Vacuum advance

[Arthurdaily]

Good morning

Could anybody tell me, please, WHEN vacuum advance is supposed to be operating? Thank you.

[bmcecosse]

Whenever there is vacuum in the inlet manifold...although some connections are designed to block the feed when the throttle is fully closed. But always in very slight throttle openings -ie when the engine is working lightly, in the cruise.

[Arthurdaily]

Right, thank you - any way of checking the actual 'suck' end? The dizzy is working fine, the vacuum advance is good - plate moves nicely when it's being sucked... but when I put my finger on the hole / tube sticking out of the carb body there is no 'suck' at idle, or with slight revs, etc.... can this not be checked like this?

[MarkyB]

Sounds fine.
Is there some reason you are looking for a problem?

[Arthurdaily]

No, no problem - but as BMC says vacuum is supposed to work at idle, but there is no 'suck' from the tube at idle... thus vacuum wouldn't be advancing would it?

[Arthurdaily]

What I meant by everything was working was it is theoretically working - but probably ISN'T in reality as there is no suck.

[bmcecosse]

Probe the connection - with throttle open, and closed. You could rig up a manometer U tube - although the vacuum could be quite high!!!

[Arthurdaily]

So you can't tell just by sticking your finger over it... this was my technical method!

[les]

The take off from the carb has such a small hole in it, I can't see how you would expect to feel suction by your finger. If the carb take off is not blocked then it will work!

[Sandun]

do u want test it - ok bring plastic vacum pipe set it to vacum unit nipal then shuk from ur mouth if it work fine move the plate

[Declan_Burns]

Probe the connection - with throttle open, and closed. You could rig up a manometer U tube - although the vacuum could be quite high!!!

Probably a bit too high for a U-tube. I have a gauge on the TD for test purposes and it reads -700mbar at idle.
Regards
Declan[frame][/frame]

[bmcecosse]

Manometer with Mercury.......in the legs

[Arthurdaily]

Right, taking advice I bought a 'Hawk Command' vintage vacuum tester - it registered nothing at all at idle, or when reving or when standing on my head... !!

[Arthurdaily]

It says in the vacuum booklet that an average car should read minus 20 lb - is that right?

[bmcecosse]

Minus 20 lb??? of what?? beans ??? Vacuum is generally measured in inches of Mercury - although furriners have their own systems......

[Arthurdaily]

20 inches of mercury. You simply replace the plastic advance tube with this and measure. Nothing. The gauge works, give a little suck and it begins to move... equally give the vacuum advance pipe a suck when idling and the pitch changes slightly...

So, this equals no sucky suck from the little hole in the side of the carb - why might this be??

[Arthurdaily]

I have, of course, stuck a pipe cleaner in, and it does not appear blocked.

[les]

You say earlier there is no problem, so why on earth are you putting yourself through all this? If the plate moves when you suck the tube it will move when connected, as a vacuum in the manifold is natural event. Now stop it!!!!

[Arthurdaily]

Haha, yes yes, I know, but the vacuum is SUPPOSED to work at idle yes? And it doesn't, nor when I rev the engine, there appears to be no vacuum - I just NEED to know why.

[les]

How do you know it doesn't? Get a see-through distributor cap and then you can see the plate move!