IDF Carb Tuning Guide

[sptcoupe]

Try making an adjustment with ignition/pump off, and then start it to see if the gauge begins to work.

[PininF]

Thanks SPT for helping out but really wouldn't know how to go about doing that…On/Off switch ?

[131]

Thanks SPT for helping out but really wouldn't know how to go about doing that…On/Off switch ?

Adjust with engine off, see if it makes a difference when it's running.

[seabeelt]

Try popping the back off of the gauge. Sometimes the arm in there gets stuck.
R/

[RoyBatty]

Re: Throttle linkage
I think I may have figured out how to use the stock linkage in my 71 BC Coupe with a little fabrication and customization, for the dual idf40's/waffle manifold setup I'm installing.
I'll post images when it's up and running.
Here they are.
I ended up cutting the lever arm from the right side of the pedal weldment and welded it to the left side.
I also cut free the weld of the short(rear) lever arm on the stock belcrank and repositioned it to give the proper amount of throw. All works great with no binding.

[MIGHTY 8]

General question. A lot of the math from the original post involved sizing down to the decimal point.
Only looked up main jets and air correcters on Pierce's website, but all were sold in only .005 increments. (I.e. .135,.140, etc.)
Are people therefor boring these smaller ones out to these minute specifications?
I realize this is very much an exact science (wizardry, personally speaking!!) but was curious if I tune only with "off the shelf" vs. exact precision, will I fail to achieve these performance goals?
Thanks!

[toplessexpat]

I don't think so. The minutiae of the original post are some of the derivative calculations, not the jet sizes.

http://www.webercarbsdirect.com/mobile/ ... =77401.XXX

http://www.webercarbsdirect.com/mobile/ ... =73401.XXX

Ive worked on sptcoupe's carbs with him - and I've never seen any jet have it's size modified.... although he does have an astonishingly vast range of jets!!

[sptcoupe]

Yep - The formulas get you in the range you need on an F11 emulsion tube. Depending on your build, you will have to experiment to settle on the jet sizes for you engine. Remember - carbs are a constant compromise throughout the different jet circuits.

There was a time when you could buy mains in half steps (142 and 147, and 153), and you can still get idles in half steps (57s, 52s , 47s, etc). On the coupe's full house engine I am running 44s with F11 emulsions, 57 idles (the lean end of the range for idles), 145 mains (about the middle of the range fro mains) and 185 airs (about the middle of the range with the 145 mains).

It starts and idles smoothly at 750-850 rpms, instantly responds to throttle with no hesitation or stumble, and pulls hard to 8000 rpms with no misses.

So forget those decimal points, I was just illustrating how you get started in jetting those IDFs.

[MIGHTY 8]

Understood. Thanks for the clarification, gents.

Seperately, I had taken a look to see what I had; carbs (40's) were purchased used and my then mechanic took car of all the installing/jetting many many years ago.

I took all four of the emulsion tubes out, and found this:
1. F67, 180 air corrector, 130 main jet
2. F67, 180 air c.., 130 main j.
3. F67, 180 air c.., 120 main j.
4 F67, 180 air c.., 112 main j.

Naturally, first question is why would you fit different size main jets, and which vehicle came OEM with F67?
Not being able to leave stuff alone, and because I wanted to be with the "cool kids," I ordered some parts.

1-4, F9, 190 air correcters, and although tried 140 mains jets all around...found that after a few test runs had to trade off the 140 to a 130 and ran another hi speed runs, and some slow traffic periods as well.
Pure seat of the pants stuff here, but leaving it as is.
Thoughts?
Only other mods are Vicks 40/80 both sides, and IAP (RIP) 4-2-1, 2.5", non Cat.

[sptcoupe]

First, there is no way you can jet each individual bore and get it right. For a garage tuner, all emulsions tubes and like jest should be the same, or you will have no idea what to chase to solve mixture issues.

I also note you didn't mention a compression bump. With those cams, a 9.8 or 10.1:1 CR will really make a significant difference in performance with the IDFs. This is something I would seriously consider. What compression/engine are you running?

The F9 emulsion tube is much richer in the mid range and much leaner in the idle jet circuit, and much leaner in the main jet circuit. I don't know what idle jets you are running, but I would be looking at a range of 47 -52. You might be able to get away with a 45. For the mains and airs on that emulsion tube, I would try a 140 main with a 175 or 180 air corrector. But honestly, tuning an IDF using a 9 or 11 emulsion tube is a different game. I frequently use F9s in 44IDFs, but remember, an F11 was FIAT's factory choice with 40s.

So for 40 IDFs, on your motor as you describe it (with no idea what kind of compression you have), I would run F11s, 35 bleed backs, 57 idles, 140 mains with 185 or 190 air correctors, a 45 pump jet and 200 needle valves, with the floats set at 10mm closed and 32 mm open (with the carb-top gasket in place). If you have a later version of the IDFs, make sure the 8mm nut on the accelerator pump actuator rod is tightened the same amount of turns on both carbs, so the velocity of the fuel stream on tip-in is the same for both carbs.

And make sure you have regulated the fuel pressure to 2.5-3.0 psi.

I have all of these jets and emulsions tubes and would be happy to send you a set up to try for a while.

[MIGHTY 8]

My many thanks again for your kind info and offer for the parts. Wanted to write you earlier, but Christmas, et al.
Truth is, I've got a new motor top-to-bottom being built and destined for install by end of 1st Q.
More importantly, these are weird carbs, which I've yet to determine the precise model ID, and after scouring the Internet for images of Weber 40's, I've yet to find anything that helps me figure out what screw is what on mine!
Sadly, I've been on the look out for years for someone who could walk me through all this and and be as helpful as most here on this board have been.
Thanks again for your kind offer, and if you know anyone possibly in the NYC area, let me know!
Happy New Years!

[toplessexpat]

I know I, and I think Brad, regularly pass through NY, NJ, and specifically LGA, EWR, JFK...

[MIGHTY 8]

That's a very kind offer!
I'm an airline guy, so spend some time on the road. But always welcome to say hello to fellow Fiat people.
Welcome any emails when in town.
Happy New Years!
Gabriel
Gavillan@hotmail.com

[SoFlaFiat]

Now that I have my motor back together, I have started earnestly working on tuning the carbs. I foresee many questions but let's start with this.
With the new 44s I went through the process of modifying the linkage to join the two together. Thanks much to Csaba for sending pictures!! I adjusted the linkage so that both carbs open simultaneously. In other words, all 4 choke butterflies start moving at exactly the same time. Common sense tells me that this is proper.
Having run a couple fan cycles, with a synchrometer, I checked the two carbs to each other. Front carb running around 10lbs with the back at around 5. Front and back barrels of each carb running fairly close before adjusting air bleeds.
My understanding is, rough adjust the carbs to each other, using the synchrometer, with the linkage between the carbs. If I do this, the rear carb chokes open a noticeable bit before the front carb. This doesn't seem right to my feeble brain but there is not enough adjustment in the air bleeds to get these synched up otherwise. Ideas?
Thanks!!

[sptcoupe]

So you are not quite there.

Seat each idle mixture screw and back them all out the same - 1 1/4 turns. Gently but fully seat the air bypass screws on all bores. Do not snug the nut up at this point

Get the car idling at around 800-1000 rpms by using the idle speed screw(s). Using the idle speed screw and the intercarb linkage, get each carb to the point that it is drawing somewhere between 5 and 7 on the synchrometer, and the motor is idling around 850 -1000. It will be rough, but just keep it idling.

Measure the flow on the #1 bore and #2 bore. Whichever is drawing the highest volume, seat that air bleed screw and snug it up with the nut. That is the highest drawing bore between the two on that the #1 carb. Mark it with a marker so you know which is the highest drawing bore. You won't be touching that one again. This is your base bore for that carb.

Now go to the other bore and slowly back out the air bleed screw until the flow on both bores is the same. When you have brought the second bore equal to the first (base) bore, snug it up with the nut. You may have to re-adjust the idle speed idle speed so that the bores are drawing somewhere between 4.5 and 5.0. But be sure the #1 bore is exactly the same as the #2 bore.

Then repeat the process on #3 and #4 bores. When you have finished, the #1 and #2 bores should be drawing exactly the same, and the #3 and #4 bores should also be drawing the same. Now you have balanced the carbs, meaning the bores on each carb are drawing equally. BUT, you still have to synch the carbs so that all four bores are drawing exactly the same.

You do this by using the inter carb linkage and the idle speed screw.

Using the idle speed screw and inter carb linkage, get the #1 carb drawing between 4.5 and 5.0 on the synchrometer, Use the #2 bore to take this measurement. Then move to the #3 bore and using the inter carb linkage, get the #3 bore drawing exactly the same as the #2.
Now the carbs are synched and balanced, and all four bores should be drawing exactly the same and the throttle plates will open at exactly the same time and at the same rate on both carbs.

If they are not, use the air bypass screws to adjust the errant bore on each carb. Remember, the only bores you will adjust are the two bores that you had to adjust earlier to get them to the same flow rate as the base bores. DO NOT move the air bypass screws on the base bores. They are your two most efficient bores and you bring the other to up to or down to them.

Now you need to adjust the idle mixture on each bore. Remember, the idea is not to have all the idle mixture screws turned in or out the exact number of turns, but rather to have each bore adjusted to the "best lean" mixture.

With car idling around 800-900 rpms (the carbs should be drawing about 4.5 -5.0), go to the #1 bore and turn it slowly until that bore begins to stumble. You are now very rich. Then turn it slowly in until the bore shuts down and the engine idle gets very rougher. From that point, turn it our slowly until the engine smoothes out. Play with that point until you can feel exactly when the engine smooths out, and the continue turning in our a tad more (maybe a 1/16 of a turn).

Repeat on each bore. When all are adjusted, the engine should be running smoothly. Then check the idle speed and use the idle speed screw to set the idle at 800-900 rpms

Using the synchrometer, recheck the balance on the bore of each carb, then the synching to ensure that all bores are drawing exactly the same. If not, use the air bypass screws and the inter carb linkage to first get the balance and then the synching correct.

Then recheck the idle mixture by slowly turning the each idle screw in until the engine stumbles. That shouldn't be more than 1/4 turn.

Take it and so a couple of full throttle runs (in second - you don't need to go to high speeds, we're just using the full range of the throttle travel to make sure a cable doesn't stretch or the linkage does change), then come back and check the synching to see that all the bores are still drawing exactly the same.

If your throttle linkage isn't done right and doesn't return the throttle plates to the same pint every time, you will not be able to get a consistent and smooth idle.

Did I miss your final jetting set up?