Robin EX21 Intake - Pg.1

Even though I really like the Robin EX engine design, I felt that the stock air box was too restrictive for what I wanted to get out of this engine.  So, I decided to switch over to an open element, higher flow K&N style fabric filter.  At first, I hesitated in doing this because the areas I drive in are typically very dusty, and I was afraid that an open element sitting near the rear wheel would quickly plug up.  My Robin EX21 Deluxe engine came with a dual air filter element.  There is an inner paper element and an outer oiled foam pre-filter.  As it turns out, that oil foam pre-filter would quickly plug up anyway.   After realizing that the stock filter element was plugging up after only about 1 hour of use, I decided to get rid of the stock air box/filter setup.  I didn't think an open filter element would plug up much worse. 

I did some research to see if there was an existing air filter adapter intended for a different engine or carb that could be adapted to work with the Robin engine.  I even bought one that I thought would work, but it turns out it didn't work out.  So, I decided to make my own.  I called the local metal supply shop and picked up some 2.5" diameter 6061 Aluminum rod stock.  I had the shop cut the rod to the approximate thickness I wanted with their band saw.  This left a very rough surface on both sides of the slice of aluminum rod.

Because I don't have any serious metal working tools at my disposal, I began the SLOW process of forming this adapter primarily by hand with more common tools.  If I had access to a lathe and mill, it would have made this whole process a lot faster and easier!  Even so, sweat and determination can be a good substitute for expensive machinery!

At first I put down piece of coarse sandpaper on a flat surface and began hand lapping the raw slice of aluminum.  I quickly realized that it was going to take too long just doing this, so I grabbed an old hand held vibratory sander and put some rough sandpaper on it.  Then I put on a glove and held the slice of aluminum against the sandpaper.  This helped knock off the rough surface faster - although it was still slow going.  After getting a relatively flat, rough surface, I took the piece of aluminum back to the sandpaper and continued to hand lap the surfaces.  I used progressively finer sandpaper until I got a nice smooth surface on the critical areas like the gasket sealing surface.  I didn't try to polish the whole piece perfectly because I knew that only the surface that mated with the carb needed to be truly flat.   

Next, I needed to put the holes in place.  Using a carb gasket as a template, I marked off the center port opening, and the mounting holes and took it to my small bench mount drill press.  For the center hole (which needed to be around 1" diameter), I used a 7/8" hole saw.  I purchased a bi-metal hole saw from Home Depot that could cut through metal.  This is NOT precision machining!  Regardless, I didn't have a milling machine at my disposal, so I needed to make the best of what I had.  I chose the smaller 7/8" hole saw size because I expected the surface to be very rough as the hole saw tried to chew through the 6061 aluminum.  I wanted the port to be undersize so I would have some extra material left over that I could hand shape and smooth to get to the final diameter that matched the carb throat.  The hole saw worked, but it was very slow going and it did make a very rough hole.