IDF Initial Setup and Adjustment

There is nothing particularly difficult in setting up your IDFs if they are working correctly. If you have a faulty carb or vacuum leak somewhere, however; you will never be able to get them to run right, so be sure that you are working with known good carbs. Most of these units are well over 30 years old and if you are not sure of their condition, it would be well to have them gone through by someone familiar with their operation and quirks.

There is nothing like spending your whole Saturday trying to chase down a lumpy idle and find out one of your jets is clogged. Generally these carbs and linkages don't really require a lot of attention. Once they are adjusted they tend to stay that way for a long time. frequent driveability problems point to the before mentioned vac. leaks or other issues. BTW the best insurance that your settings will stay is to make sure that there's no crud going to your carbs. Good filters and clean gas are absolutely required. With four individual throats that is just four more possibilities that something is going to get caught in a jet.

Tuning & Adjustment

Be sure for initial carburetor set up all air by-pass screws should be in closed position. These are not commonly used in standard carburetor adjustment. You will adjust them to ballance each individual carb throat later after you have ballanced both carbs. This is the "final" touch for your best idle.

Set the idle speed screw at º to turn in after contact with throttle lever. When doing multiple carburetors all linkage should be disconnected. All carbs should be bench adjusted to same setting.

Set the idle mixture screw to one turn out form lightly seated. When checking the seated position to make only light contact with seat, aggressive seating will damage needle and seat of carburetor.

Start engine as long as engine starts and runs do not set up idle speed first. First adjustment if possible should be to find smoothest idle with each mixture screw on all carburetors. Some prefer to do one barrel of each carburetor then come back and do the second barrel

After preliminary lean best setting of idle. Check carburetors for synchronization. Commonly done by checking lead or front barrel of each carburetor.

You will always want to bring high flow carburetor down to match the low flow carburetor. If this cannot be done you will need to recheck bench assembly for binding throttle in high flow carb. Once you have matched both carburetors you will need to set the idle to the desired idle speed setting. This will be done by adjusting both carbs up or down the same amount and re checking for synchronization.Make one last check of lean best (smoothest running position) idle on all mixture screws one last time.

Best idle should end up with the mixture screws at or near one turn off seat. If you are much off of that setting, that is an indicator that you should go to the next richer idle jet, but unless you are over 1,000ft above sea level, I would suspect valve clearance or ignition problems (plugs are always suspect during these sessions and having a fresh set available may save you a lot of time and frustration).

If your car doesn't want to idle smoothly or runs rough at partial throttle or the Idle mixture screw is more than one turn out, you may have the wrong jets installed in your carbs. Here are the settings that came in mine, and I've confirmed from a number of sources that these are the "stock" factory jets. Often the idles are too rich which causes all sorts of problems yet cures the stumble. A bad compromise. Rich idle jets foul plugs quickly and cause rough partial throttle running below 50mph. Check your plugs frequently after you first install your carbs. Black plugs mean a smaller jet is indicated. White is too lean, tan is just right.

Factory IDF 13/15 40mm w/32mm venturi. Stock Settings: Jet sizes: Idle .55, Main Jet: 125, Air corrector: 210. I know that some of you have the "Volkswagen" IDF-70s. You will have to check with a vendor for the correct jet sizes. Alternately Mike Richmond recommends .50 idle, 120 mains and 185 air correctors. Links to his and other articles are below.

These are excellent starting points, and if your accelerator pumps are functioning (they should be on the highest, tightest, setting) your carbs should work fine with either of these sets of jets up to about 600' above sea level and probably higher.

If you use an electric pump to feed these beasts be sure to regulate it to about 3-4 lbs of pressure. Too much pressure can cause massive flooding. The floats should be set with the proper tool, but a 3/8" drill bit will get you started. If you look down the throats when the car is idling and see fuel coming from the center jets, you have the float too high.

Finally, these carbs have ball bearing throttle shafts and it is common for the grease to get hard and stiff. This causes two problems, the shafts can stick, and it allows air to leak around the throttle shafts ruining your attempt to tune the carbs. If you are cleaning or rebuilding the carbs be sure to take off the linkage and smear some teflon grease into the bearing to be sure it isn't leaking.

If you want some background information about these carbs, there is more information about IDFs on Mirafiori by Mike here and Bradley Artique on twincam modifications here. Both articles are interesting reading.

Comming Soon: Fine Tuning, Adjusting the carbs using a syncrometer.

Addendum: Weber Carb jetting info.

A special thank you to Andy Jossy on the Mirafiori.com forum for finding this on the internet. There is a credit on the page about it's origin. There is jetting information about DCNFs and a couple of IDF configuration.