Combustion is chemistry... Why not balance the chemical equation... For one unit of fuel you need about 6 units of oxygen. Because air is only 20% oxygen it requires 30 units of air to chemically balance a simple chemical combustion reaction. Measure the pipe diameters, the exhaust velocity and calculate better values... Bet a clean burn about 1 unit of fuel with 30 units of air. Good luck!
LOOK at how black colored with carbon soot... more AIR to improve combustion... try adding another 2-3" diameter pipe. 1 unit fuel: 30 units air. Burn it for 30 minutes then double the fuel weight for 1 hour. You then need 30 times that weight quantity of air. If 1 cubic foot of air weighs 0.08 lbs. The fuel weight x (0.08x30)= cubicfeet of air required.
