Vapour lock?

[Logie]

I read rtvet's post with interest about fuel vapourisation problem he had on his S2 roadster.
I have a similar problem on my 1970 S2 FHC 4.2 with triple SU's.

The car itself has undergone extensive restoration with full engine rebuild, all electrics replaced - electronic ignitor ignition with new coil, plugs, leads, lucas rotor arm etc.
Cooling system in A1 condition - correct 74deg thermostat, new radiator, cooling fans working as they should and no signs of overheating.
Carbs were rebuilt by Burlens and are properly set up. SU fuel pump was renewed a couple of years ago and is working fine.

Here's the problem - car starts fine from cold and runs excellently up to operating temp but if you switch off after it has been run up to normal operating temp - eg after stopping to refuel - the car refuses to restart until it has cooled down, which takes about 20-30 minutes - it then restarts no problem and will keep going as long as you are driving along at a steady enough pace to keep the airflow going through the engine bay. As I say, there is no overheating problem with the car. When parked up with the engine hot, the carbs are no more than warm to the touch - the correct insulators are in place between the carbs and the manifold. I have checked that the correct slotted fibre washers are present to allow free venting from the float chambers.

I wonder if anyone else has come across this problem? As far as I am aware, fuel pipes are correctly routed, fuel filters were replaced and the fuel cap is a properly vented type.

I have tried insulating the fuel lines to the carbs and the float chamber bowls with foam insulation but to no avail.

I rang Burlen who suggested putting in a fuel return line to the fuel tank using something like narrow brake pipe so as not to compromise the fuel delivery to the carbs and they even supplied a banjo union bolt with a take-off on it. I have yet to go ahead with this mod though until I have explored every last option as I know not everyone is suffering from this problem so there has to be something not quite right with my set up.

Any advice most appreciated, particularly if you have had (and solved!) a similar problem.

[Heuer]

Have you checked the petrol tank sump filter? Because it is pain to clean (need to empty the fuel tank) it is invariably overlooked and the gauze filter can become blocked with suspended dirt. Leaving the car standing allows it to settle. Other thing that springs to mind is the coil. As they get hot the insulation breaks down but re-heals during cooling, and this includes brand new ones. Try a new Lucas Sport or Bosch Blue coil to eliminate the possibility.

You could also try a Filter King in place of the standard fuel filter and/or check the fuel delivery pressure. You should not need to insulate anything or run a return pipe as the standard setup works fine in all temperatures.

Have you checked the state of the plugs when it refuses as they will be a guide to what is going on?

[Logie]

Car was completely stripped down by Hofmanns of Henley in 2007. Fuel tank was removed and the fuel tank filter renewed at the time so I doubt this is the problem. Coil was replaced along with the aldon electronic ignition but I have tried a replacement lucas coil to check if that was the cause with no joy.

Standard glass bowl fuel filter has been checked out frequently and no sign of sediment or water ever found - not sure how filter king would be of benefit other than being able to adjust fuel pressure?

Plugs have been removed when not starting and found to be dry with normal light brown combustion product colour.

Found that engine can be forced to start when easystart is sprayed into airbox but will run very lumpy or will stall again until allowed to cool down a bit from normal operating temp.

[vacbag]

Logie.
Try a timing light on one of the plug leads and see if you have a regular spark when you crank it over.
Steve.
Sunny Scarborough.

[Heuer]

I had this problem on the Elan and it turned out to be the needle settings on the Strombergs. Embarrassing at petrol stations!

If the plugs are dry and brown then the mixture appears to be OK. Flooding can be ruled out as can an electrical problem, vapour lock is unlikely so we are back to fuel delivery. Presumably you are using an SU mechanical pump rather than the electronic one? Suggest you consider installing a fuel pressure gauge which will give you some diagnostics when the car will not start - only cost about ?12, details here: http://www.etypeuk.com/forum/viewtopic.php?t=164

Still think you should pull the sump filter. If the tank has been left less than full over winter rust can develop plus who knows what nasties could be in the garage fuel tanks. You should be getting around 3.5+psi from an SU pump (too much actually) but it could be dropping off when it gets hot. Alternative is to install an SU electronic pump and be done with it.

[Logie]

I've got some spark testers which i will try next time car fails to start and i will drain the tank and clean the filter out as suggested.

As I said, I'm open to all and any suggestions! Thanks for those so far - I will post back when I've done these things.

[rtvet21]

Very interesting comments, and to a certain extent at least its not just me!
I cannot see how anything in the fuel tank would lead to this problem as in my case it is very heat sensitive. I am wondering next whether I need to remove the inlet manifold and check passages etc. My problem occurs when the outside temperature is above about 25 c, and we are in traffic moving at less than about 20 mph, The engine gets worse over a period of a few minutes and unless you get out onto the open road and pick up speed, it will stop altogether. There is a spark as I have turned engine over at the time, There is fuel to the carbs, and good pressure, but it will not start for about 20 minutes
The annoying problem is it is so predictable ( as long as its summer !) and the engine definately is not boiling. My next move is a new radiator and better fans to see if this will keep her as cool as when on the move.
If the mixture was too lean, intake air was hot, would this lean to running too lean and make the symptoms ?
Plenty of work for the winter I guess

[Logie]

When I spoke to the tech guys at Burlen - they were at a loss to explain why only some vehicles of the same type seem to suffer from vapour lock issues but it may be linked to modern petrol which has a lower boiling point and some sort of anomaly within the affected vehicle (which is the thing that concerns me)
This is why they provide the banjo union with outlet as a means to recycle fuel back to the tank to help overcome any possible airlock and pressure build up caused through vapourisation which then cuts off the fuel within the float chambers.

I'm looking for that anomaly that may be contributing to this fault but I am slowly reaching the point where I have checked and double checked all the likely (and some unlikely) suspects.

If I end up fitting a fuel return recycle line and the situation still does not resolve I'll be back to pushing the damn car off the petrol forecourt whilst others sitting in their modern vehicles suddenly lose the urge to find out what it would be like to drive such a lovely car!

[Logie]

In my case, I'm pretty sure it isn't just too lean a mixture setting. When the problem occurs there is spark at the plugs but no fuel. A trick used in the old days was to give a little bit of choke to get started when the engine was very hot and fuel would evaporate too quickly leading to insufficient mixture but even this doesn't work for me. Nor does unscrewing and leaving the dashpot damper pistons loose to assist airflow.

It isn't the battery either. I have had jump leads from my (engine running) range rover attached to boost the current whilst attempting to start with the engine turning much faster than normal due to the extra kick but all to no avail.

[Heuer]

Logie

Check the earth strap between body and engine - contacts need to be clean and tight. This has been known to cause problems.

rtvet

Try running the car without the air filter and cannister. Could be you have the wrong needles fitted. My car was set up to use ITG filters which require different carb needles. I once tried putting the original system back in to reduce noise whilst touring and on a hot day it misfired hugely. Putting the ITG filter back on cured it immediately.

[rtvet21]

Thanks,I will look at the needle type and see what can be done on filters etc, problem is by the time I do this we will be in the autumn and the problem is gone!!!