Inlet Manifold Mods

NOTE: If anyone wants to buy a modified manifold and Weber carby combo, check out the For Sale page on this website.

If you want get the best results from your engine and new carburettor combination, the standard inlet manifold must be considered for modification.  The standard entrance to the plenum volume is designed specifically for the butterflies of the Nikki carburettor, and not for the use of a Weber on an adaptor plate.  The picture below shows the two cut-outs in the manifold to suit the size of the Nikki's butterflies.  The Weber's butterflies are larger, and spaced further apart. 

If you choose to use the Weber on the original inlet manifold, then as the air/fuel charge exits the butterflies, it will hit the flat surface around where the holes are.  The following diagram shows the gasket for the Weber base, with the locations of the holes in the Gemini manifold superimposed on top of them.

To overcome this problem, the inlet manifold should be modified to suit the 'outlet' of the adaptor plate.  The following diagram shows what metal should be removed in red.

The gasket can be used to scribe the area that should be removed.  The tools you will require are a die-grinder or electric drill with the appropriate metal grinding attachment as shown in the pic below.

The entire area could simply be removed with the grinding bit, but this would be a very laborious process.  It is much simpler to place to cuts in the following positions with an angle grinder and 4 inch cutting blade.

The following photo shows how the manifold can look with the adaptor plate bolted on.  Note:  The adaptor plate has also been ported significantly to improve transitional flow between the carburettor and inlet manifold.

Another small modification should be made to the adaptor plate so that it can clear the vacuum lines going from the manifold.  The following photo shows is taken from the cylinder head side and shows where a small notch should be ground away to clear the hose.