Fast idle device, 280 SE 1974

[carl888]

Thanks Oscar,

Well, I tested the AAV and its working as it should.  Nice and open when cold, closed when hot.  I think it just needed to be used.  I've put 1,000 kms on the car in two weeks, its totally fab!  It's a really ncie feeling to get into a 35 year old car that's so nice to drive and everything works. 

A couple of points.  I can't get the car to idle below 1,100 rpm.  My guess is its ingesting some false air somewhere, or the fuel pressure is too high (Haven't checked the fuel pressure) or there is some issue with the throttle position sensor.  But I can confirm the following:  Ignition timing is correct, mixture is correct, all vacuum lines intact.  I have backed off the throttle butterfly till it's closed and the idle speed adjuster on the inlet manifold is also closed and 1,100 rpm is as slow as it will go.

Thanks for any suggestions.

Regards,

Carl.
 

[Big_Richard]

im no expert on Djet, but those throttle switches are adjustable.

you should feel the microswitch inside click as soon as you move the throttle valve from the closed position, if the switch is maladjusted at the closed throttle position so that i thinks the the valve is open, the revs will sit higher - it kicks up the revs as soon as the throttle is touched basicly to prevent hesitation.

[oscar]

It has to be false air surely, because if the engine's hot and you have that idle screw all the way in and the AAV is working like you say it is, the engine will stall in no time.  Regardless of fuel delivery there simply wont be enough air.

It might be worth revisiting that vacuum switch in your first post and your dual can vacuum advance unit.  I'm not sure the diameter of the vac lines could allow enough air through to cause the high idle, but, if the diaphragm on the retard side of the advance unit is perforated, it will allow extra air through during idle.

Have you ever tried propane to test for vac leaks Carl?  Well I haven't, but have been told it's the way to go. ie, poke an unlit end of a propane torch around the plenum and other bits whilst the engine's running and if the revs run away at any point, you've found a leak.  1100rpm with AAV and idle screw closed is just way too fast not to be a decent vac leak IMO.

The TPS I believe will only lead to sustained increased revs if adjusted to an extreme (clockwise or counter'- I can't remember), one way will cause the throttle butterfly to be jammed open letting extra air in.   Although you've said it was closed in the above test.
On the fueling side of things, if there's any doubt over the TPS causing overfueling, disconnect the wiring from it and see if that makes a difference to a hot or cold idle.  It shouldn't.  But if it does, well then I'm stumped.