The “Black Art” of S.U. Carburetor Maintenance

From Moss Motoring 1983

by Richard Wood

It is not the aim of this article to present a technical thesis on the S.U. carburetor; if that is what you need, you should buy an S.U. Manual. (Moss part no. 211-345) Rather, the attempt is to supply some basic “no frills know-how” to help you regain a speaking relationship with your British motorcar. Let’s review the more common problems:

Leaking (An S.U. fact of life—like death and taxes.) Leaks usually occur where they can drip fuel onto the hot exhaust system, making a full restoration imminent. Weekly checks should be made to see that all unions and fittings are tightened fully. It’s a good idea to carry some spare fiber washers and jet seals in a small envelope in your tool box or glove compartment. If you have replaced your jet seals and installed a new jet, but you still have a leak from the jet carrier area, the jet itself could be the problem. (H-type carburetors only.) The new jets have the yoke secured to the tube by a square-headed bolt. Sometimes this is not tightened fully at the factory. Hold the tube by putting an awl or nail through the hole half-way down the jet tube and tighten the yoke with an adjustable wrench. (Unscrewing it first and putting a bit of silicone sealer on the threads, tightening it, makes sure it won’t leak.) When replacing the jet seals, make sure there are no old seals stuck in the bottom of the jet carrier. Soaking the new seals in oil for a few minutes helps them compress to their new shape and be fuel resistant.

pg5Leaking is sometimes caused by fuel building up in the venturi. This can result from several things: Excessive fuel pressure (S.U.’s need 2 to 3 psi maximum); blocked overflow pipe; incorrectly assembled overflow pipe (the fiber washer with the cutouts goes between the pipe and the cap); faulty needle and seat; sunken float; incorrect float height.

Spitting (A filthy habit, major league baseball notwithstanding.) Spitting back usually occurs when opening the throttle. The first check is to make sure the damper is filled to the correct level with 20 wt. oil. This often overlooked aspect of maintenance should be done at least once a month. If the damper has adequate oil, spitting generally indicates too lean a setting; screw the jet nuts down a few flats to richen the mixture. If this ruins your idle, its time for . . .

Adjusting the Beasts First, check the throttle shafts where they enter the body. If you can wiggle them sideways appreciably, they must be replaced before you can hope to adjust the carbs. If they are quite firm, take your trusty UniSyn, loosen the clamp that connects the two shafts, balance the idle speed of both carbs, then re-tighten the clamp. (Make sure there are no leaks where the carb bolts to the manifold, or between the manifold and the head.) Next, check the mixture by lifting the piston (with a screwdriver blade—your finger will block the air flow) about W\ If the mixture is correct; the engine should speed up, and then falter. If it stalls, the mixture is too lean; if the revs go up and stay there, it is too rich. Adjust by turning the jet nut up (to weaken) or down (to richen). Set both carbs to optimum position, and then re-check the balance and idle speed with your UniSyn. With the motor off, you should also check that both pistons will rise all the way without sticking. (Unscrew the dampers before you check this action.) If they don’t, remove the dashpot and clean the insides carefully, also the perimeter of the piston. Don’t use anything abrasive! Do one carb at a time so you don’t mix the pistons and dash-pots. Be careful where you spill the oil when you turn the piston upside down! Also, be careful not to bend the needle. If the piston does not fall with a solid “click”, your jet probably needs centering. (The book covers that procedure.) Remember to refill the piston with oil and tighten the damper after this exercise.

A Final Word of Advice The problem with your S.U. Carburetors is often your Lucas distributor! In other words, before messing around with any of the foregoing adjustments, make sure your plugs and points are clean and gapped properly, your cap and rotor are in good condition, your wires are not corroded at their ends, and your timing is correct. It doesn’t hurt to adjust the valves, either, as wrong valve clearances will never allow you to get the carbs set properly.


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'The “Black Art” of S.U. Carburetor Maintenance' has 1 comment

  1. July 26, 2013 @ 7:28 am Frank

    My first love was a TR-3 in 1961. I tried to learn everything about that puppy I could. At the time I was a pilot in the USAF and didn’t have enough time to dedicated much attention to the beast. Boy do I wish these kind of articles were available to me then. I struggled to learn what I could from others, but it was sketchy. Since then, I’ve owned two Healeys, one a Bug-Eye, an MGA, two MGBs. I’m retired now and thinking about starting a collection of British roadsters. I feel I can do a much better job in restoration and maintenance with the help of Moss Motors. Thanks for your years of service to us shadytree mechanics and British car lovers.


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