Poor ( in fact impossible ) hot starting

[christopher storey]

I am having trouble on my 4.2 S1, now 3000 miles post rebuild, with poor hot starting . After perhaps 30 miles running , a fuel stop or other 5 minute stop leads to embarrassment , with the engine spinning over with never a sign of a cough . Leaving it for perhaps 15 - 20 minutes then produces a start . There is no misfiring with hot running, so it does not immediately suggest the very common modern rotor arm problem. Equally , the float chambers do not feel any more than slightly warm , which tends to suggest that it is not vapour lock . The plugs are a normal colour so it is not unduly rich . I have put new coil and condenser on . The one thing I have not done is put new ignition leads on

All suggestions welcomed

[1954Etype]

Chris,

when you have finished cranking, take out a plug. If it is wet, it isn't fuel. I would remove the fuel filler cap (just to appease me if nothing else).

Angus

[Heuer]

Chris

As Angus says, first check the fuel cap breather is not blocked. I assume there is no misfire whilst running so that pretty much rules out the coil/leads and presumably the fuel pump ticks with the ignition on after the stop? Vapour lock is unlikely given the weather is not that hot at the moment.
Back to basics then:

1. Check the petrol tank sump filter is clean
2. Try a substitute fuel pump
3. Check fuel pressure
4. Check fuel filter and carb filters
5. Try known good HT lead set
6. Try known good coil

Have you tried leaving the car ticking over for 30 minutes whilst at home so you can do the diagnostics with tools to hand. I had a very similar problem with the Elan and that turned out to be flooding via the Stromberg carbs.

[christopher storey]

All good advice for which thank you both. The pump does not tick excessively which again suggests no vapour lock, but the (lockable) fuel cap could be the answer, although there is no indication that the tank is being sucked in which happened to us on the 2+2

[PeterCrespin]

When you come to crank it hot, put your foot to the floor before pressing the button and hold it there to see if it fires up quicker. If it does then it suggests hot flooding. If it doesn't then you might be suffering lack of fuel rather than a surfeit.

On the other hand, resistance increases with heat, so if something electrical is quietly cooking... But then you have replaced the likely culprits in coil and condenser. What happens when you pull one plug lead and put a spare plug in it resting on the head?

[Mark Gordon]

I had exactly the same problem with my '67 4.2. It drove me crazy trying different remedies. I finally sent my distributor out to be rebuilt (it came back looking like a brand new one) and once I put it back in, my hot starting problem disappeared. I don't know if the problem was caused by shaft wobbling from the worn bushings, faulty cap or rotor or a combination of ills, but once it has been started, it now restarts on the first tick.
Mark

[Heuer]

My money is on the lockable fuel cap

[christopher storey]

Peter : I have tried prolonged full throttle whilst cranking and it makes no difference - not even a cough . It is a bit strange that it never did this before I rebuilt everything!

[PeterCrespin]

Peter : I have tried prolonged full throttle whilst cranking and it makes no difference - not even a cough . It is a bit strange that it never did this before I rebuilt everything!

Hmm, the plot thickens...

I bought a set of those neon plug test thingies where you put one end in the plug leads and onto the plug and it flashes if there's a spark at the lead. I must have been bored at a street market or something. They do work but the flash is pretty weak to it's best done in darkness. But then a couple of spare plugs put in two detached leads left lying on the head so the plug body has a good earth should immediately hsow a fat blue spark when you crank so that's still a classic test I didn't read whether you had done or ont? Assume you have?

I did have an old king lead which I chopped in half and pushed each cut end into a clear piece of plastic tube so the ends were about 1/4" apart. A decent spark is easily able to jump that from the coil to the dizzy centre post, and there are six times as many of them as on a single plug lead, so it's a good and obvious test. I'd be looking at hot spark failure.

Did you do the wet plug test? I'm guessing the plugs will be dry because when the engins's hot you don't get wet plugs after cranking, not like a cold non-start situation (and even then it's not always noticeable). Since you have a points dizzy is the little wire inside partly shorting out or something?

[christopher storey]

Peter: Ishall try your great idea of a cut lead in plastic tube . I am also going to heat the rotor arm in the oven to say 100 C and see if it becomes conductive to earth.Thanks for all the ideas: I shall work through them

[MarkE]

Christopher, I've had this so many times and it always comes down to the electrics...specifically the condensor and / or coil.

On two different occasions, like you, I changed these items and it made no difference. Only after changing everything else, I changed the condensor again, and the car worked fine.

It seems that there are real problems with these components that are now made in the far East, and quite often they can be duff from new. I know several folks who hunt autojumbles to find new-old-stock or good secondhand ignition parts.

I would also change the leads, rotor arm and cap....and have a complete spare set of ignition parts in the boot.

Mark

[andy303]

If the ignition checks out, I would look to the fuel pump. Even if it is not ticking excessively it may be blocked on the inlet side. As the pump is higher than the tank it is not kept in prime and is sensitive to faults on the suction side. Which is why MG mounted took to mounting the pumps down low. I am thinking that when you stop and switch off, you are losing pump prime, and that the pump may be weak and/or overheating, which is keeping it from working again until things cool off.

When the problem happens again, try the old fuel flow test with the key switched on and the fuel line running into a suitable container. None or poor flow might point to a weak diaphram, or a clogged pickup. If you see any air bubbles this could point to a leak somewhere on the inlet side, say a break in the pickup tube somewhere above the fuel level in the tank. Air gets in, but you wouldn't know because theres no obvious fuel leak. If the fuel is hot it can vaporize enough in the tank to cause suction lift issues at the pump. But this time of year I doubt that is the problem.

Here is a link to a very informative site, written for MGs but nonetheless applicable: http://homepages.donobi.net/sufuelpumps ... icles.html

[christopher storey]

Update : I think I may have found the problem. I checked the ignition components systematically, and found that the coil HT lead had disintegrated at the coil end, with the wire being blackened and the copper washer virtually non-existent . I have replaced that, and also drilled a hole in the locking fuel cap. Thanks for all the help . We shall see!

[Tony]

Christopher,

I am pleased to hear you may have solved the problem. It is interesting that the problem only occurred when hot and not cold. I presume it is to do with expansion when the coil get hot.

Anyway there is nothing more satisfying than solving a problem that has had you pulling out your hair for the last week or so. The next stage will be the recrimination of why you did not think of that earlier. Hopefully another one for the solved problems category.

[PeterCrespin]

It is interesting that the problem only occurred when hot and not cold. I presume it is to do with expansion when the coil get hot.

Nothing to do with expansion unless there's a mechanical break, which is rare. Since you raise the topic it might help to note that many electrical issues are worse, or only present, when hot. Resistance increases with heat.

Oversimplifying *drastically*, 'heat' is when the atoms of a subtance are at high energy and electrons jiggle around in every direction. 'Electricity' is when you apply a charge and make the 'surplus' electrons in a conductor to flow in a circuit in a single direction like water in a pipe. When the atoms are calm (cold conductor) it is easier to persuade the electrons to move along the circuit from atom to atom, so you need less potential difference ('voltage pressure') to shove them along a wire or across a joint. When the atoms are manic (hot conductor) they want to ping off in every direction so you can't get the circuit to flow quite as easily and hence you get more resistance. Imagine a calm orderly queue of people being marshalled quicker through a turnstile than a panicking crowd milling in every direction. same sort of thing (ish).

Conversely, if they start off cold, conductors will get warm if they pass a lot of current, partly because the electrons are bumping into each other as they flow and losing some of their energy as heat (a bit like friction when you rub things together). There's a lot more to it, involving magnetic fields and induced currents etc. but heat, current and resistance are all linked and under-bonnet temps are unfriendly to many electrical components - hence heat-related failures.

Meanwhile, just as a narrow turnstile would give crowd flow problems earlier than a wide gate, thin wires deteriorate or cause issues faster than fat ones. Coils contain large windings of fine wire, hence they often fail in a heat-related manner. It's also why you ought to mount an oil-filled coil vertically, even though Jag didn't on the E-type. With vertical rather than horizontal positioning you get better convection cooling currents set up in the oil and the coil ultimately lasts longer.

It doesn't really apply to resin-filled coils and in any case there's not a lot in it, but that's the theory anyhow. It's why you see those odd 'in-out' pipes around the outside of sub-station transformers, like vertical handles all the way around a box - they are just allowing cooling currents in the oil which heats up around the transaformer and then rises, passes into the tubes and falls down the outer tubes as it cools and becomes more dense - like an old fashioned thermo-siphon cooling system on vintage cars.

[Tony]

Peter,


Cor Blimey, Atoms, I am impressed.I have a lot to learn

[andrewh]

that amazing Pete, thanks for the explanation. What I was about to add having only just come to this thread, and having experienced similar problems on a variety of motors over the years, is that the problem of identifying fuel or ignition, almost always comes down to ignition. I think vaporisation has got us all jumping up and down and paranoid that its at work when its not. But my experience, and that of a few engineers I know, is that your time would always be better spent, after a rudimentary check that there is fuel at the carbs etc etc, on the electrics. There is really very little to go wrong with fuel , or put it another way, its very easy to check that its working. Carburettors rarely fail to the point that they will not run, they may not run well if jets are blocked or floats have sunk but with three to chose from with an E, you would get a fire at least. Anwyay waffling now, but it seems the problem once again proved to be electrical.

[christopher storey]

Just to update. The problem returned. Guess what ? A substitute rotor arm appears to have cured it . The old one I have had for at least 15 years , and it has taken that long for it to give out!!

[Heuer]

Hope you have sourced a good original or one of the new SNG produced ones as all the others seem to cause trouble!

[christopher storey]

I've used an original which I hope is a good one! Thanks for the useful pointer to SNGB . Although I am hopeful this is the end of the saga, I'm still not convinced