PROJECT: MGTD-51: Carburetors

PROJECT 51: Carburetors. The MG (and many other British sports cars of the period) used a pair of side-draft carburetors made by SU. They were (and are) notoriously finicky to get set up and balanced, and to the American mind, are far more complex than a carburetor needs to be. There are aftermarket kits that replace them with a "normal" downdraft carb, but obviously, I can't go there. So, I am rebuilding the 69-year-old originals.

Over time (especially when the car isn't even started for many years) the seals and gaskets dry out and crack, but there aren't too many moving parts to wear out. The major wear items are the fuel-metering needle and the throttle-plate shaft.

The metering needle is a long, small diameter pin that is slightly tapered from one end to the other. It fits inside a jet body with a hole that is only a tiny bit larger than the big end of the pin. This is where the fuel is "metered" into the intake air. The farther the tapered pin is drawn out of the jet, the more fuel is delivered.

Surprisingly, the pins and jets in my carbs had been adjusted properly and showed almost no signs of wear. But since the rebuild kits came with new jets, I decided to replace the old needles, as well. The "universal" kit does not come with needles because there are "standard," "lean," and "rich" versions to choose from.

The throttle-plate shafts were a different story. Both the shafts and the holes in the throttle bodies in which they turn showed plenty of wear! There is supposed to be about .001" of clearance between the shaft and the hole; mine had more than .008". That creates a leak-path for air that makes setting a smooth idle almost impossible.

The kits come with new shafts and a set of bushings to make the holes the right size. Installing the bushings requires machining both sides of the throttle body castings (you can't do both from one side), and while I was figuring out how I was going to fixture the parts in my mill to get both bushings perfectly in line, I did some Internet searching to see what others might have come up with.

Several times I saw the name "Bob Johnson in Claremore, OK" pop up as being THE guy to send your SU carbs to if you wanted them rebuilt. I e-mailed him and asked what he would charge to just install the bushings. He replied $50 total for two bushings in both bodies. No brainer! The bodies and bushings were in the mail the next morning.

While waiting for those parts to come back, I cleaned and polished the rest of the components, and I also sent the exhaust manifold out to be ceramic coated. It should never rust or discolor, again.

The assembly of the carbs is intricate and a bit complicated, and the instructions that came with the rebuild kits are not the best I've ever seen. Because the kits will service several different models of SU carbs, there are some extra washers and gaskets … and they don't bother to tell you that. There is also an illustration that shows a "piston return spring" that was missing from both of my carbs. While cursing Brutus, yet again, I did some digging on the Internet, and I found that that spring wasn't used in the early TD carbs. (While Brutus was blameless for the spring, he did manage to lose some of the original screws and replaced them with whatever he could find.)

Between the rebuild-kit instructions, the photos that I took during disassembly, and some better cut-away and exploded views from the Internet, I was able fit all the puzzle pieces back together to factory specs.

I mounted the finished carbs and the air-cleaner to the engine for photos, but they will come back off before the engine gets mounted in the frame, to make sure they don't get damaged.