Fuel vaporisation problem?

[Graham TA]

I was wondering whether anyone has struggled as I have with what I assume is a petrol vaporisation problem on my TA.

The car runs perfectly well in cool weather, or when the engine is cold, however as soon as it starts to get warm I find the same behaviour take effect.

It appears to be petrol starvation, on occasion to the point where if idling it might actually stop and become very reluctant to restart.

I have an in line fuel filter just after the petrol tap, which when these symptoms are displayed always looks practically empty. There is just a trickle of petrol being drawn into it at this point.

I am working on the assumption this is being caused by vaporisation although could I be ‘barking up the wrong tree’. Is it possible at a certain temperature a crack opens in the inlet allowing air in? Could the pump start to fail at higher temperatures but be fine when it is cooler? Any suggestions welcome.

If it is vaporisation, so far I have tried spacers on the carbs, stainless steel braided fuel hose between pump and both carbs, and finally, a T piece in the fuel line between carbs one and two with an AMAL jet in it to allow a small amount of fuel to be sent back (apparently light aircraft use this approach in order to prevent vaporisation).

The only thing left I can think of is to insulate the copper fuel pipe between the nearside chassis and the tap where it passes across the bulkhead.

All this has not made much difference. This is an incredibly annoying problem since it is effectively preventing me enjoying the car on the most glorious days when it really should be out and about.

Any thoughts and experiences you have are very gratefully received.

Thank you

[Matthew Magilton]

Lets suppose the filter tells the story. Then there must a problem between the filter and the fuel tank. Very unlikely the pipe to the filter would get hot enough to vaporise (unless it runs alongside the exhaust or something weird like that). If you had a cracked pipe then the fuel would possibly be bubbling into the filter? I think I would be looking for a tank blockage or some other blockage which comes-and-goes. Next time its hot, put your hand carefully on the fuel line, filter, carbies etc to see where the heat is. You could also try disconnecting the pipe and running fuel into a container using the pump and guage the flow (Be very, very carefull doing this, have an extinguisher nearby). There could also be a problem with a rubber part of the fuel line collapsing internally. How good is the rubber?
Just my thoughts.
Cheers,
Matthew.

[Graham TA]

Hi Matthew

Thanks for your ideas. The rubber pipes are definately OK as they are brand new. I agree it might be a good idea to check the flow when hot.

I am also wondering whether there is a problem in the float chamber(s) as well. I stripped them down today and found one of the tangs that operate the needle valve was set way too high, therefore I assume carb one was running with a low level in the float bowl. I don't know how significant this would be.

The other problem I can't get my head round at the moment is why the pump doesn't cut out when the floats are fully raised. Instead I can feel fuel is getting forced out through the jets and into the carbs themselves. Is this an indication that the needles are not sealing the float chambers properly?

[BenTA]

Hi Graham,

It is my understanding that once the float chambers are full, the needle should cut off fuel flow (causing the pump to stop as well), with any excess fuel being forced out the overflow at the top of the bowl and onto the floor (or whatever it hits first). I dont think it should be pushing it into the carb itself.

What kind of fuel filter are you using? It is one of those plastic 'one use' things? I used them for a while and couldnt get my head around them. They are designed never to fill up! If you watch them closely they kind of pulsate and suck fuel through when required. I have no idea how they work but they do. I have now changed to a glass in-line filter (the filter is the same as the one in the bottom of the pump) and it runs full of fuel, much to my pleasure!

My two-pence worth is that if there is no fuel in the filter, it is a problem between there and the tank. With the fuel tap, are you running it on 'Main' or 'Reserve'? I was out the other day with it on Main with (what I thought was) enough fuel in the tank to easily cover the stand pipe. I was wrong, and changed it to reserve and it ran well. Is this problem happening with a good lot of fuel in the tank? If there is a difference between running it on Main and Reserve, that will hopefully narrow down your problem. Maybe its worth draining the fuel and checking the standpipe and gauze filter on the reserve line (if you have one). That may be blocked?

I hope that helps, or at lease gives you an idea!
Regards
Ben