IDF Carb Tuning Guide

[SoFlaFiat]

Jeff,
I couldn't have asked for a better guide! Thank you! I have a feeling that I am not the only one who will save this write-up!
I made choices on jetting based on your original write-up. I don't have the numbers with but will post them when I get home today.
Thank you again!!
Alan

[SoFlaFiat]

Here is what I have in currently,
Emulsion F-9
Aux venturi 4.5
Venturi 36
Mains 150
Idle jets 60
Air corrector 190

Again, these are 44s
The engine is a bit modified
Bored to 84.4
4.5 domed piston
42/82 cams
Ported intake and head
43/37.5 valves
4-2-1 exhaust
Lightened flywheel
Aluminum pulleys - cams, main, aux, water pump and alt

I am at sea level and running air intake from the front valance

Any further advise is much much appreciated!!
Thank you!

[SoFlaFiat]

I'm having difficulty with the inter-carb linkage. Maybe I misunderstand? Any adjustment I make to the linkage, takes the carbs further away from drawing the same.

[SoFlaFiat]

Well, I'm making some progress.
I swapped out the idle jets for 55s and readjusted my linkage. Actually, I disassembled the linkage, backed the idle screw all the way off and put the linkage back together making sure it didn't move the choke plates while I tightened it back up. Then adjusted the idle screw and inter-carb linkage to get them close.
This worked, probably more the linkage than swapping idle jets.
So thanks again Jeff, for stressing the importance of linkages in your original write up!
Here is what it sounds like now...

http://youtu.be/OIGtT92dEZ0

I am happy that they are at least close! It is already much smoother but #1 is a bit higher and #2 lower than 3 and 4. They are all still drawing closer to 6 rather than 4.5 so I have more work to do.

I have looked forward to the education and joy of tinkering with these and have to say, I am feeling good about what I am learning and accomplishing.

Thanks again for the input!!

[Sparky]

Having run into some hurdles with the fuel injection intake I was planning to use, I'm debating getting a pair of IDF's for my non-waffle-top intake (same as AR sells) so I can hopefully get the project on the road sometime this summer.

I've got a few questions that I haven't seen discussed yet.
Given the ~38mm runners what are the pros and cons of the 40's vs. the 44's?
I'm curious why everyone seems to be using a linkage system vs. a cable-pull?
and less of a tuning question..
What is everyone doing for their crankcase breather?

Also, a lot of the image links are dead. It would be great if someone could repost the horn/trumpet comparison pictures, or email them to me and I can repost them.

[WeberCarburetorGuys]

If you know your Lambda (Air Fuel ratio), you can do a quick calculation to figure out what jet change(s) to make to correct your #s. But if you don't have a wide band, you have to sneak up on the tune.

A change of "5" on the idle jets is a massive change, it's actually a 20% reduction in fuel flow in this case (60 to 55). I strongly recommend changes in increments of "2.5". Changes of 5 can easily bring you from too rich --------> too lean, or too lean --------------> too rich. You'll get more useful feedback with an idle jet change of "2.5". A 44 IDF with a 36mm venturi, depending on the engine's vacuum characteristics, will usually want an idle jet in the 52.5-57.5 range. So 55 is a good starting point. 60 is definitely too rich, unless the engine has horrible manifold vacuum (low mechanical compression, and/or a massive camshaft with a duration over 310 degrees.

Generally speaking the more manifold vacuum the engine pulls, the smaller the idle jets need to be, since the engine is sucking harder on the idle and progression circuits.

When you have adjusted LBI (Lean Best Idle, always set on a HOT engine), how far out the mixture screws are from all the way in, will give you a hint on your tune. What you are looking for is all adjustment screws being ~ the same, AROUND 2-3 turns out from bottomed out. If LBI is in the 0-2 turn range, the idle jets are definitely too big. If LBI is 4+ turns out, the idle jets are likely too small. If 1-2 of the screws are different from one another, there's something else going on, and you have to ID and fix that before dinking around with jetting any more. It may be a vacuum leak, tight valve, ring not sealing, etc.

Lastly, it's important to understand that the progression circuit (idle jet SIZE) will be DIFFERENT on vacuum advance engines, compared to an identical engine set up with a centrifugal ONLY distributor. Vac advance engines can run a "lean cruise" tune for more MPG and cooler running, which you cannot do with a cent only distributor. You don't have to, but you can gain a good 5-6mpg on a 4cyl engine by doing this. Main, air, and emulsion tunes will be identical on the 2 engines though.

[Sparky]

Lastly, it's important to understand that the progression circuit (idle jet SIZE) will be DIFFERENT on vacuum advance engines, compared to an identical engine set up with a centrifugal ONLY distributor. Vac advance engines can run a "lean cruise" tune for more MPG and cooler running, which you cannot do with a cent only distributor. You don't have to, but you can gain a good 5-6mpg on a 4cyl engine by doing this. Main, air, and emulsion tunes will be identical on the 2 engines though.

We really need a sticky thread on all the distributor options next! It seems every vendor has a few distributor options (including plenty of affordable electronic options), but very little information on timing curves and operation.

When purchasing some 40 IDF carbs there were a couple bits in the spec sheet that I haven't seen referenced in this thread.

550 Pump Exhaust Valve Jet w/ Delrin balls

Is this possibly another name for Bleed Backs? To be fair, I haven't even seen the term Bleed Back on any Weber diagrams.
I'm also wondering about measuring/verifying the Aux Venturi size? I'm assuming the 4.5 value recommended in this thread refers to a 4.5mm ID, but I've also read they're easily damaged during removal. Plus they're one of the most expensive parts to replace.

Below is a table of the 40mm IDF carb settings that I've gathered from this thread vs. how the carbs came vs the combination I was able to put together. These are initially going on a relatively stock 1800 engine with a Crane Fireball ignition and IAP header and will eventually transition to a higher compression (10.2:1), ported, big-valve 1800 with some 42/82 cams.

IDF 40 - carb setup | recommended | out-of-the-box | as setup
Emulsion Tubes | F-11 | F-11 | F-11
Aux Venturis | 4.5 | ? | ?
Venturis | 32 | 28 | 32
Main Jets | 132-138 | 115 | 135
Idle Jets | 52-54 | 52 | 52
Air Correctors | 195-200 | 200 | 200
Needle valves | 200 | 200 | 200
Bleed Backs | 00 | 55?? | 00
Pump Exhaust Valve Jet | ?? | 550 | 550??
Accelerator Pump Jets | 45-55 | .50 | 50

Any recommendations on a good IDF maintenance/assembly manual?
I'm also having trouble finding a fuel pressure gauge and regulator with a 0-5psi range which would make tuning to that 2.5psi target a lot easier than with the more common 0-15psi parts. I'd like to make this a returnless fuel system to keep the whole things as simple as possible.

[ebrown0104]

So for the bleed back/pump exhaust valve, I'm seeing a few different options for jet sizes. Some places list a size 000 and also a size 100. Which ones are being referred to as "00" in this list?

[ward00]

Ok - I figured out the answer to my question below. The screw that is mounted on the inter-carb linkage enables tweaking of each carb independently. I thought it was the idle adjust screw. In hindsight, I feel foolish, but its my first dual carb setup (not my first FIAT, however).

Hi folks - I took delivery of a 75 FIAT with dual IDFs this week (in Santa Cruz, Ca). Although I think I understand the concept of balancing and syncing each carb, looking at the inter-carb linkage, It's not clear to me what it is I'm physically supposed to do with the linkage to sync the carbs.

Instructions in this thread state: "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" (this part I understand). The instructions then state: "Then move to the #3 bore and using the inter-carb linkage, get the #3 bore drawing exactly the same as the #2."

What this implies to me is that after balancing the carbs, somehow the inter-carb linkage between each carb needs to be decoupled/disconnected so that the #3 bore of the 2nd carb can be synched with the #2 bore of the first carb.

However, physically looking at the inter-carb linkage, what I think I see is a rod that links all four throttle plates together, and a screw that controls idle speed of all four plates at the same time (the car is running - so I am hesitant to dismount the carbs to determine if this is true).

My question is this" what is it that one needs to do to the inter-carb linkage to physically decouple the rod between the two sets of carbs so that the step calling for "using the inter-carb linkage" above can be performed?

[seabeelt]

just back the inter cab linkage adjustment screw off until the screw doesn't touch the plate and the carbs at "independent"

[grasshopper]

I have a question. When adjusting fuel mixture on dual idf's, how many full rotations of the screw driver "out" should the mixture screws be on average ? Some manuals say 1.5,,some say 2-4 ? Are these figures full rotation of the screwdriver or half rotations.. ie 180degree rotation or 360 ? Also, one of my barrels (one closest to firewall) draws slightly less (im checking with a syncrometer). What can be causing this ? When I screw the fuel mixture screw all the way in, it doesn't seem to effect anything. The engine runs very well regardless of all this,but I cant figure out why last barrel is drawing less.. Any info appreciated thanks.

[RoyBatty]

Having run into some hurdles with the fuel injection intake I was planning to use, I'm debating getting a pair of IDF's for my non-waffle-top intake (same as AR sells) so I can hopefully get the project on the road sometime this summer.

I've got a few questions that I haven't seen discussed yet.
Given the ~38mm runners what are the pros and cons of the 40's vs. the 44's?
I'm curious why everyone seems to be using a linkage system vs. a cable-pull?
and less of a tuning question..
What is everyone doing for their crankcase breather?

Also, a lot of the image links are dead. It would be great if someone could repost the horn/trumpet comparison pictures, or email them to me and I can repost them.

I have a 75 Spider and a 71 Coupe.
I have modified both with dual IDF's and in the process have fabricated my own cable to bellcrank setup.
The purpose was to give the best linkage setup possible and keep it all as much as possible under the carbs and not routing anything over the top of the engine.
On the coupe, the setup I came up with also removed the solid linkage induced "bunny hop" of the engine in the mounts that can happen with improper clutch/throttle coordination as you leave from a dead stop.
I'll try to post some pics later. Though I think you might be able to find them already posted here in other similar threads to do with IDF installations.

[toplessexpat]

Nothing like a timely update.

See the linkage and bracket by Lofland/Wagner. Uses a strada cable which attaches to the top of the accelerator pedal (or the through firewall linkage on older cars, or the cross firewall linkage on coupe/sedan/wagons), and then make use of the cam box lobe on the driver side to mount a bracket. Works perfectly on dual carbs, and they’ve a different bracket for single carbs. Remove the entire bell crank and have a single cable from foot to carb.

[seabeelt]

Has anyone run into the issue of maintaining a good idle.

Background. I tune/adjust the carbs when the engine is at full operational temp. Set idle at 850. After driving for a bit the idle creeps up to 1650-1800 rpm.

No mater if I blip the throttle or not it will not go back down unless I adjust the idle screw to compensate........ but then it will go back down and not idle and stall because the setting is too low.

I’ve sprayed carb cleaner all around the base gasket and every where else I can think of and find no air leaks

I’m stumped

[Nut124]

The IDF that I know has no provision for cold start nor cold running.

I set mine for about 1000-1300+ idle speed when warm so they run kinda ok when cold.

The turning friction of the engine can change a lot cold vs warm. The IDFs have no way to adjust for this on the fly.

These are not like your modern EFI engines that hold constant RPM at idle no matter what.