Mighty Demon 750 Carburetor Street Tuning - Mighty Max

Achieving The Most From A Demon 750 For The Street

Sean Haggai Jul 1, 2009 0 Comment(s)
0907chp_05_z Mighty_demon_750 Power_valve 1/17

Power Valve
What does a power valve do? In a nutshell, it allows the carburetor to function much leaner for part-throttle fuel economy. However, once the throttle is in a wide-open position, vacuum in the manifold drops to almost zero and the power valve enriches the fuel mixture. The cool part is that when the engine is returned to basic part-throttle operation and vacuum rises, the power valve closes up, leaning out the mixture. This makes a power valve important because of its effect on everyday, around-town drivability. For instance, once the vehicle is put into Drive, the vacuum drops, sending a vacuum signal to the carburetor and telling it to open. This could make the engine go rich with fuel. The engine could start to blubber and run inefficiently. A rule of thumb: One power valve number is worth 6-8 fuel jet sizes.

0907chp_06_z Demon_750 With_76_83_jets 2/17

38 Timing & 76/83 jets
The 383 was making nearly 380 lb-ft at a mere 3,000 rpm with a peak of 399.6 ending at 4,900 rpm. Horsepower came in a bit later as usual, with 398.4 hp at 5,500 rpm. But brake specific fuel consumption (BSFC) is what we were after. McKindley explained, "BFSCs are important because they can define how efficiently the carburetor is working." Our goal was to make the most useable power from the least amount of fuel. Ideally, we want BFSCs in the mid to low 0.400 range. BSFC are an indication of engine efficiency, not carburetor efficiency! A finely tuned engine with a higher compression ratio can get into the low 0.400 range. A 0.590-0.600 range is appropraite for 8:1 compression engines.

0907chp_07_z 73 Jet 3/17

38 Timing & 73/80 Jets
For our second pull, we were curious to see what a leaner fuel environment would do. We swapped in smaller 73 primary and 80 secondary jets, but kept the timing at 38 degrees. To our surprise, the leaner fuel condition actually dropped power across the board. We even backed up this run with another one just to confirm the numbers.

0907chp_08_z 78_85 Jets 4/17

38 Timing & 78/85 Jets
For our third pull, we opted to swing in the other direction by adding fuel. We popped open the bowls and installed an 78 primary and 85 secondary jets., while leaving the timing at 38 degrees. Our efforts didn't go unnoticed, as the increase surpassed our baseline numbers.

0907chp_09_z 36_timing And_78_85_jets 5/17

36 Timing & 78/85 jets
During our testing we were curious to find out what lower timing numbers would produce on the dyno while still utilizing the 78/85 primary/secondary jetting in the Mighty Demon. On average, the results illustrated that we were using more fuel (average and peak BFSC were 0.590 and 0.661) through the dyno pull compared with the previous average BFSC of 0.558. Horsepower and torque numbers were minimally affected. However, because we were using more fuel and made less power, we deemed the combo inefficient.