Fuel Vapourisation

[Tangoman]

I know this subject has been dealt with before. I have read through all the threads and have gone down the route that Chris Dancey wrote about.

Using a facet pump, routing the fuel through the dismantled mechanical pump continuing round the front of the block between it and the cooling fan to a refurbed carb.

The car generally couldn't run better, but when I take it round the twisty, narrow and very hilly roads of Lancashire it eventually will only run on a very light throttle, it never boils up. Once I've parked up for an hour and let it cool down it runs as normal and if I keep it on the flat, keep moving and don't get stuck in traffic all is fine.

It's obviously vapour locking. Has anyone got any suggestions?

The car - 1936 14hp saloon.

[luli]

The fuel line has two sections: A low pressure one (between the fuel tank and the pump) and a high pressure one, between the pump and the carburetor. A leak in the low pressure section may cause the symptoms you are describing. Such an air leak will introduce air bubbels into the fuel and also degrade the pumping capacity of the pump. You can test for it by disconnecting the fuel line from the carburetor, and immersing it in a jar half filled with fuel. Turn on the pump and look for air bubbels. (This should be done very carefully since fuel fumes are explosive!). Alternatively, connect the fuel line to the carburetor with a clear hose (Tygon or similar), turn on the engine and look for bubbels.
A possible source for air leak can be the sender unit on the fuel tank. Look here for more detailes
http://wp.me/pXLKy-1nF (the pictures tell most of the story but you can also use one of the on line translators and read the text).
Luli

[chris dancey]

The symptoms you describe for sure is Vapourisation or otherwise known as Vapour Lock. I have had similar symptoms on the 10 hp Coupe ( and our MGB on a hot day ). My further modification on both was to wrap the carburettor supply pipe in exhaust tape ( and the exhaust pipes on the MGB ). Roger Collier ( P3 75 ) is having his exhaust manifold coated in a ceramic substance that advertises to reduce the heat signature of the engine bay significantly...I look forward to hearing the results.

Another worthwhile modification is to fit a heat shield below the carburettor, I have seen Roger Dix's and as far as I can tell this has a significant effect of reducing the ' dreaded lock '.

A tip that may assist ( when in a lock ) with the cooling down time is one I use. I keep a large bottle of cold water which I use to cool down the carb, inlet manifold and the supply pipe, this works, steam is generated if any gets on the exahaust manifold which causes a little bit of a display, but it does the job and one can start up and get going again.

Use of higher grade fuel also helps in a small way as its less volatile.

Luli's comments reference the bubbles in the fuel supply are valid, I have discoverd the main problem being caused by the cork pistons/ valves in the main / reserve tap not sealing ( either shrunk due to age or damaged by ethanol ), this allows air to be drawn in, Meteor Spares hold stocks on the cork and is a simple replacement, a permanent solution is to braze up the pick up, but means that there is effectively no reserve backup, the plus side is there will never be an air leak from this position. It is very easy to replace the cork valves on the saloons, but extremely difficult on the Coupe due to the confined boot. If possible keep the cork ' wet ' with fuel by switching the pump / engine on say once a week, to help prevent the cork drying out and shrinking.

[luli]

I live in a hot country. Many years ago my car too had suffered from vapour locks when climbing hills. I have noticed then that the petrol pipe is adjacent to the exhaust system all the way through, located on the right hand side of the chassis, and it is heated by it, which pushes the petrol temperature towards the boiling point. My solution was to transfer the pipe – and the electric fuel pump – to the cold (left) side of the car. Since then, for 35 years or so there were no vapour locks. Finaly they have returned, and the reason was traced to be air bubbels in the petrol, due to an air leak in the sender unit.

[Tangoman]

Thanks for your replies Luli and Chris. I'll look at all that's been mentioned. I'll re post once I've solved the problem.

John

[jp928]

I recall vapour lock issues MANY years ago in our 16 and P3s - not surprising in Oz! Best solution is to have the pump at or in the tank, so there is no or very little suction pipework. Another easy thing to do is paint the bottom of the tank (where it cant be seen) silver or white, to reflect as much road heat away as possible - the lower the fuel temperature, to less chance of vapourising.
jp

[chris dancey]

Thanks Luli for the tip that moving the petrol pipe to the nearside ( opposite side to the exhaust system ) has reduced your incidence of vapour lock. This was today reinforced by a neighbour of mine who owns a prewar Aston Martin, he has rerouted his petrol supply pipe to good effect, I shall be rerouting the Coupe's petrol pipe in a similar fashion as I experienced vapour locking on my run up to the National this year.

[Tangoman]

Update. Vapour locking was not the problem. Firstly I drained the petrol tank, both the petrol and the tank appeared clean. I then removed the filter which hadn't been in place very long and compared air flow between that and a unused one. The used one was found to be more restrictive so obviously being contaminated with something. I then compared the petrol from the tank with fresh petrol and it was not as clear. After much investigation the contamination was found to be very fine rubber dust. This was from the filler pipe which had reacted to the modern petrol. I've now replaced that pipe with one more suitable, replaced the filter and stripped and cleaned the carb. We are now running like clockwork, up hill and down dale!