Setting Up Dual Carbs

If you’re working on setting up dual carbs for your bike, check out this guide. Peg from the UK is and an avid Brit bike rider and well versed in the ways to keep them up and running. Whether you’re setting up Amal’s or another brand, she wrote up the following to get your dual carbs fully functional.

Preparation

1. Make sure you are using the correct ‘O’ ring carb mounting for your bike’s year of manufacture, you definitely do not want air leaks.
2. Set the valve clearances accurately.
3. Check the ignition advance mechanism if you still have a points setup, especially look for loose springs connecting the bob weights.
4. Set the ignition timing exactly for both left and right cylinders.
5. Change the needle jets, they will be worn out by only 10,000 miles use. You cannot tell without a microscope if they are worn; they will ruin any chance of setting up the carb nicely. They are cheap and easy to change. Count on them being worn out and change them as a matter of course.
6. Set/check the float heights, if you have not already done so.

Now you should be ready to set the carbs.
I have had good success with this method below. I use it to make a basic setup before using vacuum gauges for the final tweaks. But it is good enough to be used on its own if done carefully.

Setting Up Dual Carbs – Balance

The problem you have is that you now have no reference point for the carb settings, to balance between left and right carb once you have turned the screws.

1. On the back of the carb there are two extension trumpets if you gently unscrew these you will be able to see into the back of the carburettors.
2. If you twist the throttle, you can see the two air slides lift, make sure that you have about 1/16″ free play on the cables before the slides move.
3. Get two pieces of thin stiff wire (gas welding rod is ideal) about 5-6″ long. Wedge one of them under the cutout in the back of each carburettor slide so that the rods balance on the edge of the carb but are stopped from falling by the end tucked under the slide.
4. Now unscrew the idle speed screw (marked on your picture with the yellow arrow), the end of the rod should lift as the slide lowers. You will reach a point where the rod stops lifting as the slide is fully down.
5. Turn the screw very gently backwards and forwards, until you find the point that the rod starts to drop.

Do this for both carbs.
You now have a lower reference point where the setting is the same for each carb. Make a note, or mark on the carb of where the screw slot is, as you will use these references to set the idle speed.

Before you set the idle speed it might be a good idea to check the balance between the cable settings.
Very gently open the throttle a tiny amount (1/64″ or less) and watch both metal rods, if one rod twitches before the other then slacken the throttle cable adjuster for that side until both rods start to move at exactly the same time. The cables are now set.
Remove the metal rods and screw the trumpet extensions back on.

Go back to the adjuster screws, making a note where the screwdriver slots are, turn the screw on each carb in exactly one turn. From now on when you turn the screw on one carb make exactly the same adjustment to the other carburettor, make a note each time you move the screw as it is easy to get confused where you are in the adjustment.

There are two other adjuster screws, this is the air/fuel mixture at tickover, if you have been playing with them, then screw them very gently in until they stop, then screw them out 1 1/2 turns each.
If you have not previously moved them then leave them alone, as the bike was running fine before.

Now these old engines are not good at idling when cold, so you will need to make a basic adjustment now and another fine adjustment when the engine is hot.

The Golden Rule

When setting up dual carbs, if you move one adjuster screw, you must move the other adjuster screw exactly the same amount.

Start the engine and rev it with the throttle for a minute or two to get it warm, then lower the throttle. If the engine dies, screw in each adjuster 1/4″ of a turn.
Start the engine and try again, if it dies, screw the adjusters another 1/4 turn. If the engine revs to high back them off 1/8 of a turn.
You will get to a point where the engine will sit at idle ticking over nicely. About 1000rpm
With Triumphs, I like to have the tickover speed on the high side, as this helps maintain good oil pressure at idle.

• 6) Check your throttle clearance on the cables. After all of the adjustments you should have about 1/16″. If you have to adjust the cables again, make sure that if you adjust one cable, the other has to be adjusted exactly the same amount.
  Here I like to put the rods back in, and just double check if they start to move at exactly the same time, the opening of the throttles together is key to smooth running around town, also if they are out one cylinder is doing the majority of the work pulling away, this is not good for the engine.
• 7) Go for a 10 mile ride and get the engine hot, when you get back home if the tickover has speeded up, back each adjuster screw off by exactly the same amount until you are back to a good tickover speed.

The mixture can then be fine tuned with the engine hot, turn the mixture screw in until the engine starts to slow down, then turn it out until the engine starts to slow down-set the mixture screw half way between the two slow down points, do the same on the other carb.*
If the idle speed has changed, turn the tickover screws by exactly the same amount to get a reasonable idle speed.
Next open the throttle sharply to between 1/8th and 1/4 turn. If the engine stumbles adjust the idle mixture screws until it does not. Please note a small amount of adjustment can make a big difference here.

Most importantly, always refer to your reference marks, so that apart from the hot mixture setting (marked with a star) all turns of the screws are done by exactly the same amount each side.

Don’t expect the bike to tickover at all until it is 5 miles hot, they are not designed that way.
If at any time you think the tickover screws might be out of sync, just slip the steel rods back in and check. If you don’t have vacuum gauges, then it might be prudent to check at the end of tuning anyway.

Check out our other blogs on carbs:

1. Fit New Concentric Carbs with Old Cables
2. Triumph Carb Tuning Basics
3. Triumph Service Bulletin 10-69
4. Carburetor Mounting O-rings