Carl's Tech Tip #5: Carburettor Synchronisation

Carburettor Synchronisation

Today we are going to discuss carburettor synchronisation. Although this article is specifically aimed at the 3.0 litre V-8 engine, the principles are the same irrespective of the number of cylinders or carburettors. There is nothing complicated or mysterious about this procedure, so don't be put off. Despite what many claim, it is not a "black art". However, the job is time consuming and you will need to be patient.

Only a few tools are required:
8mm Ring spanner.
10mm Open ended spanner
Long flat head screwdriver, say 40cm.
Carburettor flow meter. (I use the SK-1 Synchrometer as it comes supplied with the downdraft Weber adapter)

Some preliminary service items to consider: Make sure your car has a fresh air cleaner. (I use a K&N, and despite the claims there was no HP increase). Check your ignition timing, make sure all your leads, plugs, points, distributor cap(s) are healthy too. A fresh set of plugs won't go astray. I use Beru UX -56's with great results. This will also give you a chance to see how the engine is running. The plugs should be a slight grey/yellow colour with no oil or wet deposits. Finally, and this is a pain, but your valve clearances and cam timing must be exact (extremely important).

Obviously, this is a fairly labour intensive job. The valve clearances are self-explanatory. However most people don't pay much attention to the cam timing. When you remove the cam covers, the stamped index marks on the cams must match exactly with the index mark on the bearing caps at top dead centre. As little as 1mm off will affect the breathing of the engine. On pre 1978 cars the cam pulleys had a 3 position vernier that made the timing adjustment difficult to get exact. The later cars had a 5 position vernier. Finally, your carburettors must be healthy. Check the float levels and make sure they are exact. (See diagram One for further information). Also, make sure the spindle bearings are not worn. If they are, synchronisation will be impossible. Under high manifold vacuum (i.e. idle) they will draw "false air" and lean off the mixture. You may wish to install sealed bearings because the fuel vapour does not dissolve the lubricant.

When we synchronise carburettors, we are adjusting them to draw the same amount of air for each of the eight throats. However, above about 1,800 rpm, synchronising has no effect so it won't improve running above this speed. What most mechanics don't consider is that we need to synchronise them not just at idle, but also at about 1,500 rpm. The reason we do this at 1,500 rpm is so that there is a smooth transition from low rpmÕs without any "stuttering" or "fluffiness."

You will need a helper to do this efficiently! Remove the rear deck (GTB's only, two catches on the hinges and don't forget to pull the pin out of the rear support first!). With the air cleaner and bottom section of the air box removed (Be careful not to drop the small 8mm nuts down the throats). Warm the engine to operating temperature being careful not to let it idle for too long otherwise you risk loading up the plugs. If you have just re-built your carburettors and want a starting position for the idle mixture, turn all the idle mixture screws fully inward, then slacken them off 3 turns (Diagram Two, C). Disconnect the throttle linkage mechanism from the cable by removing the rods that run from the pivot to the carb bodies. Completely slacken off the interconnecting synchronising mechanism that joins the carbs side by side as well. (Diagram Two, B). With your SK-1 and the linkage still disconnected, run the car at 1,000 rpm by turning the four idle stop screws on each of the carburettor bodies. (Diagram Two, A) Obviously each carb will be a little different but that doesn't matter at this stage. So, we should have your engine at 1,000 rpm. It doesn't matter which carb we do first but we have to make sure that each of the two throats are drawing the same amount of air as it's partner, but only for each carburettor at this stage. Swap the SK-1 between each throat and if there is any discrepancy, adjust the lower of the two readings up by undoing the depression balance screw on the carburettor body. (Diagram Two, D)

If the depression balance screw is nearly all the way out, you can then screw in the opposing throat screw to bring the reading down. The only carb that you will probably have to do this to is the rear left. This is because it is the venturi that supplies the vacuum for the brake booster so there will be a greater discrepancy between the left and right sides.

You may now find that the revs are slowly increasing and if so, just slacken off the idle stop screw on the carburettor body a little. If the idle stop screw is already loosened as far as it will go, screw in both depression balancing screws to bring the speed down, then increase it with the idle stop screw and re-synchronise.

OK, so now we have each carburettor body (i.e. left and right throats) drawing in the same amount of air (however compared to each carburettor they will probably be different). Now adjust the idle stop screws on the carb body so that the four carbs now have the same reading. You may have, for example, 3 carbs reading the same with the fourth too high and with the idle stop screw in the fully "out" position. If so, tighten both depression balancing screws to bring the reading a little lower than the other three carbs. Synchronise both venturis again, and then bring the reading up with the idle stop screw on the carb body to the reading of the other three. Now you should have not only each venturi per carburettor reading the same, but each carburettor should also read the same. After you adjust the idle stop screw, check each venturi again and adjust if necessary. Finally, as the carbs are connected side to side, take up the slack in the adjacent synchronising mechanism but only until the screw rests against the stop, and no further!

Now we have the idle synchronised.......great! Take a look now at the ball jointed throttle linkage. Clean the joints on the carbs and clean the old grease out of the ball sockets. Regrease the ball sockets and do the same for the cable and union where the front and rear ball linkages meet. This pivot runs on needle bearings so it is a good idea to regrease this too. Replace the linkages and adjust the ball joints until they bind, then back off 1/8th. turn and replace the locking pin. However, during this exercise, it is absolutely imperative that the linkage does not in any way move the carburettors off their idle setting. The way to do this is to loosen the locking nuts on the linkage shafts with an 8mm spanner and turn them until you can just feel the slack being taken up, and no more! You may wish to slacken off the throttle cable for this exercise. After you are satisfied that the carburettors are in the idle position with the linkages attached, start the car and check again that the idle is 1,000 rpm and the 8 venturis are drawing in the same amount of air. If not, the linkage has moved the butterflies so it will need to be slackened. Ensure this exercise is also done whilst the engine is hot otherwise the cold start device will be brought into operation and will make accurate idle speed adjustment impossible.

Now adjust the throttle cable so that it is firm. Pull the throttle pedal all the way back with your hand and give it a good pull from the engine side. Now take up the slack with the adjustment screw on top of the cam cover. Start the car and have your helper take the revs up to 1,500 with the pedal, i.e. not by pulling the linkage in the engine compartment. With the SK-1, check again the airflow. You may discover there is a slight difference between the venturi's now. A slight difference is OK, a big one is not. If there is a big difference, suspect a vacuum leak somewhere in the carb body. Make sure the front and rear banks are drawing the same air (This is important). If not, you will need to juggle the position of the throttle linkage rods until the front and rear banks are the same.

The last adjustment to check is to make sure the throttles are opening wide. Remove the fuel pump fuse (fuse 3, LHS box) and run the car until it stops. This will allow you to floor the throttle without filling the combustion chambers up with raw fuel. Have your helper floor the throttle and check the butterflies are vertical, and return to the fully closed position with no binding.

So there you have it! Not too hard but a little time consuming. They only other thing you may want to do is to set the idle mixture up with a Co meter. This only makes a difference at idle of course (as to change the mixture off idle you need to change the jets). My Co is set to give the smoothest idle which is about 6%. Some people claim that you can set the idle mixture by slowly closing the idle mixture screws until that cylinder stops firing then open them 1/2 turn. In theory this method sounds fine but I have never tried it. If you have a European spec car, these models do not have the gas analysis ports on the exhaust manifolds so your Co can only be set up by estimation. The probe can therefore only be inserted in the tailpipe where the gases are mixed from the manifold. I really donÕt think the Co adjustment is a big issue though, adjust the mixture to give the smoothest idle and that will be fine.

Good luck! Carl.


(If you read this far: Sorry for the diagrams still missing. They will be added asap. Robert)

Text for diagrams:
Diagram One: Float Level Adjustment: Make sure the Needle valve is tight in its thread (V). Hold the carburettor cover (C) in a vertical position with the float at the bottom so that the small tag on the float (Lc) gently rests on the needle valve (V) without compressing the ball (Sf). The base of the float must be 48mm from the carburettor cover without the gasket. If not, bend tab (L) to adjust. Check that the float travel (G) is 10.5mm (or 58.5mm from the carburettor cover without gasket). If not, bend tab (A) for adjustment. Tab (Lc) must be parallel to the carburettor cover.

Diagram Two:
A: Idle stop screw
B: Synchronising screw for adjacent carburettor
C: Idle mixture screw
D: Depression balancing (or bypass) screw.


Tech Tip #1: General Remarks on Maintenance

Tech Tip #2: The Brake System

Tech Tip #3: The Cooling System

Tech Tip #4: The Electrical System

Tech Tip #6: Water Pump

Tech Tip #7: Suspension

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