1971 250C carb issues
 

I have rebuilt both carbs, Zenith INAT 35/40s, sync'd AND set per the Jamiekop manual. Issue I have is the rear carb idle mixture jet makes no difference when adjusting. Changed to a different carb and have same issue. Front carb will idle down and stall when turn the idle mixture screw in. All passages are clear and vehicle will stall when covering the inlet with my hand. I'm a 40 yr plus working on cars, 10 yrs retired. I'M STUMPED......