Paul's Daimler Diary, adjusting the mixture of Stromberg and SU carburettors.

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Stromberg Page 3 - Go to page one two three four five 4. Adjusting The Mixture: Note: in the following procedure, one "flat" is the basic increment of adjustment, and refers to 1/6 of a turn of the mixture adjusting nut. This corresponds to the flat faces on the nut. These instructions are for ZS carb's or SUs with separate float chambers. You will need to check in your shop manual to see whether you turn the mixture screw to the right or the left to make it richer or leaner. (We may add the information here some day...) 1. Shut the car off and loosen the choke linkage nuts, if you have any to loosen! 2. Adjust the mixture nuts (screws) fully lean. Check your shop manual to make sure you are adjusting them the right way! 3. Now enrich each carb an equal amount -- two full turns of the adjustment nuts (screws). Then start the car. Note: In the following step, you might want to consider adjusting the carburettors one-half a flat too lean, as the mixture will be enriched when you put the air filters (which restrict air flow) on at the end of the tuning process. 4. Raise the lifting pin (or use a screwdriver if you don't have the pins) so that the piston rises no more than 1/16". Listen to the engine's exhaust note and compare it to the following conditions: · If the exhaust note rises and stays high until you drop the piston, this carburettor is adjusted too rich. Adjust the mixture one flat leaner, then repeat Step 4. · If the exhaust note falls and the car sounds as though it is going to stall, this carburettor is adjusted too lean. Adjust the mixture one flat richer, then repeat Step 4. · If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carburettor is set correctly. When you have achieved this setting for all carburetors, continue with Step 5. 5. Tighten the choke linkage nuts, if applicable, so that the choke cable will pull an equal amount on both mixture nuts when you pull the knob. 6. At this time, I find I usually have to adjust the idle again because getting the fuel mixture right usually changes the idle speed. Since you know you have the throttles synchronized, I normally just adjust the idle without loosening the throttle linkage. The easiest way is to screw one of the screws out till it doesn't' even touch the throttle stop, then use the other to get the idle speed right. When that's done, you can screw the other stop screw down till it just touches the stop on that carb and you're set. 7. Replace the air filters and go for a test drive! 5. Special Notes SU and ZS carburettors are most fuel-efficient when slightly lean, and provide the most power when they are slightly rich. You can use this knowledge to provide a certain amount of tuning for the kind of driving you do. If you learn to read spark plugs, you can get a basic idea of what your engine's condition is and make fine adjustments to the mixture nuts accordingly. If you have a ColorTune, you simply install it in place of one of the plugs, then adjust the carburetor that feeds that cylinder (the front carburettor for 1 & 2, the rear for 3 & 4). The ColorTune will let you see the colour of the flame. White flashes mean too lean; yellow flame means too rich. Blue (like a Bunsen burner) is correct, and blue with a faint orangish tinge is the best for power. You can also modify your car's throttle response characteristics slightly by adjusting the viscosity of the oil in the dashpot damper. SU and ZS carb's are set up so that a thicker oil will resist the piston's attempt to rise in the dashpot for just long enough that the engine's increased load (when the throttle is opened) will pull more fuel across the bridge; this enriches the mixture and temporarily bumps power up to help the engine achieve higher speed more readily. For light damping, Marvel Mystery Oil, or “3in1” oil is excellent, engine oil can be used for heavier damping. If you modify your engine, you will probably need to modify your needles, as it is the needle profile that determines the mixture curve for different air-fuel loads. If you experience uneven idle, hunting, or an idle that changes (rises or falls) as the engine's temperature climbs or drops, you probably have vacuum leaks. The most serious fault on most old carb's is wear in the throttle shaft area. To test for this, spray some carburettor cleaner on the outside of the throttle shaft; carburettor cleaner is non-combustible, and if the engine speed drops, it means some of this is getting into the air stream from outside the carburettor. You may also have leaks from the manifolds, from tubing such as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for locating those leaks as well. Other problems that SU and ZS carb's experience involve dirt in the dashpot and occasionally in the float chamber. The dashpot is a precision piece of machining that involves very close tolerances so that the piston doesn't stick or bind when it rises and falls. A little grit between the piston and the dashpot can make the car jerk and sputter. Take the dashpot off, wipe the insides down with carb cleaner and a lint-free, clean rag, then reinstall it, getting the screws down tight. Also, don't swap the pistons between dashpots; they're matched to one another so that the clearance between the piston and the wall of the dashpot makes a tight seal but permits easy rising and falling. Dirt in the float bowl basically shuts off that carburetor (or can make it flood open, depending on whether the dirt is wedging the valve open or closed). You can try rapping on the float bowl with the handle of a screwdriver, but your best bet is to take the cover off, clean out the valve fittings, and reinstall everything, with a new fuel filter for good measure. Some older SU models also have adjustable floats, in which you need to set the float height (which basically equals the fuel level in the float chamber) by bending a brass rod. These carburettors were replaced in the mid-1960s with carburettors that had fixed, plastic floats which are basically trouble-free unless abused. The stop at the back of the floats can break if they are installed badly, and the brass pin that holds them in place can wear an oval hole in the float pivot. New floats are fairly inexpensive and aren't a bad idea if you're doing a rebuild. Based on personal experience and many discussions with fellow Triumph owners, there are two fairly common problems which arise with Zenith Stromberg Carburettors that I would like to offer technical advise on how to correct (remember the personal experience aspect). Stromberg Page 3 - Go to page one two three four five This site is for the benefit of enthusiasts. Much of the content is my own to use as you wish. However, some I have collected from various (forgotten) sources in the public domain. If you are the owner of any material used and wish it to be removed or so accredited, please contact me. Web Site © Double Six Design

Stromberg Page 3 - Go to page one two three four five

4. Adjusting The Mixture:

Note: in the following procedure, one "flat" is the basic increment of adjustment, and refers to 1/6 of a turn of the mixture adjusting nut. This corresponds to the flat faces on the nut.

These instructions are for ZS carb's or SUs with separate float chambers. You will need to check in your shop manual to see whether you turn the mixture screw to the right or the left to make it richer or leaner. (We may add the information here some day...)

1. Shut the car off and loosen the choke linkage nuts, if you have any to loosen!

2. Adjust the mixture nuts (screws) fully lean. Check your shop manual to make sure you are adjusting them the right way!

3. Now enrich each carb an equal amount -- two full turns of the adjustment nuts (screws). Then start the car.

Note: In the following step, you might want to consider adjusting the carburettors one-half a flat too lean, as the mixture will be enriched when you put the air filters (which restrict air flow) on at the end of the tuning process.

4. Raise the lifting pin (or use a screwdriver if you don't have the pins) so that the piston rises no more than 1/16". Listen to the engine's exhaust note and compare it to the following conditions:

· If the exhaust note rises and stays high until you drop the piston, this carburettor is adjusted too rich. Adjust the mixture one flat leaner, then repeat Step 4.

· If the exhaust note falls and the car sounds as though it is going to stall, this carburettor is adjusted too lean. Adjust the mixture one flat richer, then repeat Step 4.

· If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carburettor is set correctly. When you have achieved this setting for all carburetors, continue with Step 5.

5. Tighten the choke linkage nuts, if applicable, so that the choke cable will pull an equal amount on both mixture nuts when you pull the knob.

6. At this time, I find I usually have to adjust the idle again because getting the fuel mixture right usually changes the idle speed. Since you know you have the throttles synchronized, I normally just adjust the idle without loosening the throttle linkage. The easiest way is to screw one of the screws out till it doesn't' even touch the throttle stop, then use the other to get the idle speed right. When that's done, you can screw the other stop screw down till it just touches the stop on that carb and you're set.

7. Replace the air filters and go for a test drive!

5. Special Notes

SU and ZS carburettors are most fuel-efficient when slightly lean, and provide the most power when they are slightly rich. You can use this knowledge to provide a certain amount of tuning for the kind of driving you do. If you learn to read spark plugs, you can get a basic idea of what your engine's condition is and make fine adjustments to the mixture nuts accordingly.

If you have a ColorTune, you simply install it in place of one of the plugs, then adjust the carburetor that feeds that cylinder (the front carburettor for 1 & 2, the rear for 3 & 4). The ColorTune will let you see the colour of the flame. White flashes mean too lean; yellow flame means too rich. Blue (like a Bunsen burner) is correct, and blue with a faint orangish tinge is the best for power.

You can also modify your car's throttle response characteristics slightly by adjusting the viscosity of the oil in the dashpot damper. SU and ZS carb's are set up so that a thicker oil will resist the piston's attempt to rise in the dashpot for just long enough that the engine's increased load (when the throttle is opened) will pull more fuel across the bridge; this enriches the mixture and temporarily bumps power up to help the engine achieve higher speed more readily. For light damping, Marvel Mystery Oil, or “3in1” oil is excellent, engine oil can be used for heavier damping.

If you modify your engine, you will probably need to modify your needles, as it is the needle profile that determines the mixture curve for different air-fuel loads.

If you experience uneven idle, hunting, or an idle that changes (rises or falls) as the engine's temperature climbs or drops, you probably have vacuum leaks. The most serious fault on most old carb's is wear in the throttle shaft area. To test for this, spray some carburettor cleaner on the outside of the throttle shaft; carburettor cleaner is non-combustible, and if the engine speed drops, it means some of this is getting into the air stream from outside the carburettor. You may also have leaks from the manifolds, from tubing such as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for locating those leaks as well.

Other problems that SU and ZS carb's experience involve dirt in the dashpot and occasionally in the float chamber. The dashpot is a precision piece of machining that involves very close tolerances so that the piston doesn't stick or bind when it rises and falls. A little grit between the piston and the dashpot can make the car jerk and sputter. Take the dashpot off, wipe the insides down with carb cleaner and a lint-free, clean rag, then reinstall it, getting the screws down tight. Also, don't swap the pistons between dashpots; they're matched to one another so that the clearance between the piston and the wall of the dashpot makes a tight seal but permits easy rising and falling.

Dirt in the float bowl basically shuts off that carburetor (or can make it flood open, depending on whether the dirt is wedging the valve open or closed). You can try rapping on the float bowl with the handle of a screwdriver, but your best bet is to take the cover off, clean out the valve fittings, and reinstall everything, with a new fuel filter for good measure.

Some older SU models also have adjustable floats, in which you need to set the float height (which basically equals the fuel level in the float chamber) by bending a brass rod. These carburettors were replaced in the mid-1960s with carburettors that had fixed, plastic floats which are basically trouble-free unless abused. The stop at the back of the floats can break if they are installed badly, and the brass pin that holds them in place can wear an oval hole in the float pivot. New floats are fairly inexpensive and aren't a bad idea if you're doing a rebuild.

Based on personal experience and many discussions with fellow Triumph owners, there are two fairly common problems which arise with Zenith Stromberg Carburettors that I would like to offer technical advise on how to correct (remember the personal experience aspect).

Stromberg Page 3 - Go to page one two three four five

This site is for the benefit of enthusiasts. Much of the content is my own to use as you wish. However, some I have collected from various (forgotten) sources in the public domain. If you are the owner of any material used and wish it to be removed or so accredited, please contact me.