Vacuum Advance

esray

esray

Member
Dear Forum,

I am afraid another dumb question!

I understand how the timing is advanced, first by the vacuum unit and secondly by the centrifugal force exerted by the sprung weights; however, should the whole vacuum unit move in and out as you throttle up and down? Would this not negate some of the effect of the aforementioned?

ATB Ray
 
pat180269 said:
described here http://en.wikipedia.org/wiki/Ignition_t ... on_systems
Click to expand...

Thanks Pat,

given following extract:

'Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance'........

....And in order to leave me in absolutely no doubt, this says that the return of the base-plate towards the non-vacuum setting is normal; my question is, does this happen by way of the release of the spring attaching the diaphragm in the vacuum unit to the base-plate (thereby allowing the base plate to return), or does it happen by the movement back towards the distributor of the whole vacuum unit?

Do you see my point?

Ray
 
Hi Ray,

if i understood your question correctly...

The vacuum advance port in the carburetor is placed in such a way in relation to the throttle disc, that the engine induction creates no vacuum at idle and wide open throttle positions, but shows full vacuum at small throttle openings. This is how the vacuum advance is regulated.
The connection of the vacuum advance unit to the distributor base plate is stable all the time.
 
Demetris said:
Hi Ray,

if i understood your question correctly...

The vacuum advance port in the carburetor is placed in such a way in relation to the throttle disc, that the engine induction creates no vacuum at idle and wide open throttle positions, but shows full vacuum at small throttle openings. This is how the vacuum advance is regulated.
The connection of the vacuum advance unit to the distributor base plate is stable all the time.
Click to expand...

Hello Demetris,

Thank you for your post.

The whole end of my vacuum unit with the vernier scale and the vacuum pipe moves in and out when I rev....Is this normal, please?

Ray
 
raylish said:
The whole end of my vacuum unit with the vernier scale and the vacuum pipe moves in and out when I rev....Is this normal, please?

Ray
Click to expand...

No! Whatever moves when you rev the engine (and change the vacuum...) is the diaphragm and the spring that connects the diaphragm with the base plate. All these are inside, and you can't see them moving with the engine running. The vernier scale should only be moving by turning the small knurled wheel.
 
Demetris said:
raylish said:
The whole end of my vacuum unit with the vernier scale and the vacuum pipe moves in and out when I rev....Is this normal, please?

Ray
Click to expand...

No! Whatever moves when you rev the engine (and change the vacuum...) is the diaphragm and the spring that connects the diaphragm with the base plate. All these are inside, and you can't see them moving with the engine running. The vernier scale should only be moving by turning the small knurled wheel.
Click to expand...

Thanks Demetris.

I have a video but I appear unable to upload it to the Forum?

Ray
 
I don't think that you can upload videos in the forum like you do with photos.
You have to upload them to something like photobucket for example, and then post the link here.
 
Hi Ray,

as i said before, it shouldn't be doing that. If you bought the distributor new recently, i would suggest to let the seller know about it and ask for a replacement.

If it's the original item and you feel brave enough to take it apart, you could find out why it is like this. Does the knurled wheel at the other end moves along with the vacuum module? Ideally, the knurled wheel should not have any slack on its threaded shaft that it's turning around, and the threaded shaft should be connected rigidly to the vacuum module.
 
Hi, I think you have two problems there, Demetris is right there is too much slack around
the knurled wheel and so excess movement. The other thing is the diaphragm is clearly
working because it's pushing and pulling the unit, so I think the baseplate is seized as well.
Undo the knurled wheel to release the vac unit and insert a shim (washer) between the wheel
and body. Remove dist cap and remove screws holding the baseplate to the body investigate
why it doesn't turn, either shi.. dirt has got between the two plates or a too long a screw
fixing something to it has been used.

Hope this helps!

Colin
 
colnerov said:
Hi, I think you have two problems there, Demetris is right there is too much slack around
the knurled wheel and so excess movement. The other thing is the diaphragm is clearly
working because it's pushing and pulling the unit, so I think the baseplate is seized as well.
Undo the knurled wheel to release the vac unit and insert a shim (washer) between the wheel
and body. Remove dist cap and remove screws holding the baseplate to the body investigate
why it doesn't turn, either shi.. dirt has got between the two plates or a too long a screw
fixing something to it has been used.

Hope this helps!

Colin
Click to expand...

Hello Colin,

Thank you for your time.

The base plate is fine and the vacuum is functioning – If I remove the small-bore plastic hose at the carburettor end and suck it, the base plate advances. The distributor appears to be working correctly?

But the vacuum unit moves in and out when I rev?

What puzzles me is the splined connection to the vacuum unit? I mean the simplest way for me to get rid of this movement is to thread lock (glue) the spline into the vacuum unit – But if this is meant to be fixed, why would the manufacturers go to the time, expense and trouble of machining a spline on the shaft and its receiving female in the vacuum unit?

Is the spline designed (simply) to prevent the shaft from twisting when adjusting the knurled vernier nut (as opposed to achieving this by simply (and more cheaply) welding or brazing it into the vacuum unit?

Should this spline be firmly fixed into the vacuum unit, allowing no movement at all? If so, this is the second distributor I have had with this problem? I do not want to glue it if it is not designed to be fixed in this way?

I just need to know whether the shaft connecting the knurled vernier adjustment nut at one end to the vacuum unit at the other end should be fixed or should it allow the vacuum unit to move on a spline?

A forum member with this system should be ale to tell me whether or not his moves?

Ray
 
Hi, Ray, the vacuum advance unit looks like this when removed from the distributor. I take it what you call the spline is just a keyhole shaped hole that is in the dist body. The longer rod with the threaded end has the knurled wheel/nut on it which is for fine adjustment and is located in a slot in the body. The shorter one with the 'eye' goes on a post on the baseplate, as the vac from the manifold acts on the diaphragm it turns the baseplate a little. You say that is all working correctly so that is alright. The slot that the knurled wheel run in has only sufficient clearance for it to turn. So all the fore and aft movement you see is because the dist body is worn in this area. You need to reduce this either a shim or replace the body. So in short, No it shouldn't move that much.

Colin
 

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colnerov said:
Hi, Ray, the vacuum advance unit looks like this when removed from the distributor. I take it what you call the spline is just a keyhole shaped hole that is in the dist body. The longer rod with the threaded end has the knurled wheel/nut on it which is for fine adjustment and is located in a slot in the body. The shorter one with the 'eye' goes on a post on the baseplate, as the vac from the manifold acts on the diaphragm it turns the baseplate a little. You say that is all working correctly so that is alright. The slot that the knurled wheel run in has only sufficient clearance for it to turn. So all the fore and aft movement you see is because the dist body is worn in this area. You need to reduce this either a shim or replace the body. So in short, No it shouldn't move that much.

Colin
Click to expand...

Colin,

Thank you for taking the time and trouble to post this for me. I appear to have unwittingly caused some consternation and disapproval in the forum over this post.

I do not feel comfortable in saying anything further here; perhaps you could consult Chris York (who has been extremely helpful to me) about what outcome transpired?

Ray
 
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