Discussion
on my wedge there is the usual advance pipe from the plenum chamber to the dizzy ,and in between this sits a non return valve ,WHY as it holds the dizzy at max advance instead of holding for a nano second and then letting go , seems an odd device to fit to this area. does anyone know why these are fitted
It isn't actually a non-return valve, its a slow return valve i.e. it leaks (at least it does when it is working properly).
The vac advance line is connected to a throttle edge tapping in the throttle assembly. What this means is that for most of the time it is connected downstream of the throttle plate and sees manifold depression, but at idle the throttle plate moves past the tapping so it sees normal atmospheric pressure. The aim is to give plenty of vacuum advance under cruise conditions but none at idle. But there's a slight problem with this. When you slam the throttle open from idle, this system produces a brief spike of vacuum, it takes the distributor a finite time to respond and recover from this which interferes with the throttle response. The purpose of the valve is to prevent the vacuum advance from operating unless the depression is sustained for longer than a couple of seconds to prevent this glitch.
Of course, it is all too easy to fit the valve the wrong way round, some people have even done this deliberately based on a misunderstanding of how the valve works, and anyway the valves often get clogged up completely in which case all bets are off. If in doubt I would advise you to get rid of the valve completely, since the problem it aims to cure is trivial compared to some of the problems it can cause.
The vac advance line is connected to a throttle edge tapping in the throttle assembly. What this means is that for most of the time it is connected downstream of the throttle plate and sees manifold depression, but at idle the throttle plate moves past the tapping so it sees normal atmospheric pressure. The aim is to give plenty of vacuum advance under cruise conditions but none at idle. But there's a slight problem with this. When you slam the throttle open from idle, this system produces a brief spike of vacuum, it takes the distributor a finite time to respond and recover from this which interferes with the throttle response. The purpose of the valve is to prevent the vacuum advance from operating unless the depression is sustained for longer than a couple of seconds to prevent this glitch.
Of course, it is all too easy to fit the valve the wrong way round, some people have even done this deliberately based on a misunderstanding of how the valve works, and anyway the valves often get clogged up completely in which case all bets are off. If in doubt I would advise you to get rid of the valve completely, since the problem it aims to cure is trivial compared to some of the problems it can cause.
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