vac

[max-it-out]

Hi , I thought I would try to improve fuel economy by fitting a vac capsule to the distributor , so ordered one from SNGB . Arrived today , looks ok but when I tried to move the spring with the hole in the end that fits over the post on the dist base plate , it`s solid - no movement at all . Isn`t this bit supposed to move and pull the base plate clockwise to advance the ignition when there`s a vacuum ?

I`ve fitted it anyway . I might test to see if it works using the timing light .
With no vac I expect to see around 25 * @ 3000 rpm . What should it be with the vac fitted ?

Thanks

[abowie]

Yes the baseplate needs to rotate.

The total additional advance will depend on the capsule you bought. There should be a stamping on it telling you how much advance you get at what negative pressure.

[tinworm]

Your vac unit will only move when you lift off the throttle don't forget. So over advance for fuel saving purposes only occurs 'on the overun'. It will not affect automatic advance which is effected by the centrifugal force on the balance weights (and countered by the weight springs)

Barrie

[mgcjag]

Also note that different distributors have different advance weights...the advance is stamped on the weight..Steve

[malcolm]

Yes, to se it operate on the timing light, you need to run it at, say, a steady 3500rpm then remove all throttle - you'll see a short and temporary movement on the light

[max-it-out]

The unit looks like a re-manufactured item , not NOS , no markings on it . I would expect the pressure at the take-off point close to the throttle plate to be close to a minimum level at 3000 rpm with no load , so vac advance should be close to maximum . Can`t say I`m confident though - no movement when I tried to pull the spring by hand ( I know it might only need 2 or 3 mm movement to do the job ).

[malcolm]

I might be wrong, but even with no load at 3000rpm you may not see max. advance. From my observations with my light, you need to be on the overrun (as said above) so coming from a position of load to zero load. A static 3000 rpm with no load may not do anything. I'd have thought it should be free to move by hand though.

[mgcjag]

Hi Mark....you should be able to just suck on the vac input and see the base plate move....i can on a spare iv just refurbed..Steve

[christopher storey]

The vacuum advance code which is stamped on the face of the capsule will be something like 6 -13 - 12 . This indicates that the vacuum advance starts to work when there is anything more than 6 inches manifold depression , reaches a maximum at 12 inches of depression, and at maximum produces an additional 13 degrees of distributor ( i.e. 26 degs crankshaft ) advance . Thus it works not only on overrun, but also from idle up to light cruising conditions, for the purpose of improving fuel economy . You can check it works with a timing light by running the engine under no load at say 2000 rpm when you should see something of the order of 40 to 45 degs of crank advance. If you blip the throttle, this will immediately fall back to perhaps 20-25 degs total advance and then recover to its original level

[PeterCrespin]

...when I tried to move the spring with the hole in the end that fits over the post on the dist base plate , it`s solid - no movement at all . Isn`t this bit supposed to move and pull the base plate clockwise to advance the ignition when there`s a vacuum ?

The spring shouldn’t move. ‘Solid’ is correcf unless you suck hard on the vac tube.

The throttle doesn’t have to be shut off to activate the vac advance when driving, the engine just has to be operating under light loads such as a steady cruise.

If the rod/spring moves when you suck on the vac nipple (cue John Inman, Larry Grayson, or preferably true petrolhead Dick Emery) then just fit it and fuggedaboudit after you've strobed it for full advance without the vac. 25,BTDC is way retarded for -3000 rpm IMO.

[max-it-out]

Hi Peter - Just tried again today , and it is possible to push the spring into the capsule , so there is some movement there , but I had to use a pair of pliers to do it . It suggests that there is a strong spring inside . Sucking on the intake , even with max effort , produced no movement , so I`m doubtful if it would be effective when fitted ( unless the vac produced at the throttle is very powerful .

The dist. I`m using is ex-US ( emissions ) and not a very good curve - I`ll probably advance the static so it`s more like 32 * @ 3000 .

Glad someone else remembers those old comedians - reminds me of a very old joke - man turns up to a fancy dress party with just some sandpaper wrapped around his knob , the host says " what have you come as " the answer " Dick Emery "

[mgcjag]

Hi Mark..does your base plate actually move or is it seized up.....im assumeing you have the thumb wheel adjuster and you stackend it off to fit the spring......sucking on the vac you should hear the internals of the capsuals move.....is if the spring being connected stopping it operate.....as i said above you can get different advance weights and springs to bring it up to UK spec....Steve

[MarekH]

Vacuum is indeed very powerful.

At just 1000rpm, your 3.8 litre engine is shifting 31 litres of air per second, compressing it down almost tenfold, so leaving a 28 litre "hole" in the airflow behind it. That gives a 50%drop in air pressure in its wake.

(That's what Chergar could manage and I'm guessing he had slightly bigger lungs than us humans.)

kind regards
Marek

[max-it-out]

Hi Steve , yes the base plate will move freely and overall the dist. looks to be in good condition .Originally it had no vac . ,but had a " compensator " which allowed small adjustments via the screw .
I just checked via Google and it seems a person can produce up to 18 " Hg of vacuum by sucking on a tube . This should be more than enough to move the vac capsule . The fact that no movement is produced proves it`s faulty .
Ideally I would have binned the present dist, and put in a 123 Tune . The thing is , I`ve just read an article by someone who has fitted one of the Emerald kits for EMS and fuel injection . It sounds the best long-term solution and needs no dist . so any money I spend now would be wasted .

[mgcjag]

If your after a cheap temp option then i know a few guys that are useing these with good results.. https://simonbbc.com/lucas-25d6-type-distributor/

[max-it-out]

Thanks . Yes , cheap as chips and should be ok for a while

[christopher storey]

Sorry to disagree with Google, but there is not the faintest chance that a person can exert 18 inches of vacuum! Do not think that your capsule is faulty merely because you can't suck it to an advanced position with the capsule connected to the base plate, because the spring loading is far beyond your sucking powers !
I suggest you get it on the engine, get a timing light, and check it as has been set out in this thread

[mgcjag]

Hi Christopher..i have no idea how much vacume i can produce by sucking....but i have two 22d6 on the bench and i can suck both to opetate the vac capsuals which pulls on the spring and rotates the base plate.....sucking does not make the spring stretch it just pulls on it which rotates the baseplate....Steve

[max-it-out]

Just watched someone on Utube suck on a vacuum gauge . They reduced the pressure down to 0.1 atmosphere .Not sure how that relates to inches of mercury , but it sounds quite powerfull , and at least as much as you might find at the take-off point for the vac capsule .