Monday, November 19, 2012

Carb re-build #2: New bearings and getting things aligned

With all of your parts clean, laid out, and ready to install, the first step is to fit the main throttle shaft and butterflies in our fresh carburetor body.

The throttle shaft runs on ball bearings and these should be replaced for new sealed units for 2 reasons:

1. The bearings in there are likely the original ones and over 40 years old. I was told that the bearings never go bad and this advice was NONSENSE! Every one of the bearings we removed were rough and not nearly as smooth as a new one. New bearings are less than $10 each with every carb using 2 so this is not the time to cheap out for so little money.

2. The original bearings were of an open cage design. Weber tried their best to seal out air by using a number of flat and spring washers but they are still prone to leak a little. This air leakage leans out the mixture at different throttle openings and takes away a little precision from the assembly in addition to causing noticeable irregularities in how the engine runs. New bearings (again from ) are of a sealed design, are maintenance free, and keep the air out.

With new bearings in the next step is to fit the throttle shaft. Be sure to use some oil or grease on the shaft at all of its operating points.

Next come the throttle butterflies. They need to be fitted with care as their fit is very tight in the bores. DO NOT FORCE ANYTHING! If something is stuck find the reason. It could be a tiny burr of metal or a mis-alignment. Everything should fit together nicely and force will only damage the brass plates or soft aluminum carb body.

With the throttle plates in the closed position then fit the throttle plate screws. There are two tricks here:

1. Use red Loctite on these screws. You want all the insurance you can that they will not fall into the engine.

2. Tighten them so they are 'just' snug. Tight enough so that the plates do not move easily but not so tight that they are fixed in place.

The next step is to build all of the outboard components of the throttle shaft and tighten them down well. At this stage the shaft will have shifted a little bit and, having left the plates loose enough to move, then you can loosen the butterfly screws a little to allow them to really settle into a finished position.

With the screws loose & the butterflies in the closed position hold the carburetor up to a bright light and look down the inlet barrel. You should see just a faint bit of light around the perimeter of the butterfly. What you are looking for is uneven light which indicates a butterfly that is off to one side. Once you have a nice even light pattern, fully torque the screws.

With the screws tightened check the operation of the assembly to make sure that it is smooth with no tight spots. When assembled the movement for the shaft should be almost Swiss watch like precise. This is all a function of good assembly of clean straight components and will make a noticeable difference in the operation of your engine.


  1. This is outstanding work. May I ask what products did you use the get the card bodies so clean?

    1. The carb bodies were immersed in a pail of carburetor cleaner. This can be bought at most auto parts stores and is a very powerful cleaner. It is the best way to dissolve old fuel varnish and deposits from the outside of the carb.

      Once cleaned we VERY carefully masked every opening on the outside of the carb and then blasted it with fresh glass bead at a very low pressure (20-25psi). Such low pressure acts as gentle as a plastic bristle bush to just do the last bit of cleaning and evening out to the aluminum of the casting.

      After blasting it is important to wash the carb bodies over and over again using lots of compressed air to ensure that all of the passages are clean. I hope this helps.

    2. thanks for sharing this process.
      I really enjoy watching every part of this stunning restoration

  2. Also are they new Throttle Shafts and If so where can these be found.
    Thanks for sharing your quality work.

    1. Our throttle shafts were found to be straight and had no wear so we re-used them. We did black oxide them to make them look new.

      Replacement shafts are avaialble from

    2. Thank you and keep following. There is plenty more to come.

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