Does anyone know if there is a way to manualy control the secondaries on the stock 12a carb?
I have also heard that lack of exhaust back pressure can cause the secondaries not to function.

 

The stock secondaries operate on vacuum. Not exhaust back pressure. The other means of controlling them is to convert them to mechanical so they open with throttle position instead.

 

have you converter yours to mechanical operation?
How would you go about doing it?

 

I have converted one to mechanically operated secondaries by removing the vacuum pump thingy from the carb, plugging up the holes left, and wiring the secondary throttle linkage to the primary throttle linkage. Unfortunately I cannot tell you if this worked or not because about the time I got the carb on there the motor lost a water jacket seal and became an expensive rotary water pump!

I am putting in a used engine this week and will tell you if it works or not.

Hope this helps some,

 

That's the same way I've always done it and it works fine.

 

I don't know how they did it, but the carb I got (already installed) for my '80 RX-7 SCCA car had mechanical secondaries on it, and they seem to work just fine, pull the throttle a little farther and they open right up.