Weekend Workshop
Extract perfect performance from your sporting classic engine
How to balance twin carbs.
Having two carburettors is a fine idea on a performance car, as it allows the engine to breath better and produce more power. A twin variable-jet SU or Stromberg setup was a mainstay of sporting British motoring for much of the 20th Century. The only problem is that tuning two carburettors is marginally more difficult than tuning one. Pub bores will tell you it’s witchcraft. It really isn’t.
Anyone can tune twin carburettors at home with basic equipment. Setting the correct fuel-air mixture on both carburettors is essential for optimal performance, economy and longevity. The carburettors also need to operate in perfect synchrony. If they don’t, the cylinders won’t all be receiving the same volume of fuel-air mixture. This will lead to a rough tickover and hesitation at small throttle openings.
Neither tuning nor balancing are difficult, as long as you follow a logical sequence. Although it’s theoretically possible to achieve adequate results by ear, the added variables of twin carburettors really justify investing in some basic tuning equipment. An airflow meter (from £20) is a major upgrade on the old mechanic’s cliché of putting a piece of hosepipe between the carburettor inlet and your ear. A basic gas analyser (from £100) or a Colortune (from £25) are ideal for setting mixture. Use these tools and follow our simple guide, and you‘ll be left with silky-smooth running, crisp performance and optimal fuel consumption.
Tools for the job
For accurate tuning, you’ll need tools to measure airflow and mixture. You won’t have to sell the family silver for professional kit. Get a cheap airflow meter to balance the carburettors and a DIY gas analyser or Colortune to check the mixture.
Check float and valve
The fuel level is controlled by a float and a needle valve. Remove the air filter housing and the float chamber lids. Examine the tips of the needles. Replace if there’s any sign of wear. Make sure the floats aren’t full of fuel. Clean out the chambers.
Top up dashpots
The dashpot piston has an oil-filled damper to provide momentary enrichment of the mixture during acceleration. Check the damper rod is straight and the piston isn’t worn. Top up with an SAE20 or SAE30 oil, though 20w50 will do at a pinch.
Check engine health
Ensure the engine is in good health. Set the valve clearances, service the ignition system and check the timing with a strobe. Make sure the fuel pump is healthy. Do a compression test to make certain the valves and pistons are in good order.
Set float level
The correct float level is absolutely critical. Consult your workshop manual. Use a drill bit of the right diameter to set it. Adjust by gently bending the tab on the float or by fitting a thicker or thinner copper/fibre washer under the needle valve body.
Check choke
Check that the choke cable and mechanism move freely. Lubricate with a little fresh engine oil. Make sure there’s a little slack in the cable, so the choke is fully ‘off’ when the choke knob is pushed in. Adjust the choke’s fast idle screw, if necessary.
Eliminate air leaks
Leaks between the carburettors and manifold or the manifold and cylinderhead will make accurate tuning impossible. If you suspect a leak, spray the area with penetrating oil while the engine is idling. A momentary increase in rpm confirms a leak.
Inspect dashpots
Inside the dashpot a piston moves up and down in response to manifold vacuum. This lifts the metering needle out of the jet to allow more fuel to pass through. Make sure the pistons rise and fall smoothly and the needle isn’t worn or bent.
Slacken throttle
Make sure the throttle cable or linkages move smoothly and release fully. Lubricate with a few drops of oil and dribble some down the cable, if necessary. Slacken off the throttle cable a little. Run the engine until it’s fully up to temperature.