Robin EX21 Intake - Pg.2

I used an air powered die grinder and some sanding rolls ("tootsie rolls") to smooth and shape the interior port of the adapter.  I frequently took the adapter back to the engine and placed it on the carb.  I then used the most powerful porting tool known to man (the finger), and carefully felt with my fingertip between the adapter and carb mouth transition.  I then went back to the die grinder and removed areas where there was a step/mismatch between the port surfaces.  Then it was back to the carb to check it again, and then back to the die grinder....  And so it went, little by little the port was shaped to be almost a perfect match to the carb throat.  I gently tapered the hole and radiused the leading edge of the port to give it a slight velocity stack shape.  After shaping it with the die grinder and some fine sanding rolls, I switched over to some very fine sandpaper and did the final polish of the port by hand.  Afterwards, one more time I took the adapter to a sheet of extra fine sandpaper and hand lapped both sides to polish up the surfaces a little more before mounting it.  I then installed the adapter and applied some thread lock to the carb studs and tightened down the retaining nuts. 

The Robin air box is a nice piece as far as stock engines go, but it has some restrictive passages that route the air down from the filter element(s) and then making a very sharp turn into the mouth of the carb.  Definitely not a good design for maximum air flow.  The passages were relatively small and the abrupt/sharp turn right before the carb was definitely not optimal.  One of the design features of the Robin engine that I liked was the advanced pentroof combustion chamber design.  Robin says that the EX head is modeled after the high performance combustion chamber design in the powerful Subaru WRX rally engine.

I mounted the oiled K&N style filter to the adapter and then slipped on an oiled foam pre-filter sleeve.  I went with a large K&N style fabric filter.  It's not an authentic K&N brand, but it was a good quality substitute.  I bought a nice, big 4-1/2" DIAMETER x 5" LONG x 2-7/16" INLET fabric filter for only around $10!  That's about 1/3 - 1/4 the price of a comparable K&N filter.  I also bought 4 of the foam pre-filter sleeves because they were only around $3 each.  That way, I could quickly and easily pull off the pre-filter when it is getting dirty and then just slip on another freshly oiled pre-filter and be good to go.  In this way, the inner K&N style filter should not need to be taken off and cleaned very often.  With 4 pre-filters to work with, I figured I'd have enough air filter capacity to last a full day of riding around in dusty conditions.  This high flow air filter setup gives around DOUBLE the filter surface area compared to the stock air filtering area.  In case anyone is interested in source I bought the air filters from American Power Sports.  The 4.5" DIA x 5" LONG x 2-7/16 INLET K&N style filter was part number AP1645.  The foam pre-filters were G1294.  Other places probably also carry similar (or the same) air filters.

For driving around in snow or very wet conditions, I also ordered an Outwears Pre-Filter sock that will slip over the K&N style filter (minus the foam pre-filter).  I ordered a Water Repellant pre-filter sock directly from Outwears (part number WR14-5T).  This thin fabric sock can be easily slipped over the filter in wet situations to help shed off water, slush, mud before it can get to the K&N style filter.  In addition, I have some sheet metal that I plan to use to form a simple shroud to help prevent dirt/mud/slush from coming up from underneath and hitting the air filter directly. 

So, how'd it work?  Well, I took it out for about 1 hour and I immediately noticed an improvement in performance.  The engine felt stronger and it seemed to accelerate quicker.  The torque converter felt like it was upshifting quicker and the kart felt faster.  This is all with the stock 3600rpm engine governor still in place.  I only have around 3 hours run time on the engine at the time of making this air filter change, and I plan to take it easy for the first 10 hours or so to allow the engine to fully break in.  After that, I plan to override the stock governor and let the engine rev up higher.  I also plan to re-calibrate the torque converter driven unit with a different spring that will allow the engine to rev higher before upshifting occurs.  I want to also gear down around 10% with a smaller drive sprocket (9 tooth).  All this will be done to try to allow the engine to rev up higher and take advantage of the extra RPM and power to give the kart better low end pulling (hill climbing) AND still give the kart a good top speed.  My goal is to rev this engine up to a moderate 5000-6000rpm.  At these higher than stock RPM's, I expect this high flow air filter system to make an even bigger difference in air flow & power!      

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