Service Procedures
Buyer's Guide

Carb Synch Tutorial: Ferrari 308 series
By The Birdman

If you are reading this you probably already know what carburetor synchronization is, but just to be complete, it is getting all the carbs working together as a matched set so that they all feed the same mixture to the engine. Since on a 308 there are 4 carbs (each with two barrels) and they feed different cylinders, if one carb is not adjusted the same as the others, different cylinders will be getting a different mixture and that makes your car run poorly. Synching involves setting the linkages between the carbs so they all work together at the same rate, and also usually setting the mixtures on each carb as well.

Before you sync the carbs you should be sure that the floats are set right (they control the fuel level in the bowls of the carbs) and the jets are clean. Checking the floats requires taking the tops of the carbs off and measuring them. This is outside the scope of this tutorial but here is the procedure. You should pull the main jets and the idle jets first. They can be visually inspected, or soaked overnight in fuel injector cleaner just to be sure they are clear, then re-installed. If you have a very hard time getting your carbs synched, you may have an air leak someplace such as a manifold gasket or vacuum line. The whole procedure should not take more than a couple hours, even for someone that has never done it. I recommend that you do this procedure outside, because it involves running the car for a while and standing over the engine compartment!

The major adjustments you will make are to the air bypass screws, idle mixture screws, throttle stop screws and linkage. First, here are some pictures of what you will see when you pull off the airbox.

STEP 1: Start and warm the car. The best way is a short drive.

To get the airbox off you need to pull the cover and the element, then remove the 16 nuts holding the "trumpets" down. Remove the trumpets and the airbox will lift off. You also have to remove the crankcase breather hose from underneath the airbox as you remove it. Be careful not to drop anything down the throats of the carbs!

Above is what you see when you take off the airbox. I know my carbs were dirty, but I took these pics before I rebuilt my carbs.

A carb synch is an excellent time to change the fuel lines to your carbs if they are old. Your local NAPA dealer can sell you standard 5/16" fuel line that fits the carbs perfectly. With a handful of hose clamps and $10 worth of fuel line you too can prevent engine fires. (My hose is covered with a plastic covering made for electrical harnesses. I put it on there to sheild the hose from heat a little.)

Looking closely at the top of the carbs, you can see the air corrector jets. Unscrew each of these and it comes out with an emulsion tube and main jet at the bottom. Soak overnight in carb cleaner to be sure they are nice and clean. BE VERY CAREFUL not to drop anything down the throats of the carbs. Some people put duct tape over the openings, not a bad idea.

This shows the location of the idle jet. There are two idle jets on each carb, one for each barrel. They are on opposite sides. Each has an o-ring sealing it. Typically, the ancient o-rings are cracked and/or crushed. Take one to your local hardware store and go through the o-ring bin to find replacements of the same size. Soak the jets overnight in carb cleaner and re-install. Use a little saliva on the o-rings as lube to keep them from binding as they go in. Don't use petroleum products as lube...you don't want it to gum up the jet.

These are some of the important carb adjustment points that will be referenced in this article: the air bypass screws and the idle mixture screws. The air bypass screws have lock nuts that must be loosened to adjust the air bypass screws themselves. The Idle Mixture screws have springs to keep them from turning on their own.

This pic is post-rebuild. Ooooh, look how clean it is!

There is more than one way you can go about synchronizing the carbs. I'm going to outline the procedure described in the 308 GT4 service manual, which applies to all 308 GT4/GTB/GTS series cars. Please note that this is not only the way Ferrari recommends, but it is the most logical approach to this process and I recommend that you do it the way this is outlined. I have heard all kinds of strange stories about using the air bypass screws to do the synch, etc. and these are wrong. Air bypass screws are not for synching all the carbs, they are only for synching the two barrels in each carb to each other. I'll get to that. Just follow this procedure and you can't go wrong, assuming your floats are set right, your jets are the right size and clean, and there are no air leaks. Honest, this isn't rocket science.

Once you get the airbox off and before you start tweaking, start the car up again and re-warm the engine. Do not go driving around without an airbox or air filter. At best it's not good for the car because dirt gets into the engine, at worst, your car can catch on fire because the air box is a spark suppressor. Yes, I actually know someone who had his car burn to the ground because he drove it without the airbox and it caught on fire.

STEP 3: Close all air bypass screws (unlock the locking nut and screw in the adjuster all the way).

STEP 4: Open all idle mixture screws 3 turns from closed (a good rough starting point). This means: close them all from where ever they were set, then open them again 3 full turns (360 degrees each turn) from closed.

STEP 5: Increase the engine RPM to 1500 (about) by using the throttle cable adjuster (not throttle stop screws on left side carbs) . You want the speed at this point to be set by the throttle cable pulling on the linkage ONLY. The throttle stop screws in the carbs should not touch anything. What you are doing here is making sure that the carbs are all being pulled open by the throttle cable. Some people use a piece of cardboard in the fast idle device to do this, but using the cable adjuster is much more precise.

STEP 6: To be sure that your throttle stop screws are not touching the linkage, back them out a few turns. Now your throttle linkage is being held open completely by the throttle cable, and you can adjust the linkage to synch the carbs. Good Lord that is one dirty carb!

At this point, you need your synchrometer (AKA flowmeter). This is an inexpensive tool you can buy in a number of places that just gives a reading of the amount of airflow through one throat of a carb. It's just a vane that moves a needle in the airflow.

You can get one here (among other places):

I find Weber Carbs Direct to be a good place to get carb related stuff including jets. They have good prices and fast shipping. The one you need is model SK.

STEP 7: Now is the tricky part. Using the flow meter, you need to tweak the linkage between the carbs to get them all to flow at the same rate. (Note in this pic, the engine is off for photos, so the flow is zero).

This is tricky because as you adjust, the speed of the engine will change, so you have to keep going back and forth from carb to carb with the meter to check. At this engine speed, the air bypass screws don't make much difference, so you can ignore their contribution to flow. Just worry about the linkage and how it sets the position of the butterfly valves from one carb to the next.

Start by synching the rear pair of carbs to each other. Here is the adjustment mechanism (one screw). Adjust this until the rear pair of carbs has the same flow. Then, adjust the front pair to each other with the front linkage that looks just like this one.

Then sync the front pair to the rear pair using the linkage shown here. Unlock both locking screws and the rod in the middle turns like a turnbuckle. It is threaded in the opposite direction on each end so turning it one way forces the linkage apart and turning the other way pulls it together. This is a very sensitive operation. A small adjustment makes a big difference. At about 1500 RPM you will get readings in the 7 kg/hr range. (roughly, this is not critical). Note that there is an upper and a lower adjustment rod, and either one will work for this operation. You do not need to adjust both. They each do the same thing: adjust the amount of linkage between the front and rear banks of carbs.

STEP 8: Once everything is pretty close at 1500 RPM, you are now done with the linkage adjustments and will not need to touch them again. Now we adjust the synch at idle. To do this, adjust the throttle cable adjuster in again to drop the engine speed down to closer to idle (900 RPM). Keep in mind that since your idle screws (throttle stop screws) are backed way out of the way, if you loosen the throttle cable too much, the engine will stall. The next step is to get it so that the butterfly valves are now resting on the throttle stop screws and the throttle cable is loose with a tiny bit of slack so it is not affecting the idle speed. You adjust the idle speed by setting the throttle stop screws on the front and back banks at the same point, so the engine idles at about 900 RPM.

You can get a pretty serious imbalance between the front and back carb flow at idle if you get the throttle stop screws unevenly set. They need to be set so that they each are holding the bank a little because there is enough play in the linkage that if only one is doing the holding, the other bank will be very different. Use the syncrometer to get the flow the same at idle, while holding the idle speed at about 900. This takes time. The flow at idle will be around 3.5 to 4 kg/hr per barrel. If you get a lot of sputtering and backfiring, making the idle erratic and hard to adjust, open the mixture screws another turn or two. If the mixture is too lean at idle the carbs will spit and the idle will be irratic. (If this still happens with the idle screws all the way out, read below my comments on idle jets).

Once the carbs are close at idle (you will never get them perfect so try not to be too anal retentive about it!) you can adjust the air bypass screws. Many people are confused about what these are for. These screws are used only to adjust the barrels on each carb to each other. They work by allowing some air to bypass the butterfly valve so that you can get the flow matched between the two barrels of a single carb. So what you do is pick a carburetor and measure the flow between the left and right barrel. Take the one that has the lower flow and open the air bypass screw on that barrel until the flow is matched to the other barrel, then lock them both with the locking nuts. One air bypass screw always stays closed. On any carb, there will always be one air bypass screw that is closed and MAYBE the other will be open, because you only use the one on the barrel that has the lower flow. Please note that if the flow is pretty close between the two barrels, you can leave both air bypass screws closed, which is the optimum way for them to be set. On my 308, out of 8 bypass screws, only one is open. Once they are set, remember to lock them down (gently!) with the lock nuts. Never tighten anything too tightly on these carbs. They are made of soft aluminum and it's very easy to strip threads.

Idle mixture is tough to set without a CO monitor. In general, if the car is spitting, backfiring or idling poorly, they are usually too lean. For some reason, 308 engines tend to idle better rich, so most owners end up having them run a tad rich at idle if emissions compliance isn't a factor. The general procedure without a CO monitor is to open the idle mixture screws, one at a time, while listening to the engine and monitoring the flow. As you open the screw (enrichen the mixture) the engine will run faster and faster, as that cylinder gets richer and richer, until it will actually start to get slower as you continue to open the screw and the mixture gets way too rich. You want to open it until the engine just starts to run slower (way too rich), then close the screw about 1/4-1/2 turn. Because they used different lengths of tapers on these adjuster screws at different times, there is no definitive way to say where they should be set, but most 308 owners report setting them between 2 and 5 turns out from fully closed. Now repeat this procedure with each mixture screw.

Once you start adjusting the mixture at idle, the idle speed will start going all over. Now you have to go back and tweak the idle speed again using the flowmeter and the throttle stop screws. Everything affects everything else, so this process can take an hour or so. When the carbs are synched, a 308 will idle perfectly, like a purring cat.

Turn the engine off and push the gas pedal to the floor several times to be sure that the throttle doesn't stick open and it returns to the throttle stop each time. Put the airbox back on, start it back up, and take it for a drive. Your car should be idling at about 900 RPM with all the flows the same and it should run smoothly to redline. It should run a lot better if the car was out of whack.

Contrary to popular belief, the idle jets in these Webers do more than provide the fuel at idle. The idle jets feed the idle orifice as well as the progression holes. These progression holes are tiny orifices that get uncovered by the butterfly valve as it opens. They "progressively" provide more fuel during that very "touchy" phase between the closed (idling) throttle and the point where the main jets take over. During around town driving, where your foot is on and off the throttle, most of the fuel is being supplied by the progression holes, via the idle jets, not the main jets. Opinions vary on this, but most people agree that the main jets don't start to make a significant contribution to power until between 3000-4000 RPM. What does this mean? It means that the idle jets make a big difference in how your car runs. The idle mixture screws only adjust how much fuel reaches the first progression hole ("idle orifice") which meters fuel for idling only. As you are driving around town, the idle jets provide most of the fuel to keep your engine running, and it is not adjustable with the idle mixture screws. What I'm telling you is that if your car runs well at high RPM but not as well at low RPM, the idle jets need to be looked at. They could be clogged, or too small. The late model carb 308 series (1978 and later) were fitted with small idle jets to make them run lean to pass emissions. They often pop and misfire at low RPM. Many people think that by opening up the idle mixture screws they can richen up the mixture, but it only affects the first progression hole, and rarely helps the performance of the car in actual driving. Going up a size (for example from a .50 to a .55 or from a .55 to a .60) idle jet often works wonders. It made my car run like a whole different car for $25 in new jets. I have also heard something from more than one carb expert about modern fuels needing bigger jets than '70's gasoline and that even early carb 308's often benefit from a slightly richer jetting. My car is a '77 and I went from .55 (stock) to .60 jets and it completely cured the popping and misfiring instantly. My car also prefers 1.40 Main jets to the stock 1.35s.

A lot of 308 owners report this issue. In my experiments, it seems totally related to idle speed. If your idle speed is too high, it will backfire when you shut it off, no matter how well your engine is tuned, how well the carbs are synched or how perfect your jetting and timing. It is my experience that since a lot of 308s are not properly synched/jetted, they idle poorly. Becuase they idle poorly, the idle speed is set high to compensate. The result is backfiring. You can test this yourself. If your car backfires, put the car in gear and use the clutch to slow the engine to below 1000 RPM just as you turn off the key. It won't backfire. All you have to do to cure the backfire is synch the carbs, set the idle at 900 where it belongs, and it should go away. The problem seems aggravated by the fact that 308s like to be jetted a little rich. If you lean them down until they don't backfire, they often don't run very well.

Confused about what all the stuff on the carbs does? Read this excellent article which explains it.

THE FINE PRINT: This service procedure is provided as the personal experience of the author who is in no way an expert, so take this advice with what you paid for it. I am not liable if you blow yourself up!

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