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MG MGB GT V8 Factory Originals Technical - Hot start

My GT has no problems starting from cold. But, when the engine temp. reaches "normal" it refuses to restart until the engine cools down.Once started it performs faultlessly until switch off. It's not a cranking problem, the high ratio starting motor spins fine.It just does not fire up. This is not a new issue, I've lived with it for along time now. In fact ever since I put the Rover V8 in it.
Over the years, it's had an SU pump, later changed to a Red Holley. A modified Rover inlet manifold with a Holley 350, an Offenhauser manifold with a Holley 465 & Now a Buick 215 manifold with an Edelbrock 500.
The the original Rover with a Luminition ignition system. This is the second Luminition ign. system to be fitted. It replaced a Baracuda ign. which replaced the first Luminition. The coil is a Bosch super sport with ballasted lead & is mounted on the inside of the radiator support panel. The ignition module is mounted under the slam panel. I've wired the cooling fans so that they cut out while the engine is being started. Can anyone come up with a solution or suggestion ? Barrie E
Barrie Egerton

How is your Edelbrock set up? and what size RV8 have you got?
I use the same carb on my 4.6 and have never had a hot start problem.

When you shut it off when hot, will it restart immediately and only not start after a "hot soak"? My first inclination is that the fuel is percolating out of the carb as it sits when hot.

Wayne Pearson

I'm with Wayne. You may need a phenolic carb spacer.

Oh, and hold the gas pedal to the floor when cranking.
Carl Floyd

A friend had a Holley put on his factory V8, and after that it was a pig to hot-start, even with th3 bonnet open. However some of that was down to bad connections in his main battery cable that was causing slow cranking and low ignition voltage. He wasn't really aware of this, even though I had noticed, and he though mine had no compression it cranked so quickly. We found he was losing about 3 or 4 volts. Other than that heat soak is my thought too, if it resarts immediately after switch off, but not after a few minutes more, then that will be it. Whst anbient do you have? Sydney in summer could be very different to the UK at any time, although Google reckons it is only peaking at 24C at the moment. Could be fuel vapourising in the internal passages of the carb even though the float chamber is full (although I don't know the internals of this carb and that may not be possible). Could be the *opposite* problem in that heat soak causes the fuel in the float chamber to expand which floods the carb, in which case cranking with the throttle wide open should help, although is there a accelerator pump which chucks even more down the intake? To check that, disconnect the fuel pump wire (assuming it is electric) *before* you stop the engine and let it run out to see how long it takes, then do it again but this time only run it for about half that time. SUs take a suprisingly long time to run out at idle. Reconnect the fuel pump but don't power it up, then after the usual time it takes to cause the problem switch on a start up as normal and see what happens.
Paul Hunt 2

Yes! it will restart immediately after switch off with just a turn of the key. Apart from the Edelbrock/Buick set up , the other combos have all worked satisfactorally in my Rdtsr.
The engine is a 3.5 & the jets & rods in the carby are the recommended ones for this size engine.
Thank you all for the prompt responce, I will look into the carby spacer & just hope that bonnet clearance doesn't become another issue. Barrie E
Barrie Egerton

Barry, i had a Bosch Sport coil (Blue) and the Lumenition ignition conversion on my 4 cylinder prior to conversion. the instructions indicated a 12V. direct to coil, i think. i originally had a ceramic ballast when i ran points. after the change, the Lumenition system never gave a problem. perhaps the ballast wire is the problem?
just a thought. maybe you could CAREFULLY spray a fuel in the carb when it won't start, that will indicate if if is fuel or fire.
kelly stevenson

Kelly, the Bosch coil (red)I'm using definately needs a ballast. The Luminition doesn't care whether the coil is ballasted or not, it only connects to the negative side of the coil. I can just about fit a 1/2" spacer/insulator under the carb. so I'll go down that path for starters. As for pouring gas/petrol down the carby throat ,that's on the back burner for now.It always got my old lawnmower to start so I haven't ruled it out. Barrie E
Barrie Egerton

Coils and ballast is forever popping-up as an issue. You can't go by what is printed on the coil label, I've seen some labelled '12v' that need a ballast. So-called professionals often get it wrong, I saw a car sent out by a well known V8 conversion specialist with a 12v coil and two ballasts, which meant the coil was running on 1/4 voltage/current and delivered very low HT. The only way to be sure is measure the primary resistance of the coil, and the resistance of the ignition supply (e.g. between the connector that goes to the coil +ve and the white or white/brown at the fusebox on an MGB) to see if any ballast is in circuit or not. Coils that measure around 2.5 to 3 ohms are 12v coils to be used without ballast. Coils that measure 1.2 to 1.5 ohms are 6v coils that need a ballast of approximately equal value to run off a 12v supply or they will overheat, and possibly damage the trigger e.g. points from excessive current. Coils measuring 0.5 ohms and lower are for fully electronic systems where only a very brief pulse of high-voltage energy is supplied to the coil.
Paul Hunt 2

The resistance of the coil is 1.9ohms & I reused the original ballast lead when I rewired the car last year.
I measured the clearance between the air cleaner & the underside of the bonnet very scientifically with two lumps of plastercine, one at the centre & one at the front of the air cleaner. The max thickness of a phenolic spacer I could get under the bonnet was 1/2". That's when my problems started. I could not get one thinner than 1".The guy at the speed shop said I'd have to make one out of wood! I searched the net & found that there were plenty of 1/2" ones for Holleys but none for Edelbrocks. Also some of the Holley ones were made out of some sort of composite wood fibre. So I put my woodworking skills to the test,& set about making a spacer out of 1/2" MDF using a gasket for a pattern. The second attempt turned out quite good. A lick of silver paint & some longer studs & the job is finished. I haven't given it a good long test yet, but initial results are very promising. If it is a success, then yes, I will protect it against termite infestation, Dutch Elm disease & dry rot. Barrie E
Barrie Egerton

1/2" phenolic spacers are available.
Here's one example from Summit.
Alternatively, one can stack thinner ones together to arrive at correct thickness.
Graham Creswick

Thanks Graham, initially, I thought these are all for Holleys & were the same as I could buy locally. But when I selected Edelbrock, there are 3 choices to pick from. I don't know why the thinnest one is the dearest though.
I won't be rubbing shoulders with you guys in Wisconsin this year. We'll be in the US during April & May. It's too early to tag on to the V8 meet in July. Rgds, Barrie
Barrie Egerton

Hi Guys,

Have read the thread with interest. I have exactly the same problem with a Holley 600CFM with a 3.9 litre modified MGBGT V8, I am convinced it's a heat soak problem. Have been looking at ways of curing this problem permanently.

I have seen Holley heat dissapators these are relatively cheap and are 1/4" thick and installed on other modified V8, but I think this will only help reduce the direct heat influences a little.

In addition I was considering bonnet louvres (to help hot air escape) but this is not on top of my list as it would spoil the external 'traditional' view of the car.

My final and more drastic idea is to minimise the engine bay temperature. I plan to do this by use of exhaust tape, however there is very little space for my block hugger exhaust manifolds. Has anybody tried this? I realise that I will need to purchase new exhaust manifolds ( s/s or painted steel) and/or try the RV8 exit route.

Comments and opinions appreciated.

Bill Dennison

"I plan to do this by use of exhaust tape"

That stuff kills headers. Much, much better solution is a ceramic coating like Jet-Hot.
Carl Floyd

I have Ceramic coated my headers, inside and out, and it makes a huge difference, but don't forget the heat has to go somewhere, so the rest of the exhaust gets hotter, near floor panels and Starter motors.

I have been told by a very experienced racer of MGB's and C's that bonnet louvers don't work. They don't help cooling in all situations and may cause overheating in some.
Don't ask me why, much too complicated for me, thermodynamics were never my strong suit.

D M Tetlow


That's why I had my crossover pipe ceramic coated, too.
Carl Floyd


I started my V8 up this afternoon for the first time after a rebuild.

I let it heat up to see if the cooling fans worked. (They didn't!!)

The inlet manifold was too hot to touch,(rain was sizzeling when it landed) but the carb was only warm.

I have a 1/4" spacer that consists of 2 ally plates and 3 gaskets. Very effective.

D M Tetlow

This thread was discussed between 27/03/2008 and 19/04/2008

MG MGB GT V8 Factory Originals Technical index

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