Twin Carburettors
Forget the old myth that they are hard to keep tuned.
Twin carburettors have an unjustified reputation for being hard to keep in tune. It’s origins lie in the bad practice of fiddling with the carburettor of an off-tune engine before having first checked that all else is in order – compressions, ignition timing, distributor, advance rate, tappet clearances, along with a lack of air leaks into the inlet manifold from such sources as damaged gaskets or faulty brake servos.
This is not clever regardless of whether you have one carburettor or several – the carb(s) should always be the LAST thing to adjust. However, typically, someone confronted with an out of tune twin carburettor engine readily recalls having heard that they are hard to tune and keep tuned, so assumes that’s where the problem is and sets about trying to adjust the carburettors to make the engine run better.
As the problem was probably somewhere else, the tuning attempts are invariably doomed to failure and the engine finishes the exercise no better off than it started. “Gosh!” (to be polite!) exclaims the frustrated tuner, “It’s true! Twin carburettors ARE hard to tune!!!” And so the experience is relayed on to others and the myth is perpetuated.
The actual tuning procedure is quite simple: First, check that everything else is to spec (see above) – it is a complete waste of time trying to tune out a fault elsewhere by adjusting the carburettors. Next, if you haven’t had anything to do with the car before, check the condition of the carburettors – particularly wear in the throttle spindles and also the needle valves.
Air leaking through badly worn spindles will make the idle uneven and variable, and so more difficult to set since the amount of leaking air is not constant. If worn, fix the problem or expect only limited success in setting the carbs. Having done these key initial steps, following the standard tuning/balancing process outlined in the workshop manual will quickly bring the carbs into excellent balance.
SU’s are at correct mixture when lifting the piston about 1-2 mm makes no difference to the idle speed (as described in the workshop manuals). If however you are measuring exhaust CO readings etc, they usually will be high, due to the siamesed inlet ports as much as anything, but also the lack of emulsified air in the droplets as they leave the jet. The readings clean up immediately the car is under load.
As the carbs get a bit of mileage, some wear does occur at the thick end of the needle, resulting in rich idle if the mixture is OK everywhere else, or weak mixture everywhere else if you set it correctly at idle. Simple solution: renew the jet needle. If the jet itself is looking oval, it will need replacing too.
Having set the carbs up properly in this way (including renewing spindles etc if required), my experience with each car I have kept tuned over a period of more than twenty years is that only very minimal adjustment, usually none, is needed at subsequent tune-ups if everything else is set first. (Sorry to repeat it but it is vital).
If you don’t have ready access to a workshop manual, the tuning procedure itself is:
- Adjust the throttle stops so each carb is sucking evenly (see below).
- Adjust the mixture (also see below)
- If there was significant adjustment needed for (2), repeat (1) & (2), or do a couple of loops through (1) & (2) if initial adjustment was major.
- Adjust the linkages between carbs so that both carbs are taking up at the same time when the throttle is depressed. (Note: you should set the linkages up so that there is some free movement before the spindles start opening).
- Check you are getting full throttle just as the pedal reaches full travel (so the cable doesn’t get stressed when the red mist sets in!) and adjust the cable and/or the return stop in the footwell if necessary to achieve this.
The tuning is best done with air filters removed.
More detail on steps 1 & 2:
Throttle adjustment
This can be measured on a flow meter placed across the intake of the carb, or by listening (via a hose but practiced ears don’t need one) to the level of suck at the intake. If using a hose, make sure it is held at the same point relative to the intake of each carb. Any difference in noise level not discernable by the ear is not worth worrying about.
Mixture adjustment on SU carburettors
Note: This must be done after balancing the flow, as there is transfer between the ports via the balance pipe in the manifold, and if the flow in the carbs is not about even, there will be a major effect on the opposite port when adjusting any given carb.
Adjustment mechanism
There are two means of adjusting the mixture on SU’s originally fitted to most MG’s, depending on carburettor type:
The H and HS type of carburettor fitted to prior to about 1972 have an adjusting nut underneath the carburettor, around the base of the jet.
The HIF carbuerttor fitted to MGB’s from about 1972 onwards adjustment is via a recessed screw in the side of the carb body, quite low down and at about 45 degrees to the main throat of the carburettor.
(Note that post 1972 Midgets retained the HS type, and that the change-over period was 1972-74 for the MGB so either type could be fitted to these years. The HIF does not have a separate bowl, instead it is underneat the carb, incorporated into the body.)
Checking mixture strength
Lift the piston about 1/16” (either via the piston lifter on the carb body, or with a screwdriver or other probe small enough to not disturb the air flow into the carb.) Correct mixture is when the engine speed doesn’t change or increases a little then settles back to where it was. If the engine speed reduces the mixture is lean, if it increases the mixture is rich.
This is the only reliable way to set the mixture, though you can get it roughly there by screwing the adjuster in and out till it maximises idle. However, it still pays to do the last bit by piston-lifting.
