Carb synch tutorial for 308

Hi Everyone,
Having just gone through and synched the carbs in my 308, I thought I would write up a short "how to" tutorial. I was a bit apprehensive about doing this job myself and got a quote to have it done by a local mechanic for $400. My wife said "Four hundred bucks? Can't you do that yourself?" Then I needed to save face and learn how. It's really not that difficult.

If you are reading this you probably know what carburetor synchronization is already, 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 tweaked the same as the others, different cylinders will be getting a different mixture and that makes your car run poorly. Synching involves setting the mixtures on each carb, as well as setting the linkages between the carbs so they all work together at the same rate.

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 is difficult because you have to take the carbs apart. If you are able to sync everything fine, you probably don't need to check the floats. But checking the jets is easy, so it's worth doing. You can pull the main jets and the idle jets easily. They can be visually inspected, or soaked overnight in fuel injector cleaner just to be sure.

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. 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.

This is what you will see…4 carbs. Please forgive the disgusting nasty dirty carbs in my pictures. They work, so I have no interest in taking them apart to clean the outside and make them look pretty.

 

Looking closely at the tops of the carbs, you will see the air corrector jets. Unscrew each of these and it comes out with the emulsion tube and the main jet on the bottom. They can be inspected and/or cleaned, then reinstalled.

 

This shot shows the location of the left idle jet. There are 2 idle jets on each carb, one for each barrel. They are on opposite sides. Each has an o-ring sealing it. If the o-rings are old and cracked, check your local hardware store. Most have a large section with drawers full of screws and misc hardware including a nice selection of o-rings. Find one that is the same size and replace them so they don’t leak.

 

This shot shows some of the important parts of the carb for sync. The air bypass screws and the idle mixture screws.

 

There are many ways to do a sync and everyone has their favorite way. I'm going to outline the procedure described in the 308 GT4 service manual, which applies to all 308 series cars.

First, "normalize" all the carbs so they are starting at the same point:

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

2. Open all idle mixture screws 3 turns from closed ( a good rough starting point).

3. Start and warm the car. The best way is a short drive. Keep in mind you have no air filter so not too much driving and no dusty roads.

4. 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.

 

5. Back the throttle stop screws way out so they are doing nothing and not interfering with any linkage adjustment.

 

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

http://www.webercarbsdirect.com/inc/pdetail?v=1&pid=794

 

6. 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 syncing the rear pair of carbs to each other. Here is the adjustment mechanism (one screw).

 

Next, sync the front pair to each other. It has the same adjustment linkage, but it's a little harder to reach at the front of the engine bay.

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).

 

Once everything is pretty close at 1500 RPM, adjust the throttle cable adjuster in again to drop the engine speed down to closer to idle. Keep in mind that since your idle screws (throttle stop screws) are way out of the way, if you loosen the throttle cable too much, the engine will stall. You need 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.

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 carb. So what you do is pick a carb 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 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. On my car, out of 8 bypass screws, only one is open. Once they are set, remember to lock them down with the lock screws. Never tighten anything too tightly on these carbs. They are made of soft aluminum and it's 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. The general procedure is to open them, one at a time and listen to the engine and monitor the flow. As you open the screw (enrichen the mixture) the engine will run faster and faster, until it will actually start to get slower as you continue to open the screw. You want to open it until the engine just starts to run slower, then close the screw about 1/4 turn. Because they used different lengths of tapers on these adjusters at different times, there is no way to say where they should be set, but most 308 owners report setting them between 2 and 5 turns from fully closed.

Once you start adjusting the mixture at idle, the idle speed will start going all over. Now you have to 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.

Well, you are done! Your car should be idling at about 900 RPM with all the flows the same and it should run smoothly to redline. Take a quick spin to make sure it runs right (it should run a lot better if the car was out of whack) and then put the airbox back on.

Hope this helps.

Birdman

 

I heard that Rob was going to create an area on the site just for service procedures like this but I have no idea where that stands. Of course if you KNOW this post exists, you will be able to find it in a search of the archives, but you are right months or years from now, few people will find it.

 

Sean, I agree. Those are really handy for all kinds of jobs where a socket won't quite fit. I found that a small 1/4" drive socket set makes fast work of those nuts too. I use the 1/4" drive set actually a lot more on the Ferrari than I use the 3/8" drive set just because the socket wrench is so much more compact.

All: Thanks for the kudos. I did this post thinking that it would've saved me a lot of time to read a post like this before I did my synch. Nonetheless, learning how it all works so I understood what I was doing and why was pretty interesting and rewarding. This is definitely one of those jobs that intimidates people a lot more than it should. It's really pretty darned simple, and all you need is a synchrometer that is $60.

I forgot to take pictures of what the jets look like though, so someone else should add those shots!

Birdman

 

I was thinking how much that would suck if that happened! I was just really really careful when handling the nuts, but tape is a very good idea.

 

Here's a shot of the idle jets. I stole this picture from FChat member 308GTS (from a previous post). Thanks man!