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I need some assistance from anybody who has experience with the B20E, in particular the set-up of the intake manifold and its idle air arrangement.

I rebuilt the B20E in my 142 and replaced the D jet controller with a Megasquirt. The engine starts just fine. The problem is that I cannot get the engine to idle at less than about 2500 RPM. It’s not that the engine dies if I try to reduce the idle speed, it’s that I have the idle adjustment screw bottomed and I am still getting enough air for the engine to run at 2500 RPM.

I have checked all the obvious sources for manifold air leaks, distributor diaphragm, brake booster check valve, fuel pressure regulator and MAP sensor connections and temporarily blocked off the crankcase ventilation system and blocked off the auxiliary air flow regulator. None of these were the source of the air flow. I eventually discovered a small air leak around the base of #1 injector and around the throttle plate shaft. Both of these have been taken care of and the high idle speed persists.

I have confirmed that I have eliminated the manifold leaks by blocking off the idle air inlet port in the intake manifold with my finger when the engine is running. The engine dies almost immediately so all the idle air is entering the engine through this port. I carried out a test by clamping off the hose to the auxiliary air flow regulator and screwed in the idle adjustment screw so that it was bottoming. I started up the engine and it continues to idle really high. I am now really confused. The aux air regulator is totally blocked off and the idle air screw is bottomed out and as a result there should be no air entering the engine through the idle circuit, yet there continues to be enough air for the engine to run at about 2500 RPM. Just to reconfirm that air is not entering through another path, I plug the idle air inlet port with my finger and the engine dies.

I started up my air compressor, disconnected the hose connected to the inlet side of the aux air regulator (#71 in the picture below) and connect the air supply from the compressor up to this hose so that it is blowing back through hose #71 into the intake manifold. I can feel air blowing out the idle air inlet port on the intake manifold as it should be. I plug the idle air inlet port with my finger and with the idle air screw backed out, I can hear the air blowing out the idle air outlet port on the other side of the throttle plate, again as it should. I now start screwing in the idle adjustment screw (#66 in the picture). The screw bottoms out and with my finger blocking the inlet port, I can still hear air blowing out the idle air outlet port. This is the mystery. I had been operating on the assumption that with the idle air screw all the way in, I would not have any air flow through the idle circuit. Such is not the case. I have plenty of air flow. Enough to allow the engine to run at over 2500 RPM.

The idle air adjustment screw has a tapered end. I had assumed that this tapered end would match up with a tapered port and that when the screw was totally in to the tapered port, there would be no air flow. I stuck a piece of stiff wire in to the hole that received the idle screw. Probing the hole with the wire indicates that the hole does not appear to have any perceptible taper to it. It appears to be pretty much a plug end. Unfortunately, space does not permit me to visually inspect the hole and the diameter of the hole is so small that I can’t fit any of the inspection cameras that I have access to in to the hole. The absence of taper leads me to wonder whether there is supposed to be some sort of insert in the base of the hole that would match up with the taper on the screw and that somehow, this insert has gone AWOL. However, the parts manual does not show any such insert.

As a final check, I slathered a very thick coat of machinist’s blue layout dye on to the tapered end of the idle air screw. I bottomed the idle air screw in the hole for the screw and then pulled the screw out. The machinist’s dye showed almost no marks indicating little contact of the taper with anything hard. There was a little of the dye rubbed off on one side of the screw at the very base (thickest part) of the taper. However, the taper is clearly not getting solid contact with anything around it complete periphery. I am at a total miss as to how this idle air circuit is supposed to be set up. Any observations as to what might be going on?
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