This can easily be taken care of by elongating the holes slightly with a rat tail or chain saw file or small rotary cutter in a die grinder. On some of the adapters the holes are spaced fractionally too close together to fit easily on some of the studs. It usually makes for a cleaner installation to remove all of the studs and substitute bolts - 5/16 coarse thread in most manifolds. It is important to prevent this as it can prevent the carbs from seating all the way in mount. On some manifolds the studs may be long enough that one or two will have to be removed or they will contact the back of the carb. If your manifold has 4 studs, just ignore two of the studs and use the two on each carb that make for the most convenient installation. Some SU carbs and manifolds use 4 studs, some use two studs. Note that the adapters have only two holes for mount studs. A similar type of mount is often used with SU carbs in high performance and competition applications. An significant advantage over more traditional mounting is that they isolate the carbs from engine vibration. We have used them in extensive testing over the past 10 years, including racing. These adapters are used in both automotive and motorcycle applications and are the standard mounts for these carbs. The carb adapters are the two black rubber parts pictured above - actually steel plates covered in rubber. The parts included in the dual carb kit are pictured below.
Some diagrams from that manual are included below. Basically the HSR 42, 45 and 48 are the same carb except for throat size and stock jetting. Go to the Mikuni USA website at: for a downloadable version of their HSR tuning manual, exploded parts view, etc. In addition to the intructions found here, instructions are available from Mikuni which refer to tuning the motorcycle applications, though the number sold to replace SU's on cars may now be approaching the number sold for motorcycles. Note how the linkage provided with the kit hooks up to the orginal SU linkage. An installation on a 68 Volvo 1800 is shown below. In addition to the Dual Carb Kit, Single, Tri-carb and Quad carb kits can also be ordered.
These modifications include replacement of the original throttle shafts and the installation of a smaller float needle and seat. The Dual Mikuni Kit contains carbs modified to allow them to be used in tandem to replace dual SU's (or Zenith Strombergs which were themselves an SU replacement) in an automotive application. The HSR Mikuni carbs were originally designed to replace SU carbs on Harley Davidson motorcycles but are now widely used to replace SU's on a wide range of cars. For all applications, be sure to read the tuning instructions located after the installation instructions below. Most of the information is applicable to any dual installation and much of it is also relevant to single, triple and quad installations. where they are used as a direct replacement for SU or ZS carbs. These instructions are primarily for the dual carb kits as used on Volvo 544, 1800, 122, and 140, MGB's, TR4's, etc. Mikuni Carb Kit Installation and Tuning Instructions - Updated 2/4/20
Mikuni Carb Installation and Tuning Instructions