Vacuum advance needed or not?

[Bradtot]

I have just had a look at my vac advance on my 3.5v8 Rangerover 89 fuel injected
Inline from the plenum I have a little one way valve type thing. I sucked on it and watched the dist plate but it did not move.
Further investigation showed that this little valve was marked dist side and carb side and it was the right way round.
So what it means is I have no vac advance at all and never had.
I have had the rangie for 7 years now and have never noticed this.
I got great fuel economy on petrol but am wondering if I could have got better?
So from all the books I read if u dont have any vac advance your fuel economy will be app 10% worse.
I now am on LPG and wonder if it matters as well if u dont have any vac advance.
My last Rangie 82 model with gas and holly the vac advance worked as it should.
I have now removed this one way valve and reconnected just one tube from dist to the plenum, I shall see what happens and advise.
The advance pipe connection is on the motor side of the butterfly so I only get vac when the butterfly is open, this is my understanding unless someone can tell me different
Any comments from those in the know?
should I put back the one way valve?
Regards
Brad

[Rangier Rover]

I have just had a look at my vac advance on my 3.5v8 Rangerover 89 fuel injected
Inline from the plenum I have a little one way valve type thing. I sucked on it and watched the dist plate but it did not move.
Further investigation showed that this little valve was marked dist side and carb side and it was the right way round.
So what it means is I have no vac advance at all and never had.
I have had the rangie for 7 years now and have never noticed this.
I got great fuel economy on petrol but am wondering if I could have got better?
So from all the books I read if u dont have any vac advance your fuel economy will be app 10% worse.
I now am on LPG and wonder if it matters as well if u dont have any vac advance.
My last Rangie 82 model with gas and holly the vac advance worked as it should.
I have now removed this one way valve and reconnected just one tube from dist to the plenum, I shall see what happens and advise.
The advance pipe connection is on the motor side of the butterfly so I only get vac when the butterfly is open, this is my understanding unless someone can tell me different
Any comments from those in the know?
should I put back the one way valve?
Regards
Brad

Vac advance is ment to work when on light throtle when cruising. When under full load an engine needs less advance or they "ping". There are many exeptions to this of course. Depends what ignition set up is on yours Stock or other,What engine mods(comp Ratio etc). Is it on full gas.
You say was not efected on 3.5 on ulp??? Most 3.5's are so under powered they are 80% under load when cruising uless going down hill with tail wind
Cheers RR.

[aishling]

G'Day.
It's most likely a damper rather than a one-way valve. A one way way would allow vacuum to build-up but not let go. The "damper" slows and smooths out vacuum pulses. Test by proper vac pump and gauge before discarding.
Cheers
Peter

[JDNSW]

I have just had a look at my vac advance on my 3.5v8 Rangerover 89 fuel injected
Inline from the plenum I have a little one way valve type thing. I sucked on it and watched the dist plate but it did not move.
Further investigation showed that this little valve was marked dist side and carb side and it was the right way round.
So what it means is I have no vac advance at all and never had.
I have had the rangie for 7 years now and have never noticed this.
I got great fuel economy on petrol but am wondering if I could have got better?
So from all the books I read if u dont have any vac advance your fuel economy will be app 10% worse.
I now am on LPG and wonder if it matters as well if u dont have any vac advance.
My last Rangie 82 model with gas and holly the vac advance worked as it should.
I have now removed this one way valve and reconnected just one tube from dist to the plenum, I shall see what happens and advise.
The advance pipe connection is on the motor side of the butterfly so I only get vac when the butterfly is open, this is my understanding unless someone can tell me different
Any comments from those in the know?
should I put back the one way valve?
Regards
Brad

I haven't struck one way valves (no experience with V8s though), but if the vacuum is taken on the motor side of the throttle, (normal) it will operate only when the throttle is closed or nearly - hence the effect on timing and hence economy will only be at low power.
I suspect the one way valve in fact is designed to leak off vacuum from the advance slowly, so that the timing does not change as rapidly when the throttle is suddenly opened - or perhaps the reverse, not advancing as quickly when the throttle is suddenly closed. In either case it will be part of the fiddling to make emissions meet the limits, and although probably illegal to remove, I doubt you will notice any difference - might drive better in fact.
But the delay effect it is intended to give may be preventing it working from your lungs, and, in fact, it may actually be working normally!

John

[Tank]

I have just had a look at my vac advance on my 3.5v8 Rangerover 89 fuel injected
Inline from the plenum I have a little one way valve type thing. I sucked on it and watched the dist plate but it did not move.
Further investigation showed that this little valve was marked dist side and carb side and it was the right way round.
So what it means is I have no vac advance at all and never had.
I have had the rangie for 7 years now and have never noticed this.
I got great fuel economy on petrol but am wondering if I could have got better?
So from all the books I read if u dont have any vac advance your fuel economy will be app 10% worse.
I now am on LPG and wonder if it matters as well if u dont have any vac advance.
My last Rangie 82 model with gas and holly the vac advance worked as it should.
I have now removed this one way valve and reconnected just one tube from dist to the plenum, I shall see what happens and advise.
The advance pipe connection is on the motor side of the butterfly so I only get vac when the butterfly is open, this is my understanding unless someone can tell me different
Any comments from those in the know?
should I put back the one way valve?
Regards
Brad

First make sure the dissy plate will rotate with finger pressure, then without the valve in the line suck really hard, if you can suck air through the line, then the diahpram is ruptered/holed, if not you should notice the dissy plate rotate slightly, you should be able to suck and put your tounge over the end of the pipe and it should stick.
Max. vacuum is produced at idle (throttle closed) should be about 18" of Hg (Mercury) at idle if timing and all else is correct. Vacuum advance only works at low revs and the job of advancing at higher revs is handled by the Centrifugal advance plate and weights in the dissy. the engine needs More advance as the revs get higher to compensate for the delay of the spark getting to the spark plugs from the coil via the dissy. If no form of advancing the spark firing point in the firing cycle was available then by the time the spark actually fired, the piston (at speed) would be on it's way back down the bore thereby creating a Retarded situation, the spark needs to fire at or very near Top Dead Centre (TDC) to create optimal power, if it fires After TDC (ATDC) then power is lost.
As you have LPG fitted, check to see if the LPG system installer has decomissioned the Vacuum Advnce and possibly fitted an eletronic system to advance spark, let us know how you rectify the problem, Feedback is good, Regards Frank.
BTW it takes some powerful sucking by mouth to replicate engine vacuum available to the dissy diaphram and to get it to move at all, also check that the Vacuum advance plate is not siezed up.

[lambrover]

My vacume advance was not working and when I had it replaced I noticed a difference in the bottom end pick up. When I was driving in the Snowy Mountains on top of the ridges it would back fire through the air box, I had to turn my snorkel head back ter front to prevent this, at the time I was unawear that the vac advance was stuffed I am not sure if this contributed to the back firing.

[beforethevision]

Assuming the vacuum advance works properly, it should increase power per fuel to have it functioning.

[loanrangie]

LPG likes initial advance not so much reliant on the vac advance.

[mcrover]

Unless you have lungs of steel you wont move the vac advance diaphram by sucking on it with your mouth.

Vac advance is there to increase ignition advance at low throttle application where machanical advance is there to advance the ignition BTDC (as it has to fire the spark, ignite the fuel air mixture before the piston is at the top of the cylinder so that it is at it's maximum power to push the piston down. If it fired after TDC it would not give much of a push down on the piston and most of the time will tend to burn holes in pistons or the edges off valves.)at higher revs so that the fuel gets the time it needs to burn.

This is why with LPG you can run much higher advance than on petrol engines.

The part you are talking about will be a buffer as mentioned before, inside it will be a small plastic disc with a small orafice that will restrict the flow and if you get any deterioration to the diaphram in the Vac advance or a vac leak on either side of it then it can get blocked up with crap and stop the vac advance from working.

I recomend you pull it out and piff it in the bin and then plumb it with manifold vacuum (as you have discribed you have as in the engine side of the throttle butterfly) and if you want to make sure it works then go and borrow a hand operated Vacuum pump and use that to check it.

Now that has taken me back to trade school, I rarely work on petrol engines these days.

[Bradtot]

Thanks for the input.
If I replace the hose without this restrictor in I can suck and watch the dissy base plate move just as I think it should.(I must have super lungs)
If I suck on the restrictor fron the end that has "carby' marked on it I can let it sit on my tounge and it will not leak, meaning its blocked , restricted etc.
If I suck on the end marked "dist' I can suck right through it. That is why I thought it was some sort of valve.
This was not put in when I converted it to gas it was already there. I am sure I have done the same thing before years ago and the base plate did move, so maybe its now become blocked?
I will let u know if there is any improvement as I havent driven it since I have taken this thingy out of the loop.
Regards
Brad