IDF Carb Tuning Guide

[So Cal Mark]

Many thanks to Jeff Schefferman for sending this to me.

I spent some time last night on the phone coaching a guy on the initial set up of IDFs on his spider. After about two hours, he had them running pretty well, but he needs different jets and some other items to get it right, which he has ordered. At the end, he suggested I post the approach I use to get the IDFs initially set up and running optimally. So here is just one guy’s methodology.
If you go to IDFs (or any multiple carb set up) there are certain other things you must “fix” besides getting the carbs jetted/balanced correctly, etc. You just can’t bolt them on a manifold and put them on the motor and expect to get the maximum benefit from them. And most importantly, many of the problems we have with dual carb set ups are not issues with the carbs – but other components. And the frustrating part is that we end up chasing problems by fiddling endlessly with the carbs, when they are not the problem.
Let me say that I do not recommend IDFs on a stock engine. You will be happier with one of the many single carb upgrades available, and it is a lot cheaper, too. I have had some spectacular street motors with stock cams, an early header and a single, larger carb. If you want to go the IDF route and actually get some serious performance improvements, you have to bump up the CR, get a decent header/exhaust on it, and upgrade your cams, at a minimum. Assuming you have done these things to one extent or another, and that you have all the basic tuning elements right (cam/ignition timing, etc) here are the steps I take.
Fuel delivery: You will need a good low-pressure pump that can deliver a solid 3 psi at WOT. You will also need a good regulator that can control the pressure at the carb inlet to 2.5 -3.0 psi. IDFs are extremely sensitive to fuel pressure and call for only 2.5 psi. So that means if there is 4 psi at the inlet, you are over pressurizing by more than 50%, and your carb just won’t perform consistently. The same is true if you are only getting 1.5 psi at the inlet. If over-pressured by the fuel will foam and you will get a miss at the high end of the idle jet circuit range that mimics bad plugs/ignition/fuel starvation and a thousand other things that you’ll spend a lot of time chasing. If the psi is too low, you will get a lean condition at higher rpms that, when extreme, will cause a noticeable stumble, but way before that, will prevent the engine from making the max HP. In other words, you will be giving up a lot of power BEFORE there is a symptom to tell you something is wrong. This is true of over-pressure conditions as well. Because the engine will still readily rev and do so smoothly, the only way you know this is if you are on a dyno, which is how I learned – so eliminate these two issues from the start with a good fuel delivery system. Final word – MOST electrical pumps we use on our cars will not deliver the required, steady psi at WOT, and a lot of the regulators we use won’t regulate the psi well enough. Just put a gauge on yours and watch what happens when you rev it up (with IDFs). I just recently used a commonly sold in-line regulator with a brand new gauge to do a quick initial set up, and there was a 2 psi difference in what the regulator was set at, and what the gauge said! Which was correct? For IDFs, 2 psi off is really serious.
The stock mechanical pump on earlier engines won’t work with IDFs on a modified engine.
Specifically, this is the set up I typically use. A Holley Blue fuel pump, an Inglese NG1081, 1-4 psi regulator, and a Holley 26-500 fuel gauge. There are alternatives, but these will eliminate any fuel delivery problems and have worked well for me.
Ignition: Get an electronic ignition. The MarelliPlex works fine, and there are many others that work as well.
Throttle linkage: When you get the carbs on the car, make sure you spend a lot of time on the accelerator linkage. It must consistently return the throttle plates to the same at idle position or you will be forever chasing what you think is an idle mixture problem (too lean/too rich), the engine will never idle smoothly from one traffic stop to another, and idle speed will vary constantly (sound familiar?). We curse the carbs for these issues, when it is really the linkage that is the culprit. And before you fill the carbs with gas the first time, have someone sit in the car and mash the pedal all the way to the floor, while you are looking at the throttle plates. Make sure they open 100%! I have seen many set ups where they would only open 95% or less. You are throwing way HP you have already paid for, and it can be a lot. The typical complaint here is that “I have it jetted correctly, done all those other things you said with fuel, etc, but it just stops making power at 6000 rpms”. BTW, if you see one carb’s throttle plates opened fully and the other is not, it means the carbs are not synched well, and you will be giving up power and tractability, big time. Check it periodically, as cables stretch and linkage bends, and the problem can occur over time. Finally, make sure the linkage doesn’t bring in throttle too abruptly. The accelerator pump circuit on an IDF is fairly long, and the gas weighs way more than the air, so if you tip in the throttle too abruptly, you will always get a stumble upon acceleration that you will chase as a bleed back valve or pump jet issue, and never solve. If it feels “jumpy” when driving it around town, this is likely the issue, not the carbs.
Insulators: Put some 4.5mm or 7mm insulators under the carbs to prevent fuel percolation after you shut down or vapor lock while running. The hotter the climate/higher the altitude you live in, the thicker the insulator. In Texas, its 7mms all the way! In SoCal I ran 2mm and was fine. The trade-off is the ability to get a bigger air filter box with more room over the horns. The taller the insulators, the shorter the box will have to be.
Porting: The manifolds we buy rarely actually come with 40 or 44mm bores. They are usually 38mm. This is an immense difference in flow and you are throwing away up to 10% or more of the increased flow you bought with those carbs! Be sure to hog them out at the inlet so that there is no “ledge” in the bore caused by the manifold lip, or the gaskets or the insulators. The same is true on the outlet side of the manifold, so hog them out to increase flow. But DO NOT make the manifold outlet port bigger than the head’s inlet port! Again, no ledges! You will pick up a significant amount of performance that you have otherwise given away.
Jetting: This is the most “mysterious” part of setting up your IDFs, and it doesn’t have to be.
The two most important circuits are the idle jet circuit and the main/air corrector circuits. Since both respond to the signal sent by the venture, your basic jetting is driven by the size of the venturi. So, here is the “secret” formula I learned long ago from an engine builder to get really close as it relates to jetting.
Basic set up for 40 or 44 IDFs:
Emulsion Tubes - F-11 or F-9 emulsion tubes (usually f-11 for 40s and f-9s for 44s)
Aux Venturis – 4.5 for MOST street applications
Venturis – The starting point is a venture that is 8mm smaller than the bore of the carb. So for 40’s it would be 32mm. For 44s, it will be 36mm (for MOST performance street motors). For more torque lower in the rpm range/smoother response/but less max HP on 40s, go to 30mm; for less torque/more max HP, go to 34mm. For 44s’, it would be 34 mm for more torque/less HP, and 38 for max power/less torque. I would not go beyond these limits, and I usually settle for a venture that is 8mm smaller than the carb bore.
Main Jets – Multiply the size of the venture by 4.1, and then by 4.3. This will get you in a range that will be very close to what you need. The more extensively you modify the engine and the larger the size of the engine’s cylinders, the closer you will need to be to the top of the range, and vice versa. So for my 44 IDFs on the coupe, I am running 36mm venturis. So my jet main range is (4.1 x 36) = 147.6, to (4.3 x 36) = 154.8. Because I have made extensive mods to make it breathe deeply at very high rpms and it is a bored out 2.0L, I am running 150s, and it is just about right. I may end up with either 147s, or 152s, but the 150s are just fine, and if I don’t change them, I will be very happy.
Idle Jets – Multiply the venture size by 1.6 and 1.7. So for my 44’s, my idle jet range is (1.6 x 36) = 57.6 to 1.7 x 36 = 61.2. I am currently running 60s, but also like 57s in there. For trip to FFO from Texas, I’ll run the 57s, but when I want to really have fun, I’ll run the 60s.
Air Correctors – Simply add 50 -60 to the size of your main jet if it is between140 -160, and add 60-70 to the main jet size if it is 140 or below. So on my 44s with 150 mains, I am running 200s, but will still get good results with 205s. This will ensure you get a really strong signal when transitioning from the idle to the main/air corrector circuits and get that instantaneous throttle response and relentless pull when accelerating from the 3500-4500 rpm range, to higher rpms.
Needle Valves – 200s will work for all the twin cams) (1438-1995). They usually come with 175s, which will not get those fuel bowels filled under WOT and cause a lean condition. It will still rev well and make all the right noises, but you will be down on HP and not even know it unless you are on a dyno doing comparisons.
Bleed Backs – Start with 00s. They usually come with 55s, but on a modified TC, that will normally result in a stumble on acceleration. Depending on the mods you have made, you may be able to use a 35. I just go with 00s and adjust using the pump jets/accelerator pump rod.
Pump Jets – On our TCs, 55s seem to be a good size. The smaller the jet, the longer it takes to deliver the fuel into the airstream, and vice versa.
Accelerator Pump Rods – These are found on the newer Webers, and can drive you nuts if they are not set equally. I simply back them both out to the end of the lip on the rod, and then turn them each in 10 turns. This seems to be about right as it relates to the compression of the pump. If they are 3 turns or more out from each other, you will get very inconsistent response upon acceleration.
Floats – Set them at 1mm closed (with gasket on the carb top – 10mm without), and 32mm open. Make sure the float carrier is not slightly bent and allowing the float to hit the side of the fuel bowl. This will drive you nuts
Horns – These are really important, and the way the horns are radiused is even more important. I have a chart produced by a guy 35 years ago who air flowed a variety of common horn shapes to empirically show the difference the horn shape makes. I am not at home and don’t have access to it now, but perhaps Alvon can post it (Is sent it to him), because some of our most popular shapes actually reduce air flow! The more room you have over the top of the horns, the better. As a rule, you need as many mm over the horns as the venturis are wide. So for my 44s, I need 36 mm over the horns to get a full air column. They will work with less – this is optimum.
Balancing and Synching – This is a whole different discussion, but after you have the carbs tuned (jetted) right, you have to get this right or all is for naught. You may read in some tuning guides that you don’t have to use the air bleeds to balance the carbs. This is wrong if you in fact you want to get all the cylinders drawing equally, which is one of the benefits of multiple carb set ups. It is very rare to get all the cylinders on a rebuilt engine to draw equally, so use them to optimize the performance of your engine and those carbs you just bought. And use a Synchrometer to synch the carbs. It is almost impossible to get it right with the eyeball or a tube, and it really makes a difference in performance and tractability. Imagine four trumpets playing the same note, but all four are either a little flat or a little sharp – just a little. It will sound like crap. And if your carbs are not CLOSELY balanced and synch’d, your motor will run like those trumpets sound. Get this straight forward tuning step right, and you’ll never get tired of listening to that puppy run.
Again, just one way to approach the conversion to IDFs. There are an awful lot of experienced tuners who may disagree, and there are surely a lot of other approaches, but these observations are made based on a lot of experience and experimenting, and a lot of dyno time observing the differences in power output playing around with all these parameters. If you do all these things, you will get a strong, reliable, smooth and tractable engine that brings your 2200lb car to life and puts a smile on your face, without having to go to a dyno.
At a minimum, it hopefully identifies some issues that frustrate a lot of folks that really have nothing to do with the carbs, but rather the other components involved in the conversion. But then again, sometimes I just like cussing at the cams!
Let’s get some other folks to chime in and see what we can learn.

[bradartigue]

Following the model Alfa uses for their twin-carb applications (virtually all of their European cars) you may want to consider a Bendix type pump or a fuel pressure regulator (if you're using a FIAT mechanical or those cheap square electric pumps).

[sptcoupe]

Ace - can you post that chart showing the flow increases/decreases through the various horn shapes we typically put on or carbs? I think this is an important element in getting the max out of your carbs. I know when we were experimenting with the FI on the coupe, the dyno showed up to 3 RWHP differences between horns! That is a lot. We ended up doing the custom horns in accordance with that chart, and got the best results that way. I won't be at our home in Dallas for another week, so maybe you can put it up on the this thread? Thanks.

[MIGHTY 8]

What an excellent post!

Naturally, I can only understand about 1/4 of it, but I hope to be all the way "there" someday!

That being said, let me ask one quick question that came up recently with my car.

I had the FI-to-40IDF conversion done a few years ago by a mechanic, and I felt that the car was running great-without any problems.

Recently, I had to bring the car into the shop after breaking down (literally right in front of his shop!!) (new mechanic/new city) where it was later discovered that the "set-up" was wrong in the first place.

Let me explain..

The original mechanic had placed a secondary fuel regulator (CSR brand "dial" type) just before the foremost carb in the engine bay. I don't recall which brand of elec. pump was used underneath.

The "new" mechanic stated the problem with was, this CSR brand regulator was in the wrong position..i.e. it needed to be placed after the carbs, in order to be on the return-line side of the system. By not being placed correctly, he deduced that it led to the premature failure of the elec. pump underneath the car, due to it working "overtime" to compensate for their being no regulator to asist in sending the excess fuel back.

Sounded "fishy" to me, because A, the car ran well for so many years/miles without so much as a hiccup, and B, the previous mechanic was not there to "defend" himself. (And he has owned an italian import shop for 20+ years!)

To make a long story even longer.....car is now running great, but I can say I don't really know who to trust at this point.

As a side note, I have played around with this CSR regulator, and as it is set up now, I have noticed that a setting of 1.5 PSI seems to be ideal..

Go figure!

Thanks for hearing out my rant!

G.

[124JOE]

anytime
good info

[sptcoupe]

Mighty 8 - The fuel pressure regulaor and gauge should be placed before the first carb.

I think what the new mechanic may have been saying was that with fuel return system, you have to have to restrict the fuel on the outlet side of the second carb in order for the pump to be able to create sufficient, consistent pressure. If the fuel is not restricted downstream of the last carb on the way back to the fuel tank on a fuel return system, the fuel pump will not be able to keep up and you will have low/inconsistent pressure in the carbs. And your pump will have to work overtime - which could lead to an early demise.

Usually, the restriction is in the form of a plate over the end of the fuel outlet nipple, with a very small hole drilled in it. The larger the hole, the less pressure you will have in the carbs, and vice versa. We are talking about a hole approximately .2mm in size, done with a wire drill. I guess it is feasible to restrict the fuel flow with a regulator of some sort, but I am not sure you will get any consistency, and that will cause problems.

A car can run great for years, and the moment the fuel pump gives up the ghost, it will run poorly or not at all. So it is not all unreasonable to believe that your pump died just when it did.

Unless you have a decent regulator and gauge, you will not know what your real fuel pressure is. I think I mentioned that I put on old gauge and an in-line regulator on the coupe for the initial setup, and the difference between what the regulator was set on and what the gauge read was significant - as much as 2+ psi. That kind of difference on a crab that calls for 2.5 - 2.7 psi is as much as a 100% discrepancy!

If it is running well, I wouldn't do anything but drive it and enjoy it. If you start to get inconsistent performance, then you might look at properly restricting the fuel on the way back to the fuel tank. One way to see if you have fuel starvation (too little pressure) is to run it at a steady 5200 rpms, in second or third, then after about 20 seconds, nail it. If it surges at steady throtlle and falls on its nose when you nail it, you may have fuel starvation. Of course, this won't tell you if you have excess pressure. To test that, you have to run it at 5000 + rpms for at least three miles on the freeway loaded up - in third or fourth. If it starts to stumble a bit after a few miles, it may be thaty our fuel is foaming due to excess fuel pressure.

Glad it is running well. Don't over think this stuff - just enjoy the ride.

[bradartigue]

You can use a carburetor main jet as the restriction for fuel return, install it in-line. If you ever look at the size of the return nipple on, say, a DHSA, you'll realize that the nipple is 6mm or so but the orifice is like .50 mm.

[WYSpider]

deleted

[oebb]

This comprehensive info was really great! The fuel pump info made me install an electric pump instead of the stock mechanic. This really made a difference. The carbs run much better, specially on lower revs. My setup is 2.0l w stock exhaust but open mufflers, IDF 40 and 285V (285 duration .385 lift) cams from Marc. It now pulls away great from 2000 - 4500 but the power goes away above 4500 (even if it revs higher but more reluctantly). I think my jetting should be OK (see below), any idea on where to look for more output of the higer revs?

Cheers and once more - this post was fantastic!

/Mikael (Sweden)

Jetting:
Main venturi 32
Aux vent 4.5
Main jet 140
Air Corr 210
Emulsion F11
Pumps jet 40
Bleedback 60

[sptcoupe]

You didn't mention your idle jets size, which will effect the transition signal toi your main jet circuit. Do you have a header on it? WHat is your total igniiton advance?

[oebb]

Hi again.
Head is more or less stock. I checked the idle jets and they are 55 - guess that 55 should be Ok. I also rechecked my timing and it's around 15 degrees in low idle, but I noticed it did only advance very little (like 5 degrees) in higher revs. I hooked up a hose to the vacuum capsule and I could blow through it with a little resistance . I guess this is my main problem - a blown vacuum advance. Will get a new one ASAP. Thanks a mill for the hint!

/Mikael

[sptcoupe]

Just a couple of follow-up notes.

When you are balancing the indiviual bores using the air corrector screws, listen carefully as you back out the screws to bring the air flow up to match the sister bore. If you hear a hiss, you have backed it out to the point that the air flow is no longer being metered correctly and you will get inconsistent performance. So, if you hear a hiss, stop there and turn it back in to the point that the hiss has gone away, and set it there - even if the flow is slightly different between the two bores. This way you will have consistent performance. If you keep backing it out until the bores are balanced, you will have inconsistent performance.

If you find you have to back it out way past the point you begin to hear the hissing in order to get them balanced, then you probably have other issues with that cylinder.

I often get asked how you chose between an F11 and F9 emulsion tube. The difference essentially is that an F11 will be richer at the top end, and leaner in the middle ranges. An F9 will be just the opposite, and run richer in the mid range, and leaner on the top.

The choice can mean a big difference in jetting. For instance, on the coupe with its highly modifed engine and running 44's with F11 emulsions, I have to run 57s or 60 idle jets to get the AFR ration around the 12.7-13.0 range in normal driving. But, it seems to be too rich at the top end, and if I go to bigger air correctors and smaller mains to correct that, I lose some pretty big power at the top. With F9s on the car, I have to run 47 or 50 idles to get max power and the right AFR in the mid-range, but I can keep the larger mains to maintain the top end power at the correct AFR.

The beauty of the carbs is that they can be very closely tuned to your engine and your driving style becuase of the wide range of jet combos you have with the IDFs, and going between an F11 and F9 is about the equivalent of doubling your options.

[MIGHTY 8]

test

[img]6257751666_2e4f49d992_m.jpg[/img]

http://www.flickr.com/photos/dinahmp/62 ... hotostream

Well, after some repeated attempts at posting a picture of my fuel pressure regulator set-up, it appears I was able to only post the link!

Oh well..

Point is to display the "new" location of the fuel pressure regulator.

From my previous post on this link, I mentioned that during the original set-up from a few years ago, the mechanic had installed a "dial type" CSR brand fuel pressure regulator towards the upper left part of the picture. What is displayed now, is not only the new Inglese/Holley brand recommended from this link, but it's new location which is 180 d. off from the original set-up.

Need some confirmation here please..

[sptcoupe]

I ahve seen this type of fuel routing on a number of cars, and while I have no personal experience with this set up, I recommend that you take the fuel fromn the regulator outlet and run it the front of carb #1, straight through to the inlet on carb #2, and run the return line from the back of carb #2. This eliminates two, 90 degree turns in the fuel flow. Inother words, to the extent you can, run the fuel the same way you would look at sir flow. No sharp turns, and no turbulence created if possible.

http://i721.photobucket.com/albums/ww21 ... 24/020.jpg

This is a photo of the coupe's current set up. We were just setting it up at this point, and the hose from the pressure regulator to the inlet at the fron of caqrb # 1 in not there, but you can see the inter-carb flow and the return line exiting from the back of carb #2.

Also, most hot rodders will tell you to mount the regulator so the the gauge is paralell to the ground, so that you have to tip it up a bit to read the pressure, or use a mirror. This will give you a more accurrate/consistent reading with most regulators. I'll try to find a pic and post later.

[fiat218]

i can't wait to get my engine over hauled and use the twin carbs
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