All air that is delivered to the Idle Speed Circuit is pulled in after the Fuel Metering plate. The inlet is below the plate. All air is metered and accounted for to maintain AFR. The problem with removing the Idle Speed Motor is that there is no Auxiliary Air Valve to compensate for fast idle conditions.
With the original idle speed system, the K-Jetronic manifold here was designed to use 3 brass screws and a balancing tube. Take a look:
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You'll notice that aft of the butterflies on those 90° manifold tubes, there is an additional tube plumbed in which connects both sides. The 3 screws that are present in the manifold for adjusting Idle Speed are first a feeder screw, and the final two being for the left and right cylinder banks. The tube that was included above on this earlier system allows for ΔP to occur in order for the engine to naturally correct the volumes between both cylinder banks for proper emissions. It also connected to the oil filler cap for crankcase ventilation.
Here is the problem with removing the Idle Speed Motor and using those screws to set Idle:
Once you set idle speeds, you're done. You cannot increase idle speeds beyond that point on a warmed engine. Once the advent of A/C became prevalent on cars, the additional load on the engine during idle could stall them. Thus manufacturers resorted to kicker solenoids on the carburetor throttle linkage assemblies to activate whenever the compressor clutch was engaged. But rather than install a kicker solenoid, BOSCH came up with this:
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Whenever the engine needs a fast idle condition, that valve slides open and allows additional air to bypass the throttle plates.
K-Jetronic is a modular system that accepts add-ons, and later on BOSCH came out with the updated microprocessor-controlled Idle Speed Motor that we use now. Instead of just blindly allowing air to pass through, it controls the amount through feedback and adjusts to 775± RPMs automatically.
So, if you're opting for utilizing the Idle Speed Screws instead, you need to either never use A/C again, or splice in an AAV to compensate. Even if you retain the stock idle speed control system, you have no way of properly balancing the left and right cylinder banks without measuring CO off of each manifold! The result is one side will be too lean, and the other too rich. Aside from LAMBDA now perpetually fighting your engine, you'll be robbed of power and may do other long-term internal damage to the engine.
LAMBDA also has noting at all to do with the Idle Speed system. LAMBDA only wants to measure O
2 content within the exhaust stream. It's adjustment points are within a bypass of the Primary Pressure Regulator on the Fuel Distributor by way of the Frequency Valve to control line pressures feeding the Fuel Injectors. The two systems are completely independent of on another.
Now there is another possibility for a malfunctioning Idle Speed Circuit: wiring. The test is simple though. Just test for continuity. Had this problem with my own car, grafted in a new wire to the ISM, and it worked just fine.
I also readjusted the Microswitch as well. The factory recommends setting the stop screw to only trigger the switch once the throttle plates are back at a fully closed position. Nope, that ain't happening. I lengthened the screw and bent the bracket up to bury that damn screw almost to the bottom without interfering with the throttles fully closing. The result was that the idle circuit triggered early and the RPMs would gently float down instead of rapidly falling. Same as a modern car, and far less voltage spikes from my alternator.