Intro: Dellorto SHA Velocity Stack / Air Filter Mod
Those familiar with mopeds will be familiar with Dellorto carbs; they are probably one of the most common brands. Many serious tuner dislike the SHA line, though I am drawn to the simple, solid design. Tuning a SHA is very similar to tuning a Bing carb, which I am familiar with from working with my Sachs-engined Columbia Commuter.
However, the Dellorto SHA carbs are not without major flaws.
One of my biggest gripes with the design of the Dellorto SHA line of carbs is the design of the air filter and the start of the carburetor throat. The air filter does not filter anything smaller than a bird, and only disrupts air flow. The throat opening from the air filter is a perfect right angle edge- not conducive to smooth air flow. I feel that some of the issues with the carb's performance can come from fixing that sharp edge, and air filter.
There are many ways to modify the air filter on SHA carbs, highlighted on the Moped Army site, but I wanted an even more free-flowing filter and to use the materials I have around the house. As for fixing the air flow, a velocity stack can be added. A velocity stack can be though of a funnel, but with a flare like a trumpet bell. This shape allows smooth air flow into the carb and to make the carb more efficient at drawing air in.
For the air filter:
Start by removing the metal filter from the carb. Using nothing but brute force, push the screens out the open side of the metal ring. The screens should flex and pop out. You can also use needle nose pliers to grab the mesh and pull it out one piece at a time, though this tends to mess up the mesh. I would recommend saving all the mesh circles, they could be useful later on. We want to keep the one with the biggest holes though for this project.
Next we need some filter material. For this, thin filter foam (~.25") can be used, as well as panty hoes (they work). I used speaker grill cloth, because it is durable, does not react with gasoline, and has very small holes in it. If you want to try any other materials, leave them sitting in gasoline overnight to make sure they do not react. If foam is used, it is likely you will not be able to fit a velocity stack in the carb as well.
If foam is used, cut a circle the same size as the mesh circles. If some cloth is used, cut it about .25" larger than the mesh on all sides.
If foam is used, put it in the metal filter ring first and then pop in the large holed mesh behind it. Give yourself a pat on the back, you are done with the filter! if cloth is used, spread it over the mesh circle first, and pop the mesh into the metal filter ring so the mesh is behind the cloth. make sure that the mesh is bent back so it is flat. Needle nose pliers can be used to pul the cloth tight from behind. Repeat patting process.
For the velocity stack:
The velocity stack requires more creativity. I used a plastic disk, a random plastic cylinder thing, and a plastic tube. All of these parts are things I had lying around from broken water guns, pressure washers, and wire spools (I think). The plastic tube had the same inside diameter as the carb throat (15 mm) and the same outside diameter as the hole through the plastic cylinder. The plastic disk had the same diameter as the air filter. The parts were glued together with JB Weld, and then the tube cut to length, and a Dremel used to file out the flare indicative of a velocity stack. The velocity stack has to have a cutout for a small vent below the carb throat, and be shaped to fit in with the uneven surface of the air filter area of the carb. As is apparent, THIS MOD REQUIRES THE REMOVAL OF THE CHOKE TO FIT THE VELOCITY STACK. The height of the velocity stack has to also be even with the edge of the carb, and the width small enough the air filter ring to fit over it. This allows the modified air filter to fit over the velocity stack. The velocity stack is held in the carb by JB Weld, with the small peg that acts as part of the choke stop providing an anchoring point.
This is obviously a very vague description, but this is the nature of this kind of mod. Using what I did as a basis, its up to you to figure out exactly what to use for the velocity stack and how to assemble it.
Well, there you have it! studying the pictures might make more sense than the words. This modification can be done for absolutely nothing, and requires some creative thinking. But it should serve well to help the carb function better. Unfortunately, I cannot give performance specifics, as I am kitting my moped at the moment and its not running yet. But the science of smoothing airflow is proven to help carbs.