Solex 40mm MOHD (horizontal) and 40mm MV (vertical) carburettors were originally fitted to the Alvis FWD

The links at the foot of this page contain facsimiles of original Solex Instruction Manuals and a very nice catalogue for the MV and MH types, with colour illustrations.   


The original settings for these carburettors were:

  Supercharged (MOHD carb.) Unsupercharged (MV carb.)
Choke Tube: 26mm 28mm
Main Jet: 135 155
Pilot jet: 60 60

(note: the units for the jets are 1/100 mm)


In the MOHD carburettors, the main jets were mounted in "Assembly 4" which consists of a brass jet carrier and cap, each marked with the number 4.  However, the 1931 manual for the later MO.L type carburettors states that the older MO types can be improved by fitting Assembly 12, and using the corresponding later series of main jets.  A copy of the MO.L manual (in French) can be downloaded from the links below and the relevant information is on pages 18 and 19.  It contains an explanation of the purpose of the side holes in the main jets which is not available in the earlier manuals and in particular confirms that there are four series of jets, with progressively greater mixture correction, in the order: 51, 56, 52, and 54.  This means that, assuming that the same main jet is used, and gives the correct mixture at full throttle, then the mixture at part throttle will be weakest with a series 51 jet and richest with a series 54 jet.  The physical difference is in the diameters of the side holes in the jets. 


For the main jets used with Assembly 4, the correction series are, in order of increasing correction:  44, 41, 43, and 42  (see main jet drawing at the link below for the corresponding side hole sizes).   From surviving original carburettors, Series 41 seems to have been the original selection on the FWD Alvis. 


It has been found that the blown cars go better with larger choke tubes and correspondingly larger jets.  The rules are:

            Main jet size = [choke diameter  mm] x 5  +  10 or 15

            Pilot jet size =  [Main jet size] x 0.4

So, for example, a 30mm choke would go with a 160 or 165 main jet and 65 pilot jet.  It is best to err towards the rich side.  Obviously, you will now be wanting to make your own choke tubes and jets.  Drawings for the MOHD chokes and main jets are found among the links below. 


Once you have done that, you will find that your gravity-feed petrol tank cannot supply fuel fast enough to feed your thirsty engine.  To get over that, you will have to fit a larger needle valve in the carburettor float chamber.  Once that is done, you will find that the float does not have enough buoyancy to close the needle valve when the petrol tank is full.  The solution to this is always to carry a fire extinguisher. 


The links all open in a new window.  You may need to set your browser to allow pop-ups to see them. 


Solex MOH main jet

Solex MOH choke tube

Solex MV and MH Catalogue

Solex MV & MH Instructions 32 page version

Solex MV & MH Instructions 16 page version         (contributed by John Lang - thanks!)

Solex MOV & MOH Instructions 35 page version    (October 1928)

Solex MOH sectional drawing       This is a JPG image;  best to download and view in your picture viewer. 

Solex MOH.L & MO 1931 edition    (in French)   This is a slightly later version of the MO carburettor, with improved jet assembly and mixture control, interchangeable with the earlier versions.

Solex letter to a customer      This is dated 26 September 1929.  The letterhead, complete with Olympia Motor Exhibition sticker, is rather wonderful.