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MG MGB Technical - MGB 1978 Engine

Hi all

You may not be aware but I am currently stripping down my MGB GT for a full restoration and haven't turned the engine in a few weeks. I thought today I would give it a go...

...it wouldnt crank at all so I charged the battery and we were in business. I allowed the car to idle for around 10 minutes and when I returned I noticed the idle revs were pretty high (~1500) and so I pulled it back into the garage and switched the engine off, happy that everything (apart from the fast idle) was working well.

However, I then noticed smoke escaping from the gaps between the bonnet and the wing... I lifted the bonnet and noticed that the protection between the inlet and exhaust manifold was burning at the centre (precisely between the two carbs on-top of the middle exhaust manifold outlet). I tipped some water on it and it all cooled down nicely.

Any ideas why this happened (apart from the fact it may be a little loose and possibly resting on the manifold)... is it related somehow to the fast idle? Is this a little querk (hotter middle exhaust manifold) that indicates a problem with the engine...

...or is it just one of those things?

I couldnt really do a full inspection, light was low and my 8 month old girl wanted her daddy!

Cheers,
Rick.

R Sims

Any chance of a fuel leak in the area? The heat sheild is made out of non-combustible material (asbestos?) so I can't imagine it burning unless fuel got in there.

Good thing you doused it when you did, a few more minutes and you could have had a B-barbecue.

Erick
Erick Vesterback

Depending on how long the car's been sitting for, things can plug up and create a better environment for the carb to leak. Erick's probably right about the possibiliy of a fuel leak.

I've ecperienced this when the car's stored outside all winter and there's nasty mold on the engine. But if yours has been inside, that shouldn't be a problem. Try turnign the fuel pump and then check to see if you get any drips or moist spots in and around the carb.

Hi Erick, how are thing's over in victoria? Got the car out on the road and ready for slalom this year?

~Dave
Dave Ryzuk

Rick,

Assuming your heat shield is standard and half-way in sound order, it should not be loose and certainly not by enough to be laying against the manifold. When assembled properly, carb mountings sandwich the shield in position.

I would think, from your description, there is something not right in the way the whole assembly fits. I would certainly check it against manual descriptions.

If your shield does still have the asbestos pads fitted (and it well might), and they have been in any way affected by fire, you must be very careful with anything you disturb or work on in that area. A minimum would be a mask to prevent breathing in of any fine (down to microscopic size) fibres that might be loose or made so in any work done. That stuff can be leathal to lungs in any quantity.

You must treat this stuff with the greatest of respect, through to how you dispose of it. Your local authorities may have prescriptions on how that must be done. Following those is in the interests of your entire community.

Regards
Roger
Roger T

Hi all

Thanks for all the replies.

I had a quick look at the area this morning and it certainly seems as if the middle section has cooked - its turned a grey color and is very fragile. I will have another look at this this afternoon and post some pictures on my web-log if anyone would like to look and comment (I have painstakingly included the URL in a thread in the Rust and Restoration section of this site).

Cheers,
Rick.
R Sims

My blog can be found at: http://www.richardsims.co.uk/
R Sims

Rick,

Not absolutely certain but from your photographs it is possible that what you have over the surface of that heat shield is a spray-on insulation of some sort. The original asbestos pads were on the rear side, one at each end - 'shielding' the heat shield surface behind each carb body.

The area that appears to have burnt could have been affected by a fuel leak from either carb or from a pinhole leak in the fuel hose between the two carbs. It is possible, though perhaps less probable, that dashpot oil could be finding its way onto the heatshield.

Regards
Roger
Roger T

Rick. I do not believe it is a fuel system leak. The pattern of damage is most consistent with a painted surface brought in contact with a heat source. The central darkened spot, where the exhaust manifold is closest to the heat shield and the slightly less discolored pattern radiating out from it. The damage pattern from a fuel leak should be a more up and down pattern. Add to that the fact that I do not see a source of fuel in that area, and that any leak, therefore, would have to be from a pin hole being sprayed on the heat shield, and I believe we can rule out a fuel leak. (I may, however, be completely wrong on this. My analysis is based on a conversation with an arson investigator on how they examine a scene to determine whether arson might be involved. One of his answers was a description of the patterns they see on walls when a liquid fuel has been thrown up on them before the property is burned.)

If I had to make a guess, I would say the problem is with the paint used on the heat shield. Did you install a new heat shield recently or repaint the existing one? The damage is very consistent with the use of a non-high temp paint on the heat shield and the paint catching fire. I used high temp, exhaust system paint, when I repaired the heat shield on my 68 and it has held up well.

Insulation is another thing. Unfortunately, I cannot find an original heat shield in my spare parts. The one on the 68GT has insulation over the center section of the shield, where you show discoloration and the fire happened. But, I do not remember whether it was originally that way or something I added when I rebuilt it. Perhaps others might comment?

Les
Les Bengtson

Hi Les

Thanks for your input - I have checked the area and as far as I can see there is definitely no fuel leak.

I purchased the car around 4 weeks ago and apart from driving it to my garage (around 30 miles) it hasn't been driven at all. I don't know whether anyone has painted the shield, but I've not done any work inside the engine bay since I bought the car.

I guess if this were due to the paint, it should have happened before now, i.e. on the 30 mile drive home after buying the car. Maybe its just one of those things that was waiting to happen, what with the car being stationary and idling for 10 minutes (it was quite a warm day and there was no wind) there was no cooling over the engine.

Cheers,
Rick.

R Sims

I have got the same thing on my 78. I run the engine up weekly and get it up to temp. Fine for months then noticed a water leak. Thought nothing of it just another job. Last week with the bonnet up I saw what I thought was smoke from the heatshield. Having given it more thought it could actually be steam. I am not sure if there are any core plugs under the manifold area but I am going to investigate further. It could be the same problem. Good luck.

Trev
Trev

Much like Les that pattern of burning/heat damage makes me think of a jet of flame or hot gas playing on the back of the material, like from a leaky exhaust manifold which is close by that area. Whatever, it shouldn't happen, and you won't know whether it is from a fuel or leak or manifold leak until you run it again and watch.
Paul Hunt 2

Chip board? Shield collapsing onto the manifold?

I just read the material beneath the photos on your blog. On that basis Les and Paul are almost certainly correct - manifold heat is causing the board material to burn (smoulder). Probably accounts for the absence of flame you mention.

Heat shields are made of metal and provided with insulation to reduce heat radiation directly to the carb bodies. Might be time to think about a correct heat shield.

Regards
Roger

Roger T

From Clausager the UK heat-shield at least does seem to be a composition material on late model cars and not the metal panel with asbestos inserts, probably when asbestos went persona non-grata.
Paul Hunt 2

Paul,

You got me thinking. Checking my Moss catelogue (a few years old now) I find three heat shields listed:

Heat Shield 62-73 HS Type

Heat Shield, Metal 73-76 HIF Type, and

Heat Shield, Asbestos 76 on (GHN/D5 410001 on)

My guess is that the first one would be metal also. Probably both with the asbestos backing pads.

You are correct about the later cars, although (in Rick's case) it should not be a cause of burning, unless there is a very large amount of heat being placed on it. Rick's chip board is a composit material but not one I would opt to use near manifold heat.

I have no knowledge of overseas changes regarding asbestos in cars but I think it is only in the last few years that it has been ruled unacceptable here in things like head gaskets and, presumably, heat shield backing pads. I think it went from brake pads some years ago over OH&S concerns regarding exposure of mechanics etc.

I learn something again. Great BBS.

Regards
Roger
Roger T

Hi all

Thanks for all the replies and useful information... you certainly do learn an awful lot from this BBS!

FYI I'll be removing the engine during the next month or two for an overhaul, and so a replacement heat shield (the composite board section) will be installed before replacing the engine. I probably won't get chance to install the engine for quite some time... I have a fair bit of repair/welding to do before I get to that stage!

Thanks again,
Rick.





R Sims

This thread was discussed between 03/04/2007 and 10/04/2007

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