Discussion
Various `photos in the bible clearly show some distibutors with a pipe from the body of the distributor to the plenum chamber and some without. One even illustrates a one way valve inserted in that pipe.
Mines a`95 Griffith with no one way valve , Kerridges checked another `95 Griff in their show room, again no one way valve.
I replaced the original (`cos it melted) with clear plastic tubing of a larger circumference.
Questions...does it make any difference having no in line valve on that pipe , does the size of the pipe matter and why do some distributors have vacuum advance and some dont?
Steve, where are you??
Mines a`95 Griffith with no one way valve , Kerridges checked another `95 Griff in their show room, again no one way valve.
I replaced the original (`cos it melted) with clear plastic tubing of a larger circumference.
Questions...does it make any difference having no in line valve on that pipe , does the size of the pipe matter and why do some distributors have vacuum advance and some dont?
Steve, where are you??
Yes Zippy , the new pipe sleeves ( goes over) the rubber oojamaflit that fixes on the inlet to the plenum chamber, so if its too big, presumably the pressure`s not as it should be, therefore affecting the advance mechanisim, what with no valve , it`s no wonder it`s taking so long to get anywhere !! 
quote:
Various `photos in the bible clearly show some distibutors with a pipe from the body of the distributor to the plenum chamber and some without. One even illustrates a one way valve inserted in that pipe.
Mines a`95 Griffith with no one way valve , Kerridges checked another `95 Griff in their show room, again no one way valve.
I replaced the original (`cos it melted) with clear plastic tubing of a larger circumference.
Questions...does it make any difference having no in line valve on that pipe , does the size of the pipe matter and why do some distributors have vacuum advance and some dont?
Steve, where are you??
At the end of the day, you don't need the valve, and you can get away without the vacuum advance line.
The job of the vac advance is to advance the ignition (duh!) when you're cruising at part throttle - this improves the economy and stuff. When you put your foot down there's no vacuum in the manifold and the advance is removed, so it doesn't make any difference to the power and so on - it only affects cruise economy. It seems safe but pointless to remove it completely, if you do the dizzy end can be left disconnected (open to fresh air) but you would need to blank off the manifold end.
The vac advance is controlled by a throttle edge tapping, at tickover the tapping is connected to atmosphere, as soon as you open the throttle the tapping is connected to the manifold. This means that starting from rest, as you slam the throttle open there's a brief moment of suction as it goes through part throttle. Without the valve, this would mean the vac advance comes in very briefly. The one-way valve prevents the vac advance from coming in quickly, so it doesn't react to this transient suction. I'm sure it's supposed to improve throttle response or something, but I took mine off years ago and the throttle response is fine.
If the valve doesn't work or is fitted back to front, you can find the vac advance doesn't come on, or stays on when it shouldn't. It is common to take it off just to rule it out as a problem.
Hope this helps,
Peter Humphries (and a green V8S)
Steve is on holiday at the moment. Please leave an email after the tone: BEEPPPPPPPPPPPP
Some have it some don't. Some dizzies need it, some don't. Some engines don't mind if it is not connected. Some do.
Mine does like having the advance connected but I don't have any valve. Mark Adams removed it and replaced the whole thing with a new bit of tube at the last road session as the valve was sticking and upsetting the engine a little.
Just connect a suitable bit of tubing between the plenum and dizzy. Size is not that critcal: don't use a 10 micron or 80 mm tumble drier tube.
Steve
Some have it some don't. Some dizzies need it, some don't. Some engines don't mind if it is not connected. Some do.
Mine does like having the advance connected but I don't have any valve. Mark Adams removed it and replaced the whole thing with a new bit of tube at the last road session as the valve was sticking and upsetting the engine a little.
Just connect a suitable bit of tubing between the plenum and dizzy. Size is not that critcal: don't use a 10 micron or 80 mm tumble drier tube.
Steve
'The job of the vac advance is to advance the ignition (duh!) when you're cruising at part throttle - this improves the economy and stuff. When you put your foot down there's no vacuum in the manifold and the advance is removed'
Isn't the amount of vacuum drawn related to the amount of air going through the engine, hence it would be greater at higher rpm? I thought the vac advance helped to keep the spark in the optimal position as the revs increased?
To add to this database - 97 Griff, vac line but no valve!
Isn't the amount of vacuum drawn related to the amount of air going through the engine, hence it would be greater at higher rpm? I thought the vac advance helped to keep the spark in the optimal position as the revs increased?
To add to this database - 97 Griff, vac line but no valve!
quote:
Isn't the amount of vacuum drawn related to the amount of air going through the engine, hence it would be greater at higher rpm?
It does vary with revs and throttle opening, but what I meant was as you open the throttle the depression is reduced, and at full power there is no depression and hence no vacuum advance.
quote:
I thought the vac advance helped to keep the spark in the optimal position as the revs increased?
No, the mechanical fly weights do that job. The vac advance only advances the ignition under part throttle/cruise conditions.
94 - 500
mine has both.
saw this break open at a track day on a supra twin turbo.
seriously affected the breathing/idle.
don't know if this helps/distacts!
if it runs ok have one put in next service
andy
ps size doesn't matter...so long as you get a good seal at the end ( where have i heard that before!)
the only problem may be if the pipe wall strength is too low so it compresses, but as it only runs upto 1 or 2 atmospheres i doubt that will be a problem.
mine has both.
saw this break open at a track day on a supra twin turbo.
seriously affected the breathing/idle.
don't know if this helps/distacts!
if it runs ok have one put in next service
andy
ps size doesn't matter...so long as you get a good seal at the end ( where have i heard that before!)
the only problem may be if the pipe wall strength is too low so it compresses, but as it only runs upto 1 or 2 atmospheres i doubt that will be a problem.
Thanks GreenV8S. Now I know why I don't dabble!
'the only problem may be if the pipe wall strength is too low so it compresses, but as it only runs upto 1 or 2 atmospheres i doubt that will be a problem.'
Demon Tweeks sell vacuum tubing - like normal tubing but as you say, doesn't collapse under low internal pressure. If the vac makes so little difference though, perhaps it can't be much of a vacuum.
Mind you, when my BMW's crankcase valve stuck closed (the thing that recirculates dirty air back to be burned), the engine whistled as it sucked air past the gaskets, idled like a pig, blew out smoke as it had sucked in some oil too, and the dealer couldn't get the oil cap off while the engine was running. Now that's what I CALL a vacuum!
'the only problem may be if the pipe wall strength is too low so it compresses, but as it only runs upto 1 or 2 atmospheres i doubt that will be a problem.'
Demon Tweeks sell vacuum tubing - like normal tubing but as you say, doesn't collapse under low internal pressure. If the vac makes so little difference though, perhaps it can't be much of a vacuum.
Mind you, when my BMW's crankcase valve stuck closed (the thing that recirculates dirty air back to be burned), the engine whistled as it sucked air past the gaskets, idled like a pig, blew out smoke as it had sucked in some oil too, and the dealer couldn't get the oil cap off while the engine was running. Now that's what I CALL a vacuum!
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