With Prattle, he achieved this but partially obstructing the air intake.
With Schrenk and Berger, they seem to have worked their air/fuel ratios, according to table 2, by measuring the exhaust gas volume and dividing by the fuel consumption
Furthermore to obtain satisfactory performance with gasoline engines, the fuel-air ratio is adjust so that there is too little air in the mixture for complete combustion of the fuel, thus producing considerable quantities monoxide. In contrast, Diesel engines can be operated at fuel-air ratios such that an excess of air is always present, and combustion proceeds much more nearly toward completion
The engines were adjusted in accordance wtih recommendations of the manufacturers except in the tests in which the adjustment of the fuel pump was altered to permit an increase in fuel injection
Effect of Fuel-Air Ratio on Composition of Exhaust Gas
The effect of fuel-air ratio on exhaust gases from internal-combustion engines is extremely important from the standpoint of hygienic atmospheric conditions. Figure 2 shows the relationship of composition of exhaust gas to fuel air ratios ranging from about 0.01 to 0.094 lb of fuel per lb of air. At a fuel-air ratio of 0.0679 there would be, theoretically, with the fuel used in these tests, just enough oxygen to burn completely all the fuel present, and this ratio is designated as the "chemically correct mixture" Thus, the fuel-air ratios studied included those in which air was present in considerable excess as well as those with insufficient air for complete combustion. The engines as received from the manufactuers were adjusted so that the maximum fuel-air ratio for engine A was 0.042 and for engine B 0.058 lb per lb. The excess air present under these conditions was 61 and 17 per cent respectively. It was necessary to change the manufacturer's adjustment of the fuel pump to obtain higher ratios at full throttle and this was done with engine B. The maxium ratio studient was 0.09, and at this ratio only 70 per cent of the air required for complete combustion was present
So in summary, you can get lethal levels of carbon monoxide out of diesel engine by either reducing the air intake or increasing the fuel intake. Both requiring a physical, mechanical alteration to the engine. From table 2 it appears you have to increase the fuel intake about 50% over and above the manufacturer's maximum for what is presumably a standard diesel configuration.