[DeTomaso] Carb jetting, general guidelines wanted

[Tomas Gunnarsson]

During winter I went through the carb with a rebuild kit when the car was down for maintenance anyway. I replaced gaskets, seals, needle valves, power valve (6.5, same as before) and acc pump diaphragms. No setup changes in other words. The only real change was that I tidied up where the choke air horn had previously been. Someone had removed it hacksaw style leaving a short collar about the same height as the round air cleaner flange. I now set the carb up level and milled the collar off, flush with the top of the carb housing inside the air cleaner flange, just like the secondaries are set up. After setting the float levels just below the sight plugs the engine fired right up and after warmup idles pretty much as before. My problem was that the engine would run extremely rough below 2000 rpm. I used to be able to cruise in city traffic at 50 km/h, about 1200 rpm. I took the carb apart and blew through passages in the body and metering block. No improvement. The car would run well on the highway but sneeze on sudden acceleration i.e. lean mixture. I took the car on a long road trip and the first tank stop revealed more than 10% lower fuel consumption than what I normally see on the highway. Now to my questions. Is it likely that my machining has affected the conditions around the main circuit air bleeds to cause this leaning out? The carb seems to have come with a certain offset in jet size from the factory according to jetting tables, 67P/73S. The car used to have 77P/83S and no power valve when I bought it. I installed a 6.5 powere valve and rejetted to 75P/81S. I had first tried the 67P/73S setting thinking it had been up-jetted to compensate for the plugged power valve but the car was literally undrivable. Today I changed jetting to 80P/81S and drivability is much improved but not quite where I want it. Given the mechanical change, should I try to keep the original jet size difference between P and S or should I make the difference smaller (or larger)?

Tomas