Lifetime Premium Member
Date registered: Oct 2002
Vehicle: 1967 250 SL, 1963 220Sb
Location: Los Angeles / Altadena
Mentioned: 3 Post(s)
Quoted: 102 Post(s)
The zenith carbs have a built-in switch to indicate no throttle. Under that condition (when your foot is off the gas) the transmission downshifts softly. If you don't have that switch it will clunk going into first. This is only an issue for cars with automatic transmission.
Weber is COMPLETELY useless in providing a solution to this problem. The solution is a microswitch connected anywhere to the linkage that triggers one of the three wires going to the transmission. I've seen this on the carb mounting and even in the accelerator pedal. It's hokey, but it's what you have to do if you want your car to downshift coming to the stoplight nicely.
I went through this on my 67 250S (which I no longer own) and it was a PITA.
Currently: 1967 250 SL, 1963 220Sb, 1965 300SE Lang, 1971 280SE parts car, 1972 Alfa GTV, 1965 Alfa Duetto, 1993 BMW R100R
Past: 1971 250 C, 1985 300 TD, 1967 250 S, 1968 280 S,1981 300 D, 1982 280CE, 3 Facel Vegas, Borgward Isabella Coupe, Alfa 2600 Sprint, Volvo P1900 (yes), numerous less interesting Volvos, ...