I have a question regarding the two adjustment screws atop my 90 RX-7 NA. The first screw is the small flat head cylinder right on top of the engine (sorry, no pictures). With the engine running, this screw makes no difference in the engine RPMs, up or down. What is this screw and what is it supposed to do?

The second screw is right at the trottlebody and appears to affect the slack in the throttle cable, however, with the engine running, I do not notice any increase in engine RPMs when I adjust this screw as well. What is the function of this screw and how is one to know when it is sitting in the ideal position?

Also wanting to know where the air fuel mixture screw is on this engine. Thanks.

 

S5 do not have a "air fuel" mixture screw. The first screw you are talking about sounds like it is the one on top of the throttle body. If turning it makes no difference in idle, you either have a major vacuum leak, or the passage to the screw is carboned up and needs to be cleaned.

The seconds screw I cannot tell you as it will only be a guess that what you are trying to describe. There are varies screws on the throttle body that control throttle cable operation... Maybe you are talking about the screw that is near the TPS? This one you use to adjust the TPS sensor properly...

 

Sounds like you're talking about the throttle-stop screw and the idler control screw.

 

Please see the picture below and confirm this key:

GREEN: idler adjustment screw? How exactly is this screw increasing/decreasing RPMs? Vacuum? Fuel? Should this screw be all the way screwed in when adjusting the TPS or does it matter?

BLUE: Wide band TPS screw

RED: Narrow band TPS screw

 

The green is the air adjusting screw. It sets a fixed amount of air to flow past the throttle plates. The turbo engines have that screw on the BAC valve itself. Lots of other cars have something similar, like 90s Honda throttlebodies.

You need to jumper the initial set coupler when you adjust that screw.

 

the red screw does set the narrow band TPS but the wide band is not adjustable, i cant remember but the last screw is i think secondary throttle plate stop adjustment.