Vacuum advance / distributor

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Kieranzxt

Member
Posts
29
Been poking about my 3.5 efi Range Rover today.

The little pipe that goes from the Plenum to the advance bellow on the dizzy had a black and red non return valve fitted inline. One side said "dist" the other "carb".

It was plumbed in the correct way, but what is its purpose??

It only passed air towards the dizzy (ie pressure not vacuum), but i dont understand how it would work, because the non return would lock the advance in position.:confused:

Another thing I noticed was that the dizzy seemed to "click" clockwise if I turned the rotor arm, not easily move and spring back like I have known in the past?? Is this normal??
 
firstly I believe the valve is a damper, ie it slows the vacuum advance return.

sounds secondly as if you have partially seized advance weights like mine.
 
firstly I believe the valve is a damper, ie it slows the vacuum advance return.

sounds secondly as if you have partially seized advance weights like mine.

I think you will find it is OK.

The bob-weights under the breaker plate get in the way when you try to turn the rotor arm by hand, and makes you think it's all jammed up.

In fact it isn't. When the engine is running the weights are both thrown outwards by centrifugal force, and when they do that they pull the breaker-plate backwards so that the spark comes sooner.

CharlesY
 
best check is disabled vac advance and check it works with strobe timing light as you increase revs
 
The bob weights were partially seized.

I removed the gubbins and the top plates and just doused it in wd40 and worked it back and forth. It moves nice and smoothly now.:)

I have removed that damper / check valve / non return valve unit. All I can see it doing is inhibiting the vac advance unit. It is a non return in the wrong direction?? No other vac advance car I have owned has had a similar valve.

I timed the dizzy to 10 degrees btdc and it seems a lot happier, but seems to have a slight flat spot at 3000 rpm. Would different advance springs cure this??
 
The bob weights were partially seized.

I removed the gubbins and the top plates and just doused it in wd40 and worked it back and forth. It moves nice and smoothly now.:)

I have removed that damper / check valve / non return valve unit. All I can see it doing is inhibiting the vac advance unit. It is a non return in the wrong direction?? No other vac advance car I have owned has had a similar valve.

I timed the dizzy to 10 degrees btdc and it seems a lot happier, but seems to have a slight flat spot at 3000 rpm. Would different advance springs cure this??

Changing the springs is dodgy.

There will probably be two springs, one thinner than the other, and the thin one will be tight right from the start whereas the fatter one will have one end (or both) with some longways free-play on the end pins it sits on.

On tickover or starting up the little spring stops centrifugal advance from working at all, and the engine can fire up with the ignition as far RETARDED as the static setting allows. This makes for good starting but lousy running.

As soon as the engine starts, the bobweights try to fly outwards, but to do so they must stretch the first spring. Up to perhaps 2,000+ rpm the thin spring will control the advance, and it will allow a rapid build-up of more advance as soon as you drive away and get the revs up, BUT while you are accelerating the vacuum device will hold the advance BACK a bit.
Max advance on light throttle is good for economy and cruising, but max advance on light throttle at any speed will be WAY too much advance for sudden full throttle. The VACUUM device RETARDS the ignition in those circumstances, which stops you blasting holes in pistons from over-advance at low revs with full throttle.

After that modest first rev-limit on the light spring, the stronger spring takes over, and by about 3,500 rpm it will be at full stretch and no more advance can happen. The revs limits vary between engine makes and types, so be VERY careful about changing things. It's dead easy to blast a hole in a piston if you get this wrong, especially if you enable MORE advance at lower revs.

By far the best way to time an engine is to first make sure all the bits are working (you did that) and then time the engine at a good speed, say 3,500 rpm or so, when you know that MAXIMUM advance should be in play.

Below that speed it will be less critical of small variations.

I suggest you buy a new one-way capsule if you have any doubt about it. It is there for a GOOD REASON, and you may do serious harm in the long term if you just dump it.

CharlesY
 
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Regarding advance, it runs on LPG most of the time. And since the octane of gas is a lot higher than unleaded pinking is less of an issue, so the engine will tolerate more advance.
 
Regarding advance, it runs on LPG most of the time. And since the octane of gas is a lot higher than unleaded pinking is less of an issue, so the engine will tolerate more advance.

I find that if I set her up so as she pinks ever so slightly on petrol then she runs lovely on gas. I only use petrol to start up on and only ever use a light throttle on petrol so no problem there. Dont know what my timing is set at, just kept advancing until it pinked under load and then edged it back a bit, I've always done it like this and never holed any pistons , even on the drag strip. Perhaps I'm lucky:D
 
Been poking about my 3.5 efi Range Rover today.

The little pipe that goes from the Plenum to the advance bellow on the dizzy had a black and red non return valve fitted inline. One side said "dist" the other "carb".

It was plumbed in the correct way, but what is its purpose??

It only passed air towards the dizzy (ie pressure not vacuum), but i dont understand how it would work, because the non return would lock the advance in position.:confused:

Another thing I noticed was that the dizzy seemed to "click" clockwise if I turned the rotor arm, not easily move and spring back like I have known in the past?? Is this normal??
possiblly the weights are partially seized?
 
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