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Hi Crew,
My mechanic has informed me that my 186 HT (has 186s exhaust headers, inlet manifold, and ww stromberg carby fitted) has full mechanical advance at idle. This does not seem right to me, although I have heard of makes of cars that are setup like this (why I do not know). The car is running a 2 speed stock powerglide.
Can anyone confirm that that full mechanical advance at idle is a sign of something not right (most likely faulty or incorrectly curved distributor?
Cheers
htprem
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I'm a bit rusty on dizzys,but you shouldn't be getting full mechanical advance at idle.
I'd be looking at perhaps broken advance springs or the advance weights stuck in the full advanced postion due to being gummed up.
Best to have the dizzy on the bench to sort it.
From memory and this is a stretch, you remove the vac advance plate
and the mechanical advance is under that.
Hopefully there's someone here more current on dizzys than me.
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Your mechanic is either using the wrong terminology or hasn't got a full grasp of the situation.
All old fashioned distributors have 'mechanical advance' but there are 2 types: centrifugal & vacuum. Most distributors have both, some have either 1 or the other.
All Holden engines of that era, have both centrifugal & vacuum advance built into the standard dissy.
Centrifugal advance is RPM related, i.e. the higher the RPM, the more the advance. This is by definition zero at idle, because that's where it begins.
Vacuum advance however relies on vacuum & that depends on where the vacuum is sourced. All automatic Holden engines of the era have their vacuum sourced from below the throttle plate in the carby, which in effect gives full vacuum advance at idle.
In a nutshell, that's how it left the factory.
Dr Terry |
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Hi Dr Terry,
Thanks for your insights. I understand how the mechanical advance works with the increase in engine rpm, but I am confused as to why the vacuum advance would be maximum at idle - pardon my ignorance, but I thought the purpose of all timing advance systems was to advance the ignition timing as engine rpm increased, and that vacuum advance took care of things low in the rpm range, whilst mechanical came into effect more so as the engine rpm increased, ie, vacuum advance took care of the 'blip the throttle' type of scenario and that mechanical advance took care of your driving situations beyond that initial throttle response - although I would not be suprised if my understanding is not quite right.
Cheers
htprem
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Vacuum advance works by advancing the timing under low load (high vacuum situations) to aid economy, emissions & smoother running. At idle there's next to no load, so it's a good way to improve the smoothness (and also slightly increase the idle RPM). |
Cheers,
Mick
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