vacuum advance

[dudload]

hi all,

after fiddling around with the timing of the car, I'm beginning to think that the vacuum advance unit may not be working correctly. when sucking on the pipe with a glamorous assistant looking at the distributor with the cap off, we couldn't see anything moving inside. I disconnected the pipe at both ends and there isn't a blockage - am I looking at the right part of the dizzy and if so I imagine it means a new vacuum advance unit?

if so, is it better just to change the whole dizzy or is that definitely overkill?

[bmcecosse]

I would just get a complete new dizzy from the likes of Accuspark - or Powerspark. Then you know all will be well for many many miles...

[philthehill]

Now if you had a strobe timing light you could determine if the mechanical and/or vacuum advance were working and giving adequate advance without having to second guess the situation.

[dudload]

before taking the plunge with a whole new dizzy - is looking at the dizzy with only its cap off the right place to be looking? i.e. should I see the unit move clockwise ever so slightly?

having fun learning the car bit by bit

[simmitc]

A failed vacuum unit is quite common. You are testing in basically the right way. With the cap off, look at the spring that comes from the vacuum unit and hooks over a pin on the base plate. When sucking on the tube, you should see the base plate pulled round as the spring moves. It's an obvious movement. If nothing moves, then it's not working. Replacement is straightforward and cheaper than a new dizzy - but you have to consider how worn or otherwise the rest of the dizzy is.

[Gordi Smith]

A failed vacuum unit is quite common. You are testing in basically the right way. With the cap off, look at the spring that comes from the vacuum unit and hooks over a pin on the base plate. When sucking on the tube, you should see the base plate pulled round as the spring moves. It's an obvious movement. If nothing moves, then it's not working. Replacement is straightforward and cheaper than a new dizzy - but you have to consider how worn or otherwise the rest of the dizzy is.

Quick question...
How much movement should you see in the spring that comes from the vacuum unit when sucking on the vacuum tube?
My Moggy has been misbehaving, and I've checked this and it only moves a fraction... maybe 2mm??
Should it move more than that?

kindest regards,
G

[Budgie]

Well for what it's worth I changed my vacuum advance two weeks ago just because I saw that it wasn't working. I had no starting problems and she runs like a Swiss watch. I took out the distributor and cleaned and checked it over and reset everything that needed to be reset. Put the new vacuum advance back on and hey presto, she's as good as she was before I spent £28.

[RobThomas]

Check that the fat screw holding the points on isn't clamping the base plate down. If the screw is too long or the washers are missing then it can prevent the advance from working.

[bmcecosse]

2mm movement is probably about right. There's no 'guessing' - you can either pull a vacuum or not (by sucking) and the baseplate either moves - or does not. The thing is - for £28 you got a new vacuum unit = but the rest of the dizzy is presumably worn after many thousands of miles use - so for _£50 you get a complete new unit - uprated to the better 45D model, and for a bit more you can opt for the electronic version and forget all about the dizzy in future. It may seem 'wasteful' but I think at £28 a new vacuum unit is perhaps a false economy.