Velocity Stack - Dell'orto SHB 19/19

 The original carburetor air box on the Vespa 100 Sport is a convoluted path for air to travel.  The thought was to increase efficiency of the airflow and increase engine performance slightly.  Air entering a standard straight-edged intake pipe or around corners creates chaotic swirling of the airflow.  These vortices effectively narrow the opening, making the air "fight itself" and slowing down the overall flow.

A velocity stack's curved, radius opening allows air from all angles to transition smoothly into the intake. This creates laminar flow, where air stays attached to the pipe walls, maximizing the volume of air reaching the combustion chamber.  As air moves from the wide, flared opening into the narrower intake runner, its pressure drops and its velocity increases. Faster-moving air has more inertia, which helps it continue rushing into the cylinder even as the intake valve begins to close, leading to better cylinder filling.

The design was simple enough that I could easily build it in TinkererCAD using its standard library of primitive shapes. The geometry didn't require too much intricate sculpting.

The updated design was 3D printed using PETG plastic, a material specifically chosen for its fuel-resistant properties and durability in automotive environments. PETG is naturally resistant to gasoline, oils, and greases, making it ideal for parts that come into contact with fuel.  To ensure a secure and stable installation, the velocity stack was designed to mount directly onto the carburetor’s existing threaded studs, eliminating the need for additional specialized hardware.

For the initial testing phase, a temporary protective mesh was fitted over the intake opening to serve as a primary debris shield. This screen was essential for preventing large contaminants or foreign objects from being vacuumed into the engine’s internal components, which could cause catastrophic failure during high-RPM operation.  While functional for safety, the current screen is a rudimentary fix. It provides the necessary protection for testing but a filter for fine-particle filtration will have to be employed.

The initial tests were very good.  The engine was running just a little rich prior to installing the velocity stack.  The first test ride provided a bit of a lean idle which is easily remedied.  The wide open throttle response was much improved.  There was no hesitation through the range of throttle opening.  The cylinder head temperature was slightly higher with the additional air being added.  After the suitable air filter was fitted the main jet had to increase 4 sizes for proper running.

Overall the project was a complete success.

"Project Bantam" - Carb corrosion

While this doesn't look too bad on the outside, inside lurks horrible corrosion to haunt your dreams.

Finally managing to coax the carb apart exposes the full horror of how much corrosion can actually enter a carburetor.  The float was split from freezing, and also made concave in one area due to the pressure of ice under it.

I've never met a float that I couldn't solder back together, and this was no exception.  I can solder anything.  It took a few hours of cleaning, careful straightening, and reforming.  The difficulty was in the brittle nature of the inner side walls.  They had to be tinned from inside and out.  I don't know if it will hold up in the long term, but its water tight now.

The final product looks fairly good. I spent an entire afternoon with the top of the carburetor in the lathe truing up the mating surfaces of the flanges.  The screw tabs were bent from over-tightening, and prevented the whole assembly from fitting together correctly without any air leakage.

It will be seen whether the corrosion of the thread holding the main jet (which prevented its removal) is problematic.  The only issue now is to set the float level and find a packing nut for the main jet adjustment needle.

On a side note, the main jet of the carb had to be drilled out to fit the new one I bought in a Stens 520-049 rebuild kit (original B-S kit # 394693).  The Stens needle valve kit 525-246 (original B-S kit # 99525) will have to wait until I can clean the tapped hole in the body with a 5/16-32 bottoming tap (once I get one).