Tech Articles

MGB - Dual HSR Mikuni 42 mm Installation - Replacing HS4 SU carbs

The photos below show the process of changing from SU carbs to 42 mm HSR Mikunis on an MGB. This is a task that the average home mechanic can do in an afternoon. The SU carbs are HS4 1.5" or 38mm carbs. We use 42 mm HSR Mikunis as their design and construction is superior to the Mikuni 38 mm carbs and they provide a greater increase in performance. Before and after dyno testing showed a 20% increase in power in this case.

Replacing a 38mm carb ( 1.5 " )with the  42m Mikunis ( 1.65" ) requires that the intake manifold opening, and/or spacer be enlarged slightly to match the larger carb size. This  considerably improves the air flow into the manifold and the result is an increase in power throughout the normal speed  range including at low rpms.  Left click on any of the photos below for an enlarged, more detailed view.

Stock SU carbs with Customer installed air filters.


Another view of the stock carbs, throttle linkage and choke cable.


The photo above shows one of the first installation steps - removal of the carbs.


With the carbs removed you can see the carb spacers.


This is a view with the heat shield and spacers taken off the intake manifold.

 Manifold openings enlarged.

The intake manifold inlets have been opened up to match the 42 mm carbs

 Manifold opening enlargement

A close up view of the enlarged inlet, done with the manifold removed from the engine.


 This view shows the heat shield openings enlarged to match manifold opening size.


 Spacers openings tapered  to match heat shield opening on the engine side and the size of the carb adapter opening on the carb side.


 Here the carb to manifold adpaters are being bolted on to the manifold.


                  A closeup of the carb adapter - a steel and rubber composite.


Mikuni 42mm carbs, shown with throttle shaft couplers but without the connecting shaft.


                          Same carbs show with black top air filters.


  View from the top showing the spacers and carb mounts.


A simple way to check to make sure that the carbs are parellel to each other before tightening the mounts and installing the connecting throttle shaft between the two carbs.


After installing the shaft connecting the 2 carbs it is important to adjust the linkage so that fflat slides of both carbs are opening and closing at the same time and both fully close and fully open in responce to throttle pedal movement.


Note the cable throttle linkage using the stock cable and bracket.  The bracket may have to be bent slightly in order to line up the cable with the linkage arm. In the closeup view you can see the external throttle retun spring that we recommend be part of every installation.


Side view showing the carb mounts, linkage, hoses venting into the backs of the air filters, etc.


Article Under Construction - more details and photos to come. Also see our intstallation and tuning instructions section.