VotW – Dual S.U. Carburetor Tuning

For our Video of the Week this go round I want to go back to something that gives folks a lot of headaches –VotW - Dual S.U. Carburetor Tuning tuning a dual S.U. carburetor setup. We are also going back to an old friend for this, John Twist of University Motors. While this video specifically addresses the HIF4 carbs of the MGB, the same basic principals of tuning and balancing apply to all dual SU setups. We covered the basic theory and operation of the S.U. side drafts in the past, but this video is more specific to their actual tuning.

[Corrected, see below] By the way, do you know what the “I” in the “HIF” stands for?  It means “integrated”, as in the float bowl is in-line with the carburetor itself, in this case directly under it. If the float bowl is to the side of the body, then it is an HSF carb. Oh, and that number “4” refers to the throat size. That number is translated as each 1 is a 1/8 inch, staring at one inch. So an HIF has a 1-inch throat. An HIF4 would have an inch and a half throat. Pretty simple once you know.

I may not be in the majority here, but I am quite fond of S.U. carburetors. I find the easy to understand, easy to work on and repair, and fairly robust. Once you understand the basic principals and tuning methodology, you can get them to perform quite well. Plus, unlike say a computer controlled fuel injection system, you can make quick and dirty temporary repairs on the side of the road when you need to limp home. Plus, they will operate even when in a pretty sorry state of repair. Oh, and when polished up, I find a dual S.U. setup quite attractive.

Anyway, hope you enjoy the video … and then go out and get your hands dirty!



  1. Check the HIF definition, please. I believe it’s Horizontal Integrated Floatbowl.
    And sizes are increments of 1/8 inch, beginning at one inch. H is a one inch, H2 is a 1.25 inch (MG TD); H4 is 1.50 inch, (TD MkII, TF, MGA, MGB).

    • You are very right! I was writing quickly and got my measurements wrong. My carelessness stands corrected, thank you for catching it.

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