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MG MGB Technical - Won't start when hot

Hello Everybody,
My 78 roadster starts really easily FROM COLD but once it's warm it just refuses to start. It won't even cough as if its trying. If I leave it for an hour or so then it will start without any problem. When running it seems fine under all conditions and loads and there are no signs of a problem. I've tried it over short and long distances and there is no difference. I can start it from cold with or without choke (although it prefers a little) but once warm it refuses to start no matter what I do. All suggestions / comments greatfully received.
Matt
Matt

Hi...

It could be vapour lock, ie petrol vapourising in the fuel system somewhere under the bonnet other than where it should !.

Has it been worse since the weather warmed up ?.

Don
Don

Hi Don,
This started about 2 weeks ago and the weather has been warm. It was fine up until then. Certainly the symptoms (no firing) would suggest a lack of fuel. I've also taken the plugs out straight after a minute of cranking and they're dry. It takes less than a minute to get the first plug out so should the plug at least be slightly damp with fuel?
If it is vapour lock, how would I go about checking for / rectifying it?
Thanks
Matt
Matt

Matt. Your engine needs three things for it to start--spark, compression and fuel. To find out what your problem is, you need to troubleshoot it in a systematic manner. Go out for a drive and warm the engine up throughly, then return to where you can work on the car. Try and start it after it has been off for a few minutes. If it does not start, check for spark at the coil lead, then the plugs. If you do not have spark at the coil, it is probably a low tension circuit problem. If you have spark at the coil, but not the plugs it is a high tension circuit problem with the distributor cap, rotor or leads. It would be a good idea to have a new coil lead just in case the problem is a bad coil lead. If you have good spark all the way to the plugs, it is compression or fuel.

Run a compression check and make sure you have at least 100 psi on all cylinders. If not, compression is a problem. If you have good compression, it is time to check the fuel system.

Put a test gauge on the fuel line going to the carbs, then run a volume check. If they show good, the problem would have to be with the carbs. Vapor lock on SU carbs with a properly functioning SU fuel pump is very rare. I do not have such a problem when the air temperature is in excess of 110 deg F and have never seen it with a properly set up system. If you have some form of vapor lock, it would have to be because the heat shield is bad (missing or the insulation has been lost) or the fuel bowl vent is clogged. Check these items out.

For a method of testing sparkplug wires and the Lucas fuel pump specifications, check the MG section of my website under articles. Website is www.custompistols.com/ Les
Les Bengtson

Matt,
I've had symptoms similar to you with my 77 MGB.
I think changing from the cast iron exhaust manifold to a stainless steel tubular one has allowed the problem to occur on very hot days. The tubular exhaust seems to allow a lot more radiated heat under the bonnet. When the problem occurs the carbs are very hot to touch. Also the spark plugs are completely dry.

No one's really explained to me how a vapour lock can occur. But I can see how the fuel could be vaporising in the float bowl before having a chance to enter the engine. After a short while with the bonnet open to allow things to cool a little, the car will start and run fine again.

What exhaust manifold are you running ?

Charlie.
Charles Goozee

Is your carb heat shield in place and in good condition?
R. L Carleen

Hi Charlie,

Vapour locks occur with engine or front mounted fuel pumps because they suck fuel up from the tank - on hot days the fuel vapourises in the line and the pump sucks vapour rather than fuel, it then only needs a slight leak in the pump valves for no fuel to flow until the whole thing has cooled down. The 1500 midget and Spitfire are prone to this unless the fuel line is correctly routed.

With rear mounted SU pumps, they are mounted below the fuel level and push fuel to the carbs, so vapour locks are unlikely because the only length of fuel line in which they could occur is the short section between tank and pump which is well away from exhaust heat.

It is not unusual for the fuel in the float chambers to boil away with high under bonnet temps - just wait a while for the pump to push fresh fuel through from the line.

Matt,

Sounds to me as though the mixture is rich - you should need choke to start the engine when cold even if only for a short time.

As well as Les's points, check the colour of the plugs after a hot run - just switch off the engine and don't let it idle. They should be a light tan colour - any darker brown or black indicates a rich mixture. Also check that the choke levers allow the jets to rise fully against the adjuster nuts.
Chris Betson

Hello everyone.
Ok, I've not had a chance to try everything out tonight but I will reply to everyones queries.
I have the orginal cast manifold and I have checked the heat shield. The heat shield is not loose, cracked, damaged in any way etc. The carbs were not hot, just slightly warm after my return home this evening even though the engine was definately hot and the thermostat had opened etc. I checked the plugs and they are a light tan colour. No dark brown or black. I set up the mixture about 6 weeks ago and it seems right. I can start it without choke but it is quicker and easier with it and I doubt I'd get away with that in the winter. I have an in-line fuel filter so I can see the fuel in the clear bowl part. The flow seems good and there is no shortage of fuel in the pipe and the pump seems to be working as it should. I don't have a compression tester. Is there a workround?
I need to check the LT and HT circuits tomorrow and I'll let you know. Tonight I was pushed for time but in the time I had I never experienced a problem. It started every time. Typical!
Someone has suggested that the distributor and / or points / condensor setup could be to blame as he experienced a similar problem with his Midget that turned out to be the above. His points gap was closing up when the dizzy got hot. Any thoughts or comments on this possibility? I've got a spare set of points and condensor so I could give them a try if it were worthwhile.
Cheers
Matt
Matt

This thread was discussed between 22/06/2003 and 23/06/2003

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