Vac advance pipe

[lotusandy]

Had my 1998 4.0 chim for a month now & am slowly going round it & checking everything over, have just checked the timing, it is set to about 8 degrees, however I did my usual test on the vac advance unit which is sucking on the plenum end of the pipe & checking that the timing advances significantly & that the vac advance in the dist' isn't leaking any air. However I found that I could not cause any advance at all until I removed the 'lump' in the vac pipe when it all worked as I would expect.

Can anyone tell me what this device does? it is about 1" in diameter and made of black & brown plastic, it seems to operate as a 1 way valve, allowing flow from the dist' to the plenum. However I can't see how the vac advance could ever work with it in place, can I just bin it?

Thanks Andy

[griffter]

I think it's a delay unit to damp transient changes in vacuum 'pressure' and make sure the dizzy baseplate doesn't respond to each and every fluctuation - only those consistent with part throttle for example (which is when it's intended to change the timing).

Some remove the unit and use straight pipe, some remove the pipe altogether and blank the vac advance unit and the plenum - the source of ongiong debate.

[GreenV8S]

The vac advance is connected to a throttle edge tapping. This means that when the throttle is closed there's no vacuum and hence no vacuum advance. Cruising with the throttle open a crack there's lots of vacuum and lots of advance. Full throttle there's no vacuum and no advance.

The problem with this setup is that as you slam the throttle open it goes through a brief period when there is full vacuum applied, and this causes the timing to jump as you go from no advance to full advance to no advance in a fraction of a second.

The point of the slow return valve is to prevent the vacuum from reaching the dizzy unless it is sustained for a second or so. So you don't get any advance whejn you slam the throttle open, but you do under cruise conditions.

The problem is that the valve can easily become blocked making the vac advance stay on or off at the wrong time, and it's frequently misunderstood and connected the wrong way round. These problems are far worse than the minor problem the valve was intended to fix and if in doubt I suggest you get rid and just connect a pipe straight from the throttle body to the dizzy.

[lotusandy]

Thanks for the comprehensive explanation, all is now clear, although it seem that my delay unit is shagged! Found that new ones are only 6 quid so have one on order.

Thanks Andy