Autovac

JohnWilliams6437 · Posted: Sun Oct 08, 2017 5:08 pm Post subject: Autovac

Coming to my car (3, 4/12) after an absence of a few weeks filled the carb bowls as they were empty.
Did this twice to no avail (engine just emptied them ) and had to prime the Autovac through the inlet elbow.
Should the Autovac have drawn fuel in the (admittedly) short time it takes to empty the carb bowls twice?
If you run out of fuel on the road will the Autovac self prime? and for how long would i have to churn the starter?
Interested to hear of others experience. Don't really want to fit an electric pump.
John W

Paul Spencer · Posted: Sun Oct 08, 2017 7:58 pm Post subject:

I once found the carbs and Autovac of my 3 Litre empty after a long sea voyage. I just put some fuel in the carb float chambers and that was enough to suck fuel into the Autovac. I'm still not sure why the Autovac emptied. Faulty fuel tap?

Chris Card · Guest

Although I have an autovac in place, I run off pumps but, I was under the impression that suction occurred mainly on the overrun. Also, my autovac holds about 1 gallon so, again, my assumption is that it would take more than a couple of carb bowls full to draw sufficint fuel to maintain running. My autovac has a cap which again, I assume is to prefill the tank.
I’d be interestedto know more, in case I have to change back from pumps a any time.

Chris

PS I assume that you have openned the taps, especially on the suction pipe from the manifold.

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

It doesn't take much engine turning to draw enough fuel into the autovac to fill the inner chamber - you don't need to fill the whole autovac. Once the inner chamber is full, it will release that fuel which will drip to the carbs and fill them.

You can do it without any patrol in the carbs, but you do risk draining the battery then as you'll be firing the starter repeatedly.

It also helps to close the hand throttle to maximize vacuum suction.

One last option: you can use starting fluid to keep the engine running long enough to draw fuel into the autovac.

John Murch · Posted: Mon Oct 09, 2017 3:59 pm Post subject:

Just to add weight to the above postings. On the occasions that I have run out of fuel, and changed over to the 'reserve' supply, I find that only a few seconds of churning - followed by perhaps 20 seconds for the resulting vacumn to draw up some fuel, is sufficient.
John

JohnWilliams6437 · Posted: Wed Oct 25, 2017 6:35 pm Post subject: Autovac

Further adventures with my autovac-
Thought all was well and on a sunny day ventured out.
!00 yards after turning from our quiet lane onto a busy road ran out of petrol.
The next quiet lane to get home was about a 1/4 mile which took 2 (or maybe 3) carb bowl fills. Got to a quiet lane and part filled the Autovac from my spare petrol can with 1" spout so more spilled than went in.

1.Fuel getting as far as the filter (an original Zenith featuring lots of 'washers')
Found this by sucking on the fuel pipe -nice lungful of petrol fumes.
2 Fitted plastic pipe between filter and Autovac but nothing coming througn.
Dispensed with 'filter element' and got a filter bowl full.
Difficult to believe that the fliter is preventing suction except if the Autovac is not giving much 'suck'.
3 Put my finger on 'Aotovac inlet and got some suction but didn't seem a lot-but i haven't any experience with them so can't get a feel for it's efficiency.
4 Send off to Autovac for service?
What fun.
John Williams

John Murch · Posted: Wed Oct 25, 2017 7:56 pm Post subject:

It’s easy to loose suction through a crack in a soldered joint, either on the vacuum side or petrol side. Do check all the joints before sendiing the autovac off for possibly unnecessary attention !

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

John - where is the filter? It's usually between the Autovac and the carb. You seem too suggest yours is between the petrol tank and the Autovac? Also, the valve on the inner tank might be leaking - that would cause the autovac to have inefficient suction and not work.

Chris Card · Guest

John, if you haven’t already done so, it might be worth talking to the Autovac service people - I’m told that they are very helpful. They might be able to give you a lead that will identify the problem. You can then take it on from there.

Chrisd

PS Dan has a valid point about the position of the filter in your fuel line.

John Murch · Posted: Thu Oct 26, 2017 5:07 am Post subject:

Dan, are you sure that the filter is usually between the autovac and carb, in my experience it has been between the tank and autovac, with just the mesh thimbles at the float chamber inlet.

John Murch · Posted: Thu Oct 26, 2017 6:38 am Post subject:

John, contemplating your point 3, with the engine at idle (no butterfly opening) you should be getting maximum manifold depression and hence suck.
There is a direct connection between the vacuum inlet and fuel inlet (into the autovac) until the fuel level in the autovac rises and triggers the drop valves.
So why little suck? Have you had the pipe off from manifold to autovac to check for any blockage?

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

John M,

On all my Autovac cars (4 of them), the original filter is between the Autvac and the carb. They all have modern filters before the Autovac, as that makes far more sense to me.

John W:

You might want to disconnect the vacuum line from the autovac and connect a manual suction device (hand brake bleeder, electric vacuum etc) - makes it much easier to hear the Autivac cycling than with the engine running. Can also connect a clear plastic line from the fuel supply to the autovac to see what the fuel to to Autovac is doing.

Robert Craven · Posted: Thu Oct 26, 2017 11:19 am Post subject:

My main filter is positioned like John Murch's in the supply to the autovac. As has been mentioned in a previous post lemon juice can be a useful cleaner.

Dan's suggestion of substituting clear pipes for diagnosis is a good one. I did it when I was having fuel starvation problems at high speed and on a rolling road it showed the autovac was working perfectly sucking in petrol in regular cycles (fascinating to watch) and the problem was one of the carburetter float needles had slipped.

I think the Technical Facts suggests the pipe between manifold and autovac can get clogged and may need cleaning, but I've never actually found it to be obstructed.

John Murch · Posted: Thu Oct 26, 2017 12:07 pm Post subject:

in the 3 Ltr instruction book, it clearly states that the filter is situated before the Autovac.
Perhaps in later cars things were different.

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

Thanks John. I would guess then mine are “restoration” modifications in the Bentley’s. The RR cars were likely original as one of mine is unrestored.

JohnWilliams6437 · Posted: Thu Oct 26, 2017 6:08 pm Post subject: Autovac

Thank you gents- will report back after weekend investigations.
John W

JohnWilliams6437 · Posted: Tue Nov 21, 2017 5:36 pm Post subject: Autovac

Autovac serviced quickly and looks good - (not tested yet)
Recommend new owners :www.autovac.co.uk
JW