We have been interested in the performance of these carbs for many years and they have always presented challenges to ensure that they operate correctly.
One of the issues has always been concerned with air leaks between the shafts and the PTFE strips that act as shaft supports/bearings.
Over the years there have been several solutions used to resolve these problems, some involving the use of bronze or Oilite bearing replacements and other providing Rotary Shafts seals using either R21 type seals, O Rings or seals from Mikuni Carbs.
We have tried a variety of solutions and the common denominator has always been a lack of consistency in terms of seal friction which always seems to affect idle behaviour and can create temperature sensitivity.
Over the last 12 months we have been working on a solution that allows the use of a precision ground shaft which we have electroless nickel plated to provide a wear resistant finish and should provide a very good quality seal.
We selected a material for replacement bushes manufactured by Quadrant Plastics which is sold as Ketron HPV PEEK
It is a very stable, chemically resistant to gasoline and ethanol and extremely wear resistant due to the carbon fibre reinforcement.
http://www.quadrantplastics.com/eu-e...on-R-peek.html
We have custom made bushes to suitable size to allow their installation in a Solex 40PI casting.
We quickly discovered that using an h6 shaft and an h6 reamer provided excellent results in terms of shaft fit and quality of operation BUT does cause problems.
We reamed a set of carburettor bodies on afternoon in February, throttle operation in terms of smoothness of operation was excellent but the next morning our workshop temperature was -2 degC and the shafts did not rotate. As the workshop warmed to 5 degC operation was once again perfect.
Clearly this was predictable and we hadn't allowed sufficient clearance to deal with the contraction of the main casting.
We still wanted to use a 'minimum clearance' so we used an industrial freezer and reamed the bearings with the carb bodies at -20degC.
We have adopted this as a standard practice.
Stage 1 - Machine the body of the casting to a suitable profile to allow the fitting of a replacement bearing. We have slot drilled the housing so we do not break into the bore of casting and have manufactured suitable tooling to allow this work to be carried out consistently.
Stage 2 - Insert Ketron Bush into the body of the casting
Stage 3 - Ream the bush to the correct size using a custom made Carbide Reamer with a pilot to ensure precise alignment.
The reamer is an h6 tolerance.
Install a custom made felt seal which has virtually zero friction and although an old-fashioned idea provides very repeatable results.
Results so far have been consistent and idle behaviour is extremely repeatable and shaft sealing is consistent.
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