Racingartist

I am building a 12 A. I did the bridgeport on it lightened the rotors and installed an aluminum flywheel. I run carbon apex seals and it is balanced. I have a 48 mm Weber idf down draft . I need to tune the carb. it is running way to fat at low RPM under 3K. I have to keep it at 4K to 5K and it's running 15 air fuel ratio and down to the lower RPM it go's way fat 9.2:1 and lower. So it won't run there (surprise). So is there anyone out there that is a Weber grue out there that can offer so Ideas. What jets sizes they come with maybe a good set up to start with. I live up at 4300 above sea level and race at about 100 feet above sea level. What air control jet and main jets and emulation tubes are most likely a good starting point?

aeenox

Take a look at this PDF that Racingbeat created it might help get you started

https://www.racingbeat.com/manuals/webercomponents.pdf

Andrew-s

iTrader: (1)

When I first started running a Weber IDA 48, I also had super rich areas over the rpm range. I first started changing jetting but if I had to do it over again a better starting point is to really nail down the float level first.

In my opinion the float level is hard to set using the 'aluminum gauge plate' tools that you can learn how to make via google, especially if you are using a non-stock needle/grose jet.

If you continue to google, you can find that the VW and Porsche IDA guys use a little measuring tube that screws into the float bowl and will give a direct measurement of the running float level for ~$40.

The 80s RX-7 MFR competition manual states that they ran their 48 IDAs with a fuel level that is 21-23mm down from the top edge of the main body of the carb/float bowl which works really well on my RX-7 race car. I run a fuel pressure of 4.5psi but for an all-out racing IDA I have read that 6psi is better to ensure that the bowl can quickly refill.

The Cobra guys with IDAs also have a way to directly measure the fuel level via a depth caliper and an open main fuel jet well which I have also used in a pinch, this dimension should be ~43mm: 48 IDA Float Settings - Club Cobra

Racingartist

Thanks guy I am use to working on Sprint cars with fuel injected Alcohol engines or stock cars with Holly carbs.

j9fd3s

iTrader: (3)

i didnt know anyone actually made a float level tool, its worth it! you basically want the float as high as possible, without dripping (a little drip is ok in a race car)

for an idle jet, i seem to end up around 65, but its +/- at least 10. i'm not sure what you have, but try everything. for a track car, you probably want the richest one that runs ok. engine will get mean, but richer = more lubrication to the seals at low throttle openings.

for the mains, you should be in the 180-220 range. i'm not sure what jets and stuff the IDF uses, but i like the F7 or F8 Etube, instead of the lean spot hesitation, when the mains come on, you can get so rich it stops running.

use the air corrector to fine tune the transition between the idle and main circuit, and you're good. if i was to make a product i would make an air corrector with no hole, or a very small hole, out of plastic. so you'd just drill it out until the car ran, measure it, and then buy a real jet.

Racingartist

I was playing with the race car this afternoon I was getting as fat as 9.5 to 1 and then it would die. I changed the main jets (at the end of the emulsion tube) from a 150 to a 135 and got it to run much better with 14.2 to 1/ 15.0 to 1 @ 2000 RPM idle. I plan to fiddle with it tomorrow. It runs in the 15 to 1 air fuel ratio at 5,000 or 6,000 rpm range. But it has a hesitation when I hit the throttle and I have to pump it a little to get it to catch.

speedturn

iTrader: (1)

Jet sizes depend on Venturi diameter!!

What size venturies do you have in your 48 IDA Weber carb?

finger lock

iTrader: (6)

Hello,

I run a 12 bridgeport with a 48 IDA carb. I do not have experience with the 48 IDF but you might be able to see what I'm doing.

40mm choke (rules mandate this size, if you ran a 42mm or 44mm there would be more power to be made and you would have to re-jet accordingly)
Main 200-210
Air 125-135
idle 70
emulsion tube F8
I set the floats as described in the Weber manuals
timing 25-27 degrees advanced

I have an O2 sensor in each of the exhaust runners and I log the AFR while on track. I look at the AFR at the end of the longest straight at WOT and make jet adjustments based on that AFR (it is not uncommon to have different air and or main jets for the front and rear rotors).

My motors make max power in the 12.9-13.1 AFR range. I usually set the idle somewhere around 1500-1700 and it idles super fat.

I hope this helps,

Guy