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Post by Penguin45 on Oct 23, 2015 11:53:45 GMT
Here's a little one to ponder. The Austin idles at about 800rpm. If I put my foot on the brake pedal, the revs rise by perhaps 50 rpm - certainly an audible difference. Presumably related to the action of the servo (Powerstop IIB), but what is actually happening?
Chris.
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Post by dave1800 on Oct 23, 2015 13:11:39 GMT
I can only guess! Does it stay higher? It it does perhaps it is weakening the mixture slightly somehow - similar to loosening the oil filler cap. I've actually experienced the opposite in the past caused by a combination of the brake lights loading a dynamo and some poor wiring. I assume there is no mechanical drag across the pedals. If it only rises momentarily then falls back again maybe that is just taking some load off the engine as the servo operates? My brain hurts  . I'm sure someone here knows the real answer. David |
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Post by tommydp on Oct 23, 2015 13:49:16 GMT
Don't know the technical explanation, but as far as I know it's normal for the idle speed to rise if you pump the pedal and must have something to do with the brake servo and the manifold vacuum. My guess is also that it weakens the mixture slightly. It should return to normal instantly when you let the foot off the pedal. I believe that if the mixture is correct, it will rise like this. If it stumbles and idle is down, it's too weak. Seems logical, as the symptoms are as the lifting pin.
Mine runs on if I stop it when brake pedal is in. Also if the heater is on. Never found the reason, but it must be something electrical. An issue in the green, ignition oerated wires I guess.
Tommy
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wolseley1800
Member
Posts: 127
Attribute: The Voice of Doom!
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Post by wolseley1800 on Sept 24, 2016 21:01:08 GMT
Lets work this through.
1, Pressure on pedal.
2, Vacuum valve opens.
3, The air in vacuum body is sucked into the manifold after the carb.
4, This will reduce the airflow through the carb - there is a set volume of air dragged into the cylinder.
5, Increasing pressure in venturi.
6, Causing a reduction of fuel.
So yes it runs weaker, but only for a moment then all returns to normal until the pedal is pressed further or the next operation.
Ironically, letting off pressure will increase petrol.
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Post by Penguin45 on Sept 24, 2016 23:59:10 GMT
Aha - hello Gordon! Catastrophic servo failure was the root of all this evil. It got much worse..... Did my head in - hence the Lockheed (Well, Powertune) now fitted with the Indiana Jones mounting brackets.
Chris.
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. I'm sure someone here knows the real answer.
