of a straw holding a column of water ,, a bigger air jet is like lifting the finger higher ,, and lets the water drain from the straw,, faster ( earlier )

Bearzah

iTrader: (6)

Awesome info, thanks.

Keep it coming guys!!

j9fd3s

iTrader: (3)

basically the simple way to see what the idle circuit is doing is just to remove the main stack and go for a drive. it will be immediately obvious what the idle circuit does.

testing the main circuit is also easy. cruise along at an rpm where you're still on the idle circuit, and floor it. if everything is right, it'll just go. if you're too lean it'll act like you just turned the key off.

once you have the boundaries of each circuit in the carb its pretty easy to figure out what you need to do. if you want the main to come on sooner, use a smaller air. if you want the idle circuit to work to a higher rpm use a smaller air.

additionally. the 3.0 needle and seat is a good idea. i would also try the 65 idle jet, and if that is better maybe even a 60. if its falling off at the top of 4th, its likely to need more fuel pump.

for the venturi, you should measure what you have

i haven't done much experimentation with emulsion tubes on my car, but if you can't get it to run by changing the airs, it might want a different E tube. i'm running an F7 in mine

Bearzah

iTrader: (6)

Thank for the info guys...

At this point, the owner wants to just order a setup that will work better "out of the box"... Looking to order new chokes (looking for size suggestions) and all new guts where needed.

Seems a 37-42mm is about the range used most commonly on a 12a... Given the engine's porting, race-only use and higher rpms seen, I'm leaning towards a larger choke, but, am open to suggestions.

Also, it's running a Carter fuel pump and Holley fuel pressure regulator (still playing with pressure, but, currently set at 4psi static)

j9fd3s

iTrader: (3)

there is no out of the box. you need to pick the air flow hardware (engine, exhaust, intake, venturi) and then tune the fuel. you have to play with the jetting

4psi of fuel pressure is fine, as long as you can maintain it to the top of 4th.

for the choke, since you don't even know what it has, step one is to measure it/look at the size. if you gave me a carb with no choke, i'd try a 40mm, but if the choke in there is between 37-42mm i'd probably just leave it

FB_rad

an afr meter would take a lot of guess work out of trying to tune this.. make sure the accel pump jet is a 0 bypass. its in the bottom of the float bowl

snivley whiplash

iTrader: (4)

try running a smaller choke with the same jet set up and smaller air corrector. I had a 42 choke in the dellarto on a six port running open on all ports. Had poor tip in (transition to main jet), went to smaller choke and air corrector and had great results. The low speed to mid range hesitation went away and the top end pull was better and lot lean on the wide band reading, so bigger is not always better.

you have to take into consideration how much air the engine will actually need. Sometimes this is less than you think. Have you looked at Gruntled.com for jetting suggestions? Take a look great info there.

Just my .02 worth, had lots of success with delorto jetting and the carbs are not much different other than jets. Btw try running about 3 psi fuel pressure and see if it helps

Joe