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MG MGB Technical - Will Not Start When Hot

I have a 66B that will not start when at normal running temperature. Once the engine cools down to 180 it will restart. Since I have replaced every thing in the on the ignition side coould i have a "vapor lock" situation? It almost seems to have started when ethanol started to be used in the fuel and now accounts for 10% of the mix. Any ideas out there that I can deal with this situation? Thanks in advance...
steve ohanessian

Steve-
Chances are the fuel is boiling in the float bowls of your carburetors when sitting. Check the state of the insulation on your heat shield. New insulating material can be obtained at any Speed Shop frequented by the local Hot Rod set. Take care if any of the original insulating material is still on the heat shield. Be very circumspect about removing it as it contains asbestos that is a tremendous health hazard. Do not do anything to it that will cause it to produce dust that can be inhaled, as this asbestos dust can cause mesothelioma, a medical condition that is something that you definitely do not want. If you must do anything with this material, put the heat shield into a bucket of water and do whatever is necessary in order to remove it while it is submerged so that no airborne dust is generated, and then put the residue in a plastic bag and seal it before disposing of it.
Stephen Strange

Have you checked for spark when it won't start. Stephen's suggestion about potential heat problems in the carburetor bowls is quite likely. Also, a cracked distributor cap can cause similar results. When heated up, the crack expands increasing the distance between the rotor and terminals in the cap. Both are worth looking into. Next time it won't start, check for spark also.
Rick Penland

My MGB Mk1 wouldn't start when hot or, with embarrassment,cut out in traffic jams. After leaving it for half an hour it would start up up again. Prior to cutting out it would misfire. If I got going before it cut out it would carry on misfiring and then clear once on the open road.

My newish temperature gauge gave no indication that it was over heating.

At the end of the day I took off the twenty year old radiator which was full of sludge. Changed it for a recondtioned one and had no problem since.
S P J Wiggins

I've had the "opportunity" to give this subject considerable thought lately. First I made sure the fuel and ignition systems were up to standards. When I still had problems in the heat, I theorized that the fuel was vaporizing or at least expanding in the fuel line, causing the line to become pressurized above the normal few p.s.i. To counter the heat, I added insulation to the fuel line from where it enters the engine compartment up to the rear carburetor. I'm attaching a photo from another thread I started in this forum.

After testing the past few days, I can report that the insulation is a very big help. Of course, insulation just delays the transfer of heat. So if I stop the engine when it is very hot, the fuel line will eventually become hot enough to cause problems. The advantage of the insulation is that once you get fuel flowing, the fuel line rapidly cools enough to prevent problems.

To be sure you start the car with plenty of fuel in the float bowls, turn the ignition switch on and let the fuel pump run for 20 seconds before starting the car. -G.


Glenn G

Expansion is about the only thing that can happen, but only while the engine is stopped. That could force the float valve open and so raise the level in the float chamber, but in the past (using a manual switch to turn the pump on and off when a float had sunk) I have found float level makes very little difference to driving. It could make the mixture a bit rich for hot starting. Fuel vaporisation causing a problem i.e. fuel starvation is pretty-well a non-starter (ho ho), as soon as the float drops and the valve opens any vapour in the pipes will be pushed straight out by the action of the pump to be replaced by fresh fuel. Fuel would have to be boiling at the rate of a pint per minute or more to defeat the pump. Claims of vaporisation keep cropping up, even in a UK summer, but as I keep saying these cars run perfectly happily in desert states. I don't dispute hot starting and hot running problems, merely that is it a design issue - it isn't, it is a fault that can be fixed.
PaulH Solihull

I recentley put the 25D dissi back on my car for other reasons. It wouldnt start hot. Then the 45D went back on and no problem. It can only be heat soak into the capacitor, although it measured fine on my meter, infinite resistance and 0.2 uF. When you attempted a hot start it would just throw the Bendix back out. There was fuel in the inline filter and in the float bowls. There was pressure in the fuel line when disconnected, but there has to be to stop the pump running.
Stan Best

When attempting a hot start , does the starter motor spin the engine at the normal speed ? If not ,too far advanced ignition can cause such a problem. Barrie E
B Egerton

"throw the Bendix back out"

Implies it was firing but not catching. You did do before and after timing measurements and make them the same, I trust?
PaulH Solihull

This thread was discussed between 09/05/2011 and 06/06/2011

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