Ok so it's pretty obvious how to adjust it but how do you know how far to adjust it? And what does the "accelerator pump mod" have to do with mech secondaries?

 

Screw the adjustment nut in all the way. Open the throttle a bit by hand. The idea is to get the AP to start shooting fuel as soon as the throttle opens, but you sacrifice volume if the nut is screwed out too far. So back the nut off (and test) until it shoots just a little bit when the throttle is opened. You kind of need experience to know the right spot just by seeing the pump shot, so you may need to test it on the road.

There are several AP mods you can do, so it depends on which one you mean. The AP exists because air is lighter than fuel, so when you open the throttle quickly, more air enters the engine much faster than more fuel, so the mixture goes lean. This normally only occurs when the primary throttles open, because the vacuum secondaries always open relatively slowly. But with mech secondaries, if you slam the pedal the secondaries jerk open as well causing another (or extra) lean condition. So you want to modify the AP to help reduce this lean bog when the secondaries open.

The only useful AP mod IMO is to increase the displacement of the diaphragm, either with extra gaskets as per Sterling's mod, or by cutting back the stops on the main carby body side (also you can cut some material off the inside of the AP cover). This gives you a fuel shot that lasts longer in the primary throttle's movement, which helps reduce the lean bog on transition. You still have to press the pedal slowly though.

There are other mods such as re-jetting the AP and extending the lever to make the shot last longer. I would not bother with these because unless you increase the displacement of the AP, making the shot last longer will only give you a leaner acceleration mixture. And the drop in power is noticeable.

 

So basically is so the car doesn't fall on its face when you start to give it throttle? I was referring to the mods that stirling does. Basically the double gasket and the extended arm. I read in one thread that it's a must when you switch to mech secondaries but the accel pump stops spraying by the time the secondaries open.

 

Yes correct, you can disable the AP and try opening the throttle. The car will probably die unless you press the pedal very slowly.

Double gaskets is a good idea, don't bother with the extended arm. The latter will make your acceleration mixture leaner which means lost power, when I tried this it was very noticeable.

Like I said before, a longer AP stroke helps reduce the bog on transition, but doesn't always eliminate it. The best thing I have been able to do to get a smooth transition is getting the secondaries to open earlier with the mod linked below.

https://www.rx7club.com/1st-generation-specific-1979-1985-18/improved-mech-secondaries-mod-969355/

 

I managed to eliminate the bog without altering the arm. I just drilled out the AP jet a bit and used the gasket method to increase the volume. Then I adjusted the nut with a little trial and error until I got a lasting shot of gas.