IDF Carb Tuning Guide

[seabeelt]

There is a “additional fuel” spring loaded arm on the side of each carb. It’s not a choke but adds more fuel to the mix when opened. Mine aren’t in use but I might try connecting the choke pull wire to them and see if that works better for cold and then perhaps I can turn the idle dow just a bit when fully warm.

[Nut124]

I have been working on adjusting, troubleshooting dual 44IDFs since last fall in my -78 Spider.

Not going to post a complete report, but will address the air bypass bleed screws.

Common instruction googled up suggests these should be left alone, in fully closed position. No reason provided.

With my 44IDFs, there is a dead spot between the idle/progression ckt and the start of the main ckt. The idle/progr ckt runs out of fuel before the main ckt starts. This is annoying and a very common IDF complaint. I can compensate by running larger idle jets but this makes the idle ckt way too rich.

So, I opened the bypass air bleed screws 3 turns each. This allowed me to fully close the throttle plates so that the progression holes are not active at idle. Now, idle air is coming thru the bleed screws rather than past the plates. Now, my idle mixture screws can be open as much as 2.5 turns, vs 1/2 to 3/4 turn w/o the air bleed. Idle speed is also more stable and repeatable from nite to nite.

Also, the extra idle mix screw helps sustain the idle/progr ckt a bit longer making the dead spot barely noticeable w/o having to run extra rich idle jets. This is because the bypass air stops as soon as the plates are into the last progression hole, so the extra idle mix screw fuel adds to the progression ckt duration.

So, I say, open the air bleeds 3-4 turns ea and do not mess with them.

Another quick comment: I have been using dual webers in Fiats for 20+ years and never used nor owned one of those air flow measurement devices.

[mamsterla]

3 to 4 turns seems like a lot - I have seen a number of online guides recommend full close or out 1 turn. 3-4 seems like you might be getting that sucking noise on all cylinders. I am currently dealing with an issue where after a WOT, my car stumbles on returning to idle. Seems to go very lean for a bit before correcting. I have these fully closed and was thinking of opening them a turn.

I have been working on adjusting, troubleshooting dual 44IDFs since last fall in my -78 Spider.

So, I say, open the air bleeds 3-4 turns ea and do not mess with them.

Another quick comment: I have been using dual webers in Fiats for 20+ years and never used nor owned one of those air flow measurement devices.

[Nut124]

3 to 4 turns seems like a lot - I have seen a number of online guides recommend full close or out 1 turn. 3-4 seems like you might be getting that sucking noise on all cylinders. I am currently dealing with an issue where after a WOT, my car stumbles on returning to idle. Seems to go very lean for a bit before correcting. I have these fully closed and was thinking of opening them a turn.

I have been working on adjusting, troubleshooting dual 44IDFs since last fall in my -78 Spider.

So, I say, open the air bleeds 3-4 turns ea and do not mess with them.

Another quick comment: I have been using dual webers in Fiats for 20+ years and never used nor owned one of those air flow measurement devices.

A few comments:
- This adjustment is likely to be sensitive to the amount of vacuum the engine draws at idle. This depends on cams, header, etc.
- Also, there are air bleed screws where the tip angle or pitch varies. I have a set of chinese IDFs and a set of the Red Line (Spanish) IDFs. The bleed screws do not look exactly the same. At the time I wrote that I was running chinese.
- The point of the post was that giving the carbs idle air via the screws seemed more repeatable and constant from day to day vs having the bleeds closed and trying to set the butterflies to a tiny sliver of an opening. Also, at idle, the plates need to be closed to the point that all progression holes remain well above the plates, so that fuel only flows thru the idle mix screw. Using the bleed helps achieve this.

What is that sucking noise you refer to?

How do you figure the stumble is due to lean condition? Most IDFs run way rich at idle. My car has stumbled, and failed to restart after hard WOT running when shut down abruptly. I have heard this complaint from others as well. I have always assumed this was due to excess rich condition, flooding the plugs.

[mamsterla]

The sucking noise is when you hear that there is a lot of air going through the air bypass. It is very audible to me from closed to wide open.

I have an wide band O2 on my car from AEM and I can watch the AFR go lean as it stumbles. I will be working on a few things over the weekend including going to a 123 distributor. Hopefully I can report on what I see.

3 to 4 turns seems like a lot - I have seen a number of online guides recommend full close or out 1 turn. 3-4 seems like you might be getting that sucking noise on all cylinders. I am currently dealing with an issue where after a WOT, my car stumbles on returning to idle. Seems to go very lean for a bit before correcting. I have these fully closed and was thinking of opening them a turn.


What is that sucking noise you refer to?

How do you figure the stumble is due to lean condition? Most IDFs run way rich at idle. My car has stumbled, and failed to restart after hard WOT running when shut down abruptly. I have heard this complaint from others as well. I have always assumed this was due to excess rich condition, flooding the plugs.

[Nut124]

The idle bypass is only active when the throttle is closed. Once the throttle starts to open, the inlet of the bypass is left below the throttle plate and will no longer do anything. Any sucking sound much beyond idle is something else, I think.

Keep in mind that the wideband does not measure actual AFR but O2 level. A misfire can read like lean when unburnt mixture with O2 hit the sensor. Just something to consider when troubleshooting.

Let me know how the 123 works out. Been thinking of getting one myself.