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The Not-So-Black Art of SU Carburetors
Phil Singher

If you've lived with a set of SU carbs for some time, you are no doubt aware that these hold an element of mystery for many otherwise sensible mechanics. Even at our local Volvo dealership, the one technician who deals with them is regarded as somewhat of a Guru, filled with arcane knowledge and lore.

Actually, SU carbs are simple devices and can be dealt with readily by the home mechanic armed with a Haynes manual, a little information and some common sense. Our 122S was giving poor performance, gulping gas and fouling spark plugs on moderate hills. As for passing a California smog check...

SU carbs wear out where the throttle shafts meet the carb body. In a high-vacuum situation (throttle closed), air leaks through these spots and this causes the piston and needle to rise prematurely, giving a rich mixture. Tuning a car with this condition for proper idle mixture will result in excessive leanness when the throttles open and the leakage becomes insignificant; conversely, tuning (by trial-and-error) for good driving on the road results in an overly rich idle and plug fouling under load. It is easy to spot this problem by spraying some carb cleaner on these junctions while holding the throttles slightly open. If the engine speed drops appreciably, you are in for a rebuild. Volvo has a kit, consisting of throttle plates, shafts and bushings, just for this purpose. Take a deep breath, find your Haynes manual and prepare to become a Guru.

Before you dismantle anything, make sure that the ignition timing on your car is in the ballpark and that the valves are adjusted correctly (adjusting them warm works best for us. These factors will affect the tuning of your rebuilt carbs and it is easier to get them rounded up in advance. When you remove the carbs, put the clip from the linkage balljoint back on the joint so it doesn't get lost. Similarly, remove the little cylinders that connect the choke cables and screw them back on the cables. Drain the oil and fuel from the carbs into an environmentally appropriate container instead of onto your kitchen table.

Disassemble the carbs just like the manual says. Remove the needles from the pistons and treat them kindly; any sort of ding here is disastrous. Inspect the jets very closely; under magnification, if possible. If you find any wear at all, replace the jets and the needles, noting the two-letter code stamped on each needle so you can get the correct replacements. You can leave the choke mechanisms on. Before giving everything a thorough scrubbing with solvent and a toothbrush, remove the rubber O-ring that seals the jets to the float bowl. Notice that the throttle plates have angled edges and make a drawing of which way they go -- it is OK to install them upside down, but not front-to-back.

The carb bodies will need to be bored out and the new bushings pressed in. Also, the new shafts will need to be drilled for the pins that hold on the actuating arms. Accuracy is important in both these operations and they need to be done by a real machinist. Don't waste your time with carburetor rebuild shops (they are mystified by SUs and won't work on them) but find a good automotive machine shop. We wound up at a Harley place, which did a fine job. Have them reverse the actuating arms when they pin them, so you have a nice new surface to mate to the linkage later.

The one critical part of reassembling the carbs is to get the needle centered correctly in the jet. Do this before hooking the jets to the float bowls, and follow Haynes exactly. If you remove the piston assemblies after this, you will need to recenter the needles. Haynes warns against over-tightening the jet-float bowl connection (it's only a brass fitting), but get it good and snug. Loctite is a good idea.

Having spent this much money and effort, you might as well replace the float-bowl fuel valves. Volvo supplies Grose jets to replace the original style valves. The washers that come with them are meant as adjustments for the float level -- it is fine to install them with no washers if that gives the correct result. You must use a paper filter in the fuel line with these valves.

Put everything back on the car, remembering that there is a second set of gaskets between the manifold and the heat shield. You are now ready to tune. Tuning a set of SUs is actually two different operations: 1.) setting the idle mixture, and 2.) synchronizing the carbs.

The Haynes procedure for setting mixture is good, so go ahead and do it their way. Raising the jets into the carb body leans the mixture; backing them out richens it. This is ambiguous in the edition we have.

Synchronizing the carbs (making both carbs do the same thing at the same time) is a matter of tinkering with the clamp-on arms on the part of the linkage which sits between the two carb bodies. You must adjust these so that both piston/needle units begin to rise at the same instant as the throttle is opened. You won't need any special meters or gadgets; just watch the pistons carefully, be picky and don't get frustrated. When this has been achieved, adjust the shaft with the two ball-joints for proper free play at idle.

Now, go back a step and fine-tune the mixture and idle speed, then put on the air cleaners.

On our car, this rebuild resulted in a night-and-day improvement in driveability and economy. The smog check technician was gratified and amazed, but then again, he's not a Guru.

Total cost including machining was about $158, with new jets and needles.

The text of this article originally appeared in the Volvo Sports America Western States Magazine, March/April 1994.

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