MGB Tuneup Part 3: SU Carburetor Balance and Mixture Adjustments

One of the biggest misconceptions about old cars is that a tune-up on a Triumph TR6 or an Austin-Healey involves changing the plugs and turning a screwdriver on the carburetors. That's simply not enough, and it's a large part of the reason why they have a reputation for being unreliable. We take an MGB through a more comprehensive procedure.

In this video, we do a quick check of ignition advance, and then finally look at a set of SUs. Balancing carburetors seems like an art, but honestly it's no more difficult than hanging a picture frame to make it level. You just need a tool so that you know what "level" is, and adjust what you're doing accordingly. On most SUs, adjustments are made by simply turning a screw to change balance, or by turning a nut at the bottom to change mixture.

It IS important to check one thing with SUs that I didn't do in the video. After your adjustments, re-tighten the linkage. Then, with the air cleaners off, rev the throttle and look at the behavior of the air pistons. They should rise and fall at the same rate. If they don't, you need to tweak the linkage (not the throttle stop or the mixture) by loosening it, repositioning slightly, then tightening it back up. What's happening is that the linkage may be acting on one carburetor before the other... so it could be off throughout the entire rev range except where it was just set at idle.

We'll continue through until we (finally) get to adjusting the carbs... but follow along. You can tweak those SUs as much as you like, but whether you have an XKE or a Spitfire, your car can run terribly if the distributor isn't working properly!

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