RE: JC 28 CC surging
CATA
If the cowl makes a big difference to the way the engine runs, you have one of (or both) the following issues.
A - The effective Inlet area is bigger than the effective outlet area and you are building pressure in the cowl. That pressure will change depending on a few factors but especially related to airspeed. To overcome this, you may want to look at cowl entrance baffling and/or cowl exit baffles.
B - Because of pressure fluctuations in the cowl, the carb ambient pressure reference will get an incorrect reading. To correct this you may want to run a tube form the reference point (the little hole on the cover with 4 screws on it - most solder a tube in there) to a static pressure area such as the fuselage or outside the cowl via a rear facing tube.