Under bonnet temperatures and modern fuel

[a8hex]

Does anyone have anyone have any knowledge of the behaviour of modern, not quite petrol, fuels as the under bonnet temperature rises.
My XK150 has developed a bad habit after sitting in traffic for some time. It doesn't over heat, but instead when you escape the holdup it doesn't want to pull properly as the revs rise. Its happy to spin OK without with the clutch depressed or in neutral, but with a load on the engine it doesn't want to rev. Low revs are OK and still plenty of grunt then, but not as revs rise. The first time it happened I was in the middle of overtaking someone when it started to miss fire, changing up let me complete the manoeuvre safely.
Someone suggested that the fuel in the float chambers might be getting too hot and starting to bubble. Santa brought me a suitable thermometer and a quick static test the other day showed the front float chamber at about 55degC. Petrol should have a boiling point of 95, Ethanol IIRC about 75, but there all sorts of other st in modern fuel so I'm not sure what sort of behaviour I could expect. I don't image the vibrations from the engine help either.
Any ideas anyone?

[MJK 24]

Ignition coil starting to break down?

[bstark]

I'd be looking at coil, leads or capacitor starting to break down first, and also the battery - especially if still running a dynamo that won't give any charge at idle. One way or another it sounds like a weak spark.

[//j17]

Does the problem clear quite quickely, after 30 seconds or so of clear air driving? If it does then it sounds like fuel vaporisation.

It's not so much the fuel getting hot in the float chambers and bubbling but the float chambers and carb absorbing heat from the hot engine bay so when you start to use it a percentage of the fuel is actually evaporating inside the carb, so not atomising in to a mist and ending up as a burnable mix in the cylinder. 30 seconds of clear air driving drops the engine bay temps massively and the problem goes away.

Don't know the XK150 specifically but check if there is/was/should be a heat shield between the carbs and engine.

[eccles]

//j17 said:

Does the problem clear quite quickely, after 30 seconds or so of clear air driving? If it does then it sounds like fuel vaporisation.

It's not so much the fuel getting hot in the float chambers and bubbling but the float chambers and carb absorbing heat from the hot engine bay so when you start to use it a percentage of the fuel is actually evaporating inside the carb, so not atomising in to a mist and ending up as a burnable mix in the cylinder. 30 seconds of clear air driving drops the engine bay temps massively and the problem goes away.

Don't know the XK150 specifically but check if there is/was/should be a heat shield between the carbs and engine.

I used to get this exact problem with my Mini Jem on motorway runs. When I used to stop for fuel, the engine bay temp would vaporise the fuel in the pump and lines but the float chamber on the SU would be ok.
This lead to a few heart stopping moments as the fuel in the float chamber would run out as I accelerated up the slip road to rejoin the motorway. After a few coughs and splutters it would clear and I could pull away from the inevitable fast approaching juggernaut!
Once I realised what the problem was I used to pull off the pumps, rev the car up a bit until it ran cleanly, and all would be well.

[spitfire4v8]

Can also be caused by waxstat jets .. these are meant to help by very slightly altering the relative position of the jet in the carb as engine bay temp increases and decreases. they're usually troublesome after a while ..

[V8 Fettler]

OP, do your fuel lines recirculate back to the tank?

[a8hex]

bstark said:

I'd be looking at coil, leads or capacitor starting to break down first, and also the battery - especially if still running a dynamo that won't give any charge at idle. One way or another it sounds like a weak spark.

I don't think it's the battery. The car has as alternator in place of the original dynamo, but after parking up for a couple of hours it started easier and then ran like a train. I'd be in trouble if I lost the electrics as she only has an electric fan, the mechanical one was removed to make space for an oil cooler when the engine was upgraded.

With the coil, leads and capacitor are these likely to cause a problem after sitting at idle for a while but be OK again a little later on after a rest?

//j17 said:

Does the problem clear quite quickely, after 30 seconds or so of clear air driving? If it does then it sounds like fuel vaporisation.

It's not so much the fuel getting hot in the float chambers and bubbling but the float chambers and carb absorbing heat from the hot engine bay so when you start to use it a percentage of the fuel is actually evaporating inside the carb, so not atomising in to a mist and ending up as a burnable mix in the cylinder. 30 seconds of clear air driving drops the engine bay temps massively and the problem goes away.

Don't know the XK150 specifically but check if there is/was/should be a heat shield between the carbs and engine.

The problem doesn't clear quickly. It has continued to be an issue for 10->15 minutes after getting clear road. I fact I'm not sure I've ever driven till the problem clears. Parking up for an hour or so clears it OK.

In terms of the XK150 under bonnet area, think in terms of a Mk2, they aren't that dissimilar in terms of the amount of space available. My Xk150 has a breathed on 4.2 with triple carbs.

V8 Fettler said:

OP, do your fuel lines recirculate back to the tank?

No, the fuel lines just run through to the carbs. The overflows then drip onto the floor. There was no smell of fuel to suggest that they were overflowing. Fuel consumption is difficult to tell, the car drinks like a fish anyway.

[V8 Fettler]

Do you have SU carbs? For Holley 4 barrel and equivalent, a return fuel line to the tank reduces vapour-lock in the pipework.

[Plank]

My X/19 suffers from fuel vaporisation when sat in traffic, and will not restart for 5 minutes if turned off after idling for a long time, especially if I use Shell V Power. Its not as bad on std 95. Perhaps V power is more volatile.

[a8hex]

V8 Fettler said:

Do you have SU carbs? For Holley 4 barrel and equivalent, a return fuel line to the tank reduces vapour-lock in the pipework.

It's a set of 3 SU HD8s.
When I measured them, the front one was about 55degC while the other 2 were a tad cooler but still over 50.

[Mopar440]

As others have said, I'd be looking everywhere except the fuel quality.

In the ‘50s and the ‘60s ethanol blended fuel with at least 15% ethanol was widely available and would have been used by many owners of the time. Including XK150 owners, and very probably your own car. The fact that there aren’t a wealth of recorded problems should be reassuring to us all that perhaps a lot of today's perceived problems are overstated.

Feel how hot your ignition coil is when the symptoms next present themselves.

[Ubendum]

Not sure if this is of any help, but a while ago there was someone on one of the classic car forums (Post War Classics if memory serves) that had very similar problems. Might be an idea to post the same question there.

[a8hex]

I'm planning on going to the Phoenix on the 1st so I'll be getting her out of the garage to make sure she starts OK tomorrow, so I'll have a go at measuring the coil temperature. I don't have a gadget to measure the output of the coil. Coils aren't too expensive and so a replacement would be an easy test. I'll also try and pull a plug or two when she's hot and see if there is anything happening there.
Thanks

[RichB]

Replace the condenser before the coil. I had this once on a Midget, drove me mad and it turned out to be a tuppenny-ha'penny part.

[Vanin]

Coil starting to breakdown is a possibility but go for the cheapest option first and that is to replace the black Lucas rotor arm with a red one from America. Accuspark on eBay I believe about £7.00 if you have not done so already.

When I had the old type on my XK120 I had the same problem every time I went for a M.O.T as the car would sit idling for some time during the test and would nearly always start to misfire a few hundred yards up the road. Next morning it would start perfectly.
It was not until I took it to Goodwood where it became really hot when the problem became more apparent and ruined my day there by constant misfiring. We changed the rotor arm but after a few laps the problem occurred again. I bought about five rotor arms from the local parts and they all failed. They were probably even worse than Lucas and made in the Far East.

I felt that there was another problem so stripped the whole system on my return and discovered that the carbon plug leads had not been crimped correctly at the plug end leave a gap over which the spark could jump. Over time this had burned a gap over a quarter inch which had induced a greater spark over the already weak rotor arm.
This causes an internal track to form usually from the little rivet which holds the brass plate on down to the stem of the distributor.
Sometimes this is so severe that all the spark disappears down the stem.

Do not discard these rotors as they are excellent Anti theft devices!


I suspect on your car that there is a partial breakdown and as temperatures rise the track opens up.

The red rotors have cured not only all my problems, but those of many TR, Healey, MG, Aston owners who I have encountered on various events. It make me very popular as I keep a few spares in the tool box and you need all the friends you can get running an old car!

[V8 Fettler]

a8hex said:

V8 Fettler said:

Do you have SU carbs? For Holley 4 barrel and equivalent, a return fuel line to the tank reduces vapour-lock in the pipework.

It's a set of 3 SU HD8s.
When I measured them, the front one was about 55degC while the other 2 were a tad cooler but still over 50.

If it is a fuel vapourisation problem then it's more likely to be occurring in a fuel line than a carb body. Typical symptoms are failure to hot start, eg after a fuel stop. Once the fuel is flowing then the fault generally clears fairly quickly.

Given the low costs, perhaps try a scatter-gun approach (as mentioned by others) for electrical components? Do you have a ballast resistor? If failing under heat stress then you'll get a hot start with the resistor bypassed (as should occur) but erratic low tension voltage to the coil when running.

All flexible fuel pipes in good order? Modern fuel does nasty things to old flexi-hoses.

[Vanin]

However, in case I am wrong about the electrical side and to address your fuel issue I had a problem with my 1950 petrol Little Grey Fergie tractor. This has the same engine as the Triumph TRs and I have also had the rotor arm go on it!

One quite hot but not extreme day I was mowing the Churchyard , something I have been doing for many years with this tractor, when the engine stuttered and stopped .
I had a look round and there was a loud hissing coming from the fuel tank which sits above the engine. The hissing was coming from the tiny hole in the brass cap which is the vent.
I undid the cap and was greeted by a plume of petrol which covered me and the engine and hot exhaust and which luckily did not catch fire.

I stood well away for enough time for the petrol to evaporate and then went back and was astounded to see the petrol boiling in the tank. I touched the tank to find it cool enough to put the back of my hand on it. I telephone an engineer who arrived ten minutes later and it was still boiling!
We deduced that the pressure in the tank had forced fuel past the float and flooded the engine. This was confirmed by the engine starting again and running for half an hour before stopping for the same reason.

This called for an experiment!

Armed with an electric kettle, a milk bottle, and an infrared temperature gun I filled the milk bottle with petrol and inserted it into the kettle which was full of water and placed the experiment in the middle of the lawn at the end of an extension cable.
I think that it was not too dangerous as the worst that could happen would be a small fire, ruined kettle, blown fuse and hole in the lawn!

I flicked the switch and waited some distance away but was amazed when the petrol started bubbling at an indicated 35 degrees centigrade.
I turned off the kettle and went over to find that I could pick up the milk bottle which was only warm and the petrol continued to bubble with quite large bubbles for some time.

I put the bottle back and turned on the electric again and it bubbled for a time and then stopped. Some time later at about 70 degrees C it started to bubble again but this time with a fine string of bubbles very different to before.

Conclusion was that there are a whole load of additives to make the unleaded stuff perform, ethers, esters and other things I have forgotten since school which are highly volatile and which turn to gas at blood temperatures.
The petrol itself boils at about 70 Degrees C but it is the former which causes all the problems in old classic cars.

I have asked questions about the formulation of different brands but made no progress so perhaps the Petrolhead forum will live up to its name and come forward with some information.
I am sure that there is a great difference between different brands and also between winter and summer grades.

So another cheaper trial for your car having changed the rotor arm would be to change the brand of fuel having emptied the tank.

[bstark]

a8hex said:

With the coil, leads and capacitor are these likely to cause a problem after sitting at idle for a while but be OK again a little later on after a rest?

they can all start to break down with temperature and the temp under your bonnet may be just reaching that cusp when sitting there for a while. Once moving you are asking these components to work a bit harder but also the airflow will be cooling them down.

said:

I suspect on your car that there is a partial breakdown and as temperatures rise the track opens up.

This is what I'm thinking, and the rotor arm is another good call. Likewise the points if you still have them. If they are very close they can close up almost completely when hot (had this on our Fiat earlier this year - typically the last thing I checked as well...)

Bob

Edited by bstark on Wednesday 31st December 08:39

[nta16]

Vanin said:

replace the black Lucas rotor arm with a red one from America.

better still get it from the UK from an originator Distribution Doctor - http://www.distributordoctor.com/red-rotor-arms.ht...

Vanin said:

Accuspark on eBay I believe about £7.00 if you have not done so already.

these are Dissy Doc originated - http://www.accuspark.co.uk/rotors.html