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Published: May 11, 2012

Fitting Guide for twin Carbs..

Advantages of twin carbs are that they are a very cheap form of modification (a whole conversion can be done for around £400 or maybe less!). They look and sound great and can give power increases of around 30bhp, and can also be tuned for ‘max performance’ or ‘max economy’ making them ideal to most car modifiers.

Typical parts needed:

· Twin Carbs.

· Air Trumpets & Filters.

· Throttle Linkage.

· Low Pressure Fuel Pump.

· Inlet Manifold.

· Fuel Regulator & Filter.

· Anti Vibration Fitting kit.

Step 1: Remove original Inlet manifold.

Remove the original air filter, inlet manifold and injection system. This is best & most easily done by following the instructions as per a Haynes manual.

Note* Once done it may be best to tie up the unused wiring with cable ties somewhere tidy~

Or if you are good with electrics then cut away all unused wiring (e.g. injector wires) to make the engine bay easier to work around but beware-the ignition system still requires a lot of these wires so be careful which ones you cut!!

If you’re original throttle body had water pipes connected to it then connect these in series. Step 2: Mounting carbs on your ‘New manifold’.

Your Manifold should look similar to this.

And your anti vibration sealing kit should look like :

You will need to connect your brake servo to one of the holes in the manifold. The studs (which should be supplied with your manifold) should be screwed in with preferably thread lock. Once all the anti rattle seals have been put on, the carbs can be mounted on the manifold:

Step 3: Now the carbs & manifold can be located on the cylinder head (note* use a new ‘inlet manifold gasket).

Secure all the nuts & connect up the distributor and brake servo vacuum pipes. (You may need to buy some pipe and‘t’ pieces to connect up the distributor pipe).

Step 4: Throttle Linkage: These are available in many different types e.g.:

Once fitted it should look like this:

Step 4 cont… Connecting throttle cable

It is always best to buy new throttle cables when fitting your new Carbs & these are available for around £8-13 depending if you choose single or twin (Our linkages come supplied with twin cables).

Step 6: Fuel issues. This is probably the biggest part of the whole installation if your car had fuel injection (although it may be best to also use a new pump even if you have a Carb fuel pump). Because fuel injection systems needs fuel pressures of around 50-70psi, and carbs cannot take anymore than 5psi max, your original high-pressure fuel pump must be removed (As per a ‘Haynes manual’) and replaced with a fuel pick up (8mm copper pipe or similar to suck up the fuel from where the original pump did.

Step 6: Mounting new pump:

It is usually best to mount the new pump near the tank-although make sure it has some sort of protection. The original pump wiring may still be used with the new pump. Connect up the pipes (you may need to buy extra rubber fuel pipe) One going from the original pump casing outlet - to the new pump inlet~ , then from the new pump outlet to your engine bay fuel filter inlet (via the original copper fuel line underneath the car).

Step 7: Fuel Regulator

It is always best to buy a new ‘fuel filter regulator/filter’, these cost around £40, and is necessary for regulating the fuel supply pressure.


Now you should be able to check all your work, put on you air trumpets & air filter & start the car!:

Step 8: Initial setting up.

Starting method for ‘Twin Dellorto Carbs’ (Cold engine) .

Allow the float chambers to fill by just having the ignition on so the pump is heard, filling should take about 5-10 seconds. Then fully depress the accelerator rapidly four times, Now turn the engine over (whilst gently pumping the accelerator), if it does not start within 10 seconds , repeat the procedure three times. The engine should fire, but may need 'nursing' for a minute or two before it will idle ( Note* Most dellorto’s have a cold start choke type mechanism, we never use this as it becomes very easy to ‘flood’ the engine using it ( It is best to just ‘ pamper’ the accelerator pedal until the engine is happy to idle).

Setting the Idle speed.

‘Lumpy’ running and engine rocking is normally down to the idle mixture and balance screws being set incorrect. Here is a technique to get a clean idle and progression. Before adjusting the carbs in this way you must make sure that the following settings are correct.

1) The engine is at normal operating temperature
2) That the throttle return spring is working fine.
3) That the engine has sufficient advance at the idle speed (between 12 and 16 degrees- set by the distributor)
4) That there are no air leaks or electrical faults.

A reasonable idle speed for a engine on Dellortos is between 900 and 1100 RPM.

Primary setting up: Screw all of the idle mixture adjustment screws fully home and then out 2.5 turns. If you are using DHLA carbs then start at one turn out. Start the engine and let it reach normal operating temperature. This may mean adjusting the idle speed as the engine warms up. Spitting back through the back of the carburettor normally indicates that the mixture is too weak, or the timing is very retarded. If this happens when the engine is warm and you know that the timing is OK, then the mixture will need trimming richer on that cylinder. Set the idle as near as you can to 900RPM.

Using an airflow meter or carb synchroniser adjust the balance mechanism between the carbs to balance the airflow between them, if the rearmost carb is drawing less air than the front, turn the balance screw in a clockwise direction to correct this. If it is drawing more air, then turn the balance screw anti-clockwise. If the Idle speed varies at this point, adjust it back to 900 RPM, to decrease idle speed screw in an anti-clockwise direction, to increase, screw in a clockwise direction.

When you are sure that the carbs are drawing the same volume of air, visit each idle mixture screw, turn the screw counter clockwise (richening) in small increments (quarter of a turn), allowing a good 5 - 10 seconds for the engine to settle after each adjustment. Note* it the engine speed increases continue turning in that direction and checking for engine speed, then the moment that engine speed starts to fall, back off a quarter of a turn. If the engine speed goes well over 1000RPM, then trim it down using the idle speed screw, and re-adjust the idle mixture screw. If engine speed decreases then turn the mixture screw clockwise (weakening) in small increments, again if engine speed continues to rise, continue in that direction, then the moment it starts to fall, back off a quarter a turn. The mixture is correct when a quarter of a turn in either direction causes the engine speed to fall. If that barrel is spitting back then the mixture is too weak, so start turning in an anti-clockwise direction to richen. During this procedure, the idle speed may become unacceptably high, so re-adjust it and repeat the procedure for each carb cylinder.

After all the mixture screws have been set, the idle should be fairly even with no unacceptable 'rocking' of the engine, if the engine is pulsing, spitting or lunging then the mixture screws will need further adjustment. If the engine is rocking or shaking then the balance is out, so revisit with the airflow meter. No amount of adjustment will give a good idle if the throttle spindles are bent or leaking air, or if the linkages are loose on the spindles!

Diagrm of a typical carburettor

Once the car is running, a trip to a performance expert / rolling should be able to accurately set them up to get peek performance. Don’t necessarily expect ‘instant’ increase in performance as carbs can lose your engine just as much BHP as they can gain it if not correctly set up.

This Article is written completely by XLR8. It is our property & is protected by copyright 2005. Any unauthorized copying is strictly prohibited.