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Re: "Running - On"

Subject: Re: "Running - On"
From: (Graham McCann)
Date: Sat, 11 May 1996 09:34:41 +1000 (EST)
>I've learned my first lesson in replacing something that doesn't need
>replacing. My 80 B has been "running-on" so stupidly checked the run-on
>valve. It was not working so ordered a new one. I just replaced it & took the
>car for a ride to warm it up. I couldn't wait to get home to shut it off.
>When I did, yup you guessed it, it still dieseled. What's the answer? I have
>found I really like to tinker with it. Thanks in advance!
>80 MGB

Running on is often caused by having the idle speed set too high.  What
happens is that the fairly poorly machined inside of the head remains hot
enough once the motor is turned off to be able to ignite any fuel sucked in
by the motor remaining turning over.  You thus have a diesel engine.  By
reducing the idle speed as low as possible (below 700 rpm if possible) you
shut off this air/fuel supply and the motor should stop.  If you have SU
carbies lack of oil in the dashpots can contribute to the problem.

In the same way an air leak into the inlet manifold can contrive to suck
sufficient fuel into the manifold to run on - even if the carbie butterfly
is against the closed stop.  This leak might be around the inlet manifold
gasket or at one of the gaskets which go between the carbie body and the
actual inlet manifold.  If you spray some WD 40 or the like around the
mainfold you might see it being sucked in.  Depending on the wear within the
carbie itself - shafts for example, air might be getting in there as well.

Finally running on is sometimes caused by the engine running at higher
temperatures than it is designed for.  Check engine timing.

Hope this helps,

Graham McCann  Rivett, ACT. Australia
06 2889055

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