15 replies to this topic

[lewBlew]

Anyone know how to test if mine is working? Apparently you can suck on the pipe but does this produce the required pressure?

Mine doesn't 'hold' suction if I suck on the pipe so I think the internals may be shot. No double entendres intended

[Boycie]

Easiest way is to remove the dizzy cap, suck on the small-bore plastic pipe and see if the dizzy baseplate turns. It won't move much but should rotate clockwise a little.
The diaphragm in the advance unit does fail with age, I replaced one a couple of weeks back that had given up/ split.

[lewBlew]

It doesn't move at all as far as I can see! Don't quite understand how I could make it move just by sucking on the pipe but ok.

[Boycie]

The pipe puts vacuum from the inlet manifold to a spring-loaded rubber diaphragm that is located in the unit screwed onto the side of the distributor.
Connected to the other side of the rubber diaphragm is a small rod that attaches to a pin on the distributor's base-plate. When vacuum is applied by the manifold, the diaphragm moves outwards with the vacuum and pulls on the connecting rod.
This rod, in turn moves and rotates the distributor baseplate, effectively altering the ignition timing until the vacuum disappears (ie, you apply the throttle and accelerate or stop sucking!)

[lewBlew]

Thanks for the info, think I need a new one then :)

[lewBlew]

Oh just want to say, despite being unable to get the dizzy base-plate to move via the sucking method, the unit itself does affect the engine speed at idle when I plug/unplug it with the engine on. If it's broken, would it still do this?

[Boycie]

Yes it probably would still do what you're describing to the idle speed, broken or not. Try doing the same test, but this time plugging the vac. hose with your finger. The revs will go up a little because when the hose is unplugged, you effectively have an air leak into the induction fanimold, albeit a small one.

[lewBlew]

When I plug it with my finger, or a screw or whatever, there's hardly any change. But plug it into the vacuum unit and the revs drop. This right?!

[lewBlew]

Woah, hang on, can't remember if they drop or go up. Let me go and check...

[bmcecosse]

If you can't suck a vacuum on the pipe - it's not working........(unless the pipe itself is burst??) New vacuum units are not inexpensive - you may want to consider a complete new dizzy from simonbbc or accuspark which comes with a new vacuum unit along with everything else....... I would just get the 'points' dizzy -but they also do electronic versions - for rather more money of course!

[lewBlew]

Right been outside to settle this once and for all!!!

Suck on the vacuum pipe coming out of the dizzy, for all it's worth, and the baseplate doesn't move. Nor does the pipe 'stick' to my tongue at all.

Start the engine and connect the pipe, idle speed goes down (sounds worse). Disconnect the pipe, idle goes up. HOWEVER.... plug the pipe coming from the carb with my finger or a screw and I can feel and hear it sucking but the idle speed doesn't change at all. Hmmm...?

[lrostoke]

Rev's normally go up when you connect the pipe..

No idea why yours are dropping, disconnect it from the carb end and suck on it when the engines running, see what happens.

The baseplate movement is normally pretty noticable..

[lewBlew]

I'm really confused about this. Tempted to get a new unit and distributor (Accuspark seem to do the 59D4 replacement units for less than £40) but there must be something else wrong for nothing to happen when I plug the vacuum pipe coming from the carb, but for revs to DECREASE when I plug it into the vacuum unit!

Edited by theboybradley, 07 June 2012 - 10:54 AM.

[Boycie]

Stop worrying about the revs, you're trying to fix a problem that isn't there. Your first job must be to get the vacuum advance working properly, whether that means replacing the diaphragm unit on the side of the dizzy or freeing-up a seized baseplate; all will be revealed when you get the dizzy out, clean it up and dismantle it on the bench.
If you need a new vacuum unit, this one from Minispares will do the job http://www.minispare...25.aspx|Back to
but I normally just raid my parts supply or a scrap yard for a suitable replacement :)

[lewBlew]

OK. Vacuum unit doesn't hold suction when I suck on it and do the whole tongue thing (!) so I assume it's broken rather than a stuck base-plate. But I will investigate both.

Oh the JOYS!