vapour lock ideas

[BMWChris]

Did a rally on the weekend new engine good (I think - got paranoid about noises on way home).

Only problem was, i'm fairly sure, vapour lock. After several hours of gentle driving we would queue for five minutes and then do several tests (car working very hard) of about a minute each, with maybe another short queue before each one. By the third tests the car was stuttering and missfiring for, maybe 30 seconds before clearing. We drove for 13 hours with only a few stops and the car was fine as long as it was moving. It did the same thing after stopping for fuel on the way home.

I'm not sure where the vapourisation takes place. Is it in the carbs or fuel line, for example?

The car is a Midget 1500 and is already fitted with a heat sheild between the standard SU carbs and exhaust. I'm not allowed to modify the bonnet so louvres or similar are out. I use super unleaded and an octane booster.

On a differnt car in an imense climate I once cured it by soaking rags in water and tying them to the fuel pipe but I was thinking about a cheap hand held fan or two and a switch in the car.

An other ideas?

[Huntsman]

You have the electric pump at the back of the car? Your fuel lines should have pressure in them so that should help.

Can you re-route the pipes away from sources of heat?

Not your HT coil going duff?

[BMWChris]

Thanks for your reply. I have a high performance fuel pump at the back which is clicking away loudly and a flow regulator to keep it under control. Coil is brand new. Issue is def heat related.

The air flow at 20mph is enough to stop the vapour locking but I don't know if that is air flowing over the carbs or flowing over the radiator (temp gauge goes up slightly when stopped after long run) which then heats water-heated manifold too high?

[woodytype S]

My MGA suffered badly from vapor lock.I covered the back of the heat shield with silver foil,problem sorted.

[lowdrag]

I've always found that vapor lock wasn't manifold related but usually heat soak from the engine. You'll have to try and insulate the carbs better; perhaps there are modified gaskets between the water rail and the carbs available? I suppose a Kenlowe wouldn't hurt either.

[BMWChris]

woodytype S said:

My MGA suffered badly from vapor lock.I covered the back of the heat shield with silver foil,problem sorted.

Thanks for the reply. Was it normal kitchen foil?

[SB - Nigel]

BMWChris said:

Coil is brand new.

I’ve had many brand new car components that turned out to be faulty – I assume you have a sports coil (for higher temps)

BMWChris said:

Issue is def heat related. . . .The air flow at 20mph is enough to stop the vapour locking but I don't know if that is air flowing over the carbs or flowing over the radiator (temp gauge goes up slightly when stopped after long run) which then heats water-heated manifold too high?

The temp will go up when you stop as the water pump and fan have also stopped and there is no air flow so residual heat builds up

As there’s no ducting to the carbs the air inside the engine bay will get even hotter whilst the car is not moving

Having had many problems with my previous Triumph’s carbs (2 sets) and with my current 1275 Midget can I also suggest - dirt in, or worn, carb float needles and seats may be causing, or contributing to, the problem

I’d also recommend a thorough clean and flush of the whole cooling system and components, the simplest of things can have the best effects and this is worth doing anyway and refill with 4-LIFE coolant that is ideal for clubman racing, amoung other things it has 10 year life, warns of H/G or combustion failure - and better still, it shows up where leaks are coming from – it’s saved me more than once

ETA: while you're at it checked the fan is secure (and balanced, there's a balancing piece)

It would be a good idea to fit a new set of hoses at the same time too (preferably silicone with round edge clips)

Taking short cuts usually works out to be the long and expensive way of doing things I found more than once

On my current 1275 Spridget I put up with rough carbs and having them sorted bit replaced and refurbed and a brand new set of needles and seats to find one of the needles faulty – I finally gave up and done what I wanted to do at the start but was advised against and that was to buy a brand new set (not refurbed) of carbs, what an improvement - not that I'm suggesting you need to replace your whole carbs

BMWChris said:

Thanks for the reply. Was it normal kitchen foil?

you really do believe in budget motoring

Edited by SB - Nigel on Tuesday 1st June 21:57

[woodytype S]

Hi Chris,the foil that I used was the sort that pipe laggers use,but I am sure kitchen foil will do the same job.

[BMWChris]

woodytype S said:

Hi Chris,the foil that I used was the sort that pipe laggers use,but I am sure kitchen foil will do the same job.

Thanks - I wasn't even aware that thiss type of foil existed. It has also been suggested I wrap the exhaust manifold.

I'll let you know how I get on.