Fuel Vapour Lock

[David Morris]

Hi All,

I have suffered from petrol vapour lock in the past, and the only cure seemed to be to let the engine cool off, until the vapour lock had cleared. Since then I have learnt that more modern petrol systems incorporate a 'bleed' pipe from the carbs back to the tank, so that the excess fuel is returned to the tank. This ensures the fuel waiting to be used in the carbs is cool, as the warmer, but unused fuel, is now safely back in the tank.

Our Jowetts never had this feature and I am wondering if we should consider adding this feature to avoid vapour lock? Is it a feature of more modern fuels? Certainly the petrol we are now using doesn't have the same performance characteristics of traditional fuels. It seems to lose it's volitility rapidly and becomes 'stale' quite quickly.

All the best,

David

[Keith Clements]

The delLorto carb that I use on the Jowett has a fuel shut off valve in the delivery pipe and this prevents one cause of hot engine issues as you can get siphoning through the float valve.

The next issue is the temperature of the petrol in the float chamber can cause a rich mixture.

These two causes of over rich mixture when hot prevents starting. I have never had vapour locks in my Jav or Jup as the petrol pipework is not really affected by heat.

Also having driven in USA and Australia in temps not usual for UK we did not have issues except once when it was perhaps due to modified location of petrol pump close to exhaust.

[PaulRM]

Interesting. I'm running my Javelin on standard Zeniths. Vapour lock has never been an issue, even in 40 degrees Celsius. Vapour lock was a terrible issue on my previous car - an early Volvo PV 444 K slope back. Downdraught Zenith on that, with mech fuel pump on side of engine, and pipe running around the front of engine to carb. I reckon they did this set up in period to cut down on carburettor icing, with old style fuel, in Sweden.

I've been running my Javelin about in the rain and cold. No prob starting and running in traffic in about 4 degrees c or below, during rain, but as soon as I get on the motor way in severe rain, after about 20 mins the engine cuts out when you slow down, Thought this was electrical - took all the precautions to cure it. Then I thought: it's carburettor icing. The full radiator muff doesn't seem to cure the issue. I've read on this site about carb icing and cures - very useful, so now wondering whether the carb pipe air intake pipe should be moderately heated for winter driving. or perhaps some sort of lagging should be placed around the float bowls and even the fuel pump, in winter.

Itching to ditch the dreadful Zeniths and replace with Webers, as my friend has successfully done on his Jupiter, but costs preclude fitment at the mo.

[David Morris]

Hi All,

I have experienced both vapour lock and carburetter icing, although obviously at different times of the year! The first was a feature that showed itself if you left the Javelin standing for more than 15 minutes in warm sunshine. I called it the 'Morrison's effect. If you just stopped to fill with petrol, say for a few minutes, then she would restart immediately. However, if you took longer with the Javelin sitting in the sun-drenched car park, perhaps doing a weekly shop, then be prepared to wait until the vapour lock had cleared.

Carburetter icing was quite amusing, as Paul says, under load and in cold weather, the Javelin would start to stumble and hesitate. Looking under the bonnet showed both carburetters shrouded in pink ice. Just wait for a few minutes, and the warmth from the cylinder heads would thaw the ice and you could restart easily.

I have never tried Webers, having always used the original Zeniths.

Happy days!

All the best,

David