Vacuum unit full of petrol

[Dru]

I finally got round to taking the distributor off because I've suspected for ages that the vacuum advance wasn't working. I've tried sucking on the vacuum hose to see if it does anything, and not seen any perceptible movement in the distributor.

On dismantling, I found that the vacuum unit was full of petrol.

Most odd.

Drained it out, checked for integrity of diaphragm by sucking and sticking finger over the orifice. It's OK.

But on reassembly, it's a real pig to suck hard enough to make the distributor plate move. Is that normal? -I know it's hard to quantify the amount of pressure obtained by sucking on the hose, but should it be relatively easy?

And how on earth did it fill up with petrol, I wonder?


By the way, I saw that the last person to go into the dizzy had put a blob of paint on it to locate its position relative to the clamp. I did a more precise job by putting a splash of tippex across the two of them, and marking the position with a fine line pen. On reassembly, it seems to run just fine; I can't check the dynamic timing because I'm waiting for a strobe to come by post. Sorry if I'm saying nothing new; I've got a CSE in Stating the Bleeding Obvious.

Now, to get rid of the taste of petrol. ECCCCHHHHH....

[bigginger]

On dismantling, I found that the vacuum unit was full of petrol.
Most odd.
...And how on earth did it fill up with petrol, I wonder?

Via the pipe connected to the carb, I imagine. Sorry that's not helpful
a

[MoggyTech]

Common problem and yes the fuel can wreck the diaphram over time.
Does your car have the fuel trap fitted in the carb to dizzy vacuum line? This trap was used on earlier cars with the copper line, but the later cars with plastic line don't seem to have it fitted.

What I did, was got hold of the fuel trap, and fitted it into the vacuum line circuit using thick wall small bore plastic pipe. Another trick, is to use a longer vaccum line, and leave a section of pipe coiled up into a loop to act as a fuel trap.

[Dru]

Thanks for that response. The line is plastic, and without a fuel trap. I wonder if there's such thing as a pattern non-return valve that could be fitted in-line? We used to use a very simple but effective one on vacuum toilet systems at sea; a rubber disc in a push-together fitting. V effective at keeping the urrrk out of the works.

Which leaves the question of - just how much pressure should be needed to operate the diaphragm? -easy, or suck like billy-oh? I'm wondering if I should replace the unit...

[MoggyTech]

A non return valve would prevent the vacuum unit from closing when throttle moves. The vacuum needed to operate the unit does need a fair old suck on the pipe to move the diaphram. Vacuum advance only comes into play under very light engine load and on over-run, the load advance is handled by the mechanical bob weights in the dizzy.

The little fuel traps works quite well. Oh, and to get rid of the taste of petrol try whisky

[Dru]

A non return valve would prevent the vacuum unit from closing when throttle moves.

dohhhhhh

I've found that a glass of red wine is doing the job.... followed by another... hic...

[RogerRust]

I have found this a problem over the years with lots of series A engines. I don't think the petrol stops it working, but I have a phobia about petrol anywhere it is not suposed to be.

Look out for a second hand metal one and find some tube to adapt it as MT says above.

[bmcecosse]

If the diaphragm doesn't move easily with sucking - then it's gone 'hard' and is useless. New vac unit required.

[alainmoran]

Oh, and to get rid of the taste of petrol try whisky

Top tip ... any excuse I guess, but I'll try ANYTHING to get rid of that taste ... it just repeats on you for HOURS ... right, I'm feeling the need to siphon my fuel tank now before I get to that welding on the nearside back wing ;D

note to self: try not to burp while welding!!!!!!

[Dru]

it just repeats on you for HOURS

o god, ain't that the truth. I can still taste it...

Plan of action, then: I'll look out for a fuel trap, and replace the vac unit. Then I can do a compare and contrast. I wonder what's inside a fuel trap?

[wanderinstar]

The little fuel traps are getting very hard to find nowadaays. Rob Thomasson once told me to get one of those little jam or marmalade jars that you finf in motorway service areas. Drill 2 holes it top and Araldite the plastic pipe in, having cut it in two first. Advantage of glass jar is that you can see if there is any petrol in it.

[bmcecosse]

'Fuel trap' was dispensed with many years ago - really not necessary.

[MoggyTech]

The fuel trap contains a fine meshed gauze which creates a very large surface area much like that in a catalytic convertor. The large surface area promotes cooling which causes any fuel in the line to condensate to the bottom of the trap when the engine is stopped. Then when you start the engine, the vacuum draws the fuel into the carb to be burnt by the engine. They do reduce the fuel getting into the dizzy vac unit, but don't stop it completely. I use a vacuum sook pump to draw fuel out of the vac lines (Dizzy and Vacuum Gauge) once a month.

[Dru]

Having read and learned, I've cobbled together this thing

<a href="http://www.flickr.com/photos/belvedere/1723725573/" title="Photo Sharing">
<img src="http://farm3.static.flickr.com/2305/172 ... b4f978.jpg" width="246" height="500" alt="fuel trap" /></a>


...which I shall install today. The sparkly stuff is brass gauze. The test tubes and bungs came in packs of ten, so there's some spare if anyone is interested...

[bigginger]

I've played with the link and here's a version that actually goes somewhere. Can't seem to see a pic there though, but maybe that's just me
http://www.flickr.com/photos/belvedere/1723725573

[alex_holden]

Looks like a prop from a B movie. Are you going to wire a Jacob's ladder into the ignition circuit too? ;)

[Dru]

Neat. Beats heck out of those funny blue light thingies. But I think I'll stick to the Van der Graaf generator mounted on the roof... keeps the seagulls off a treat.