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MG MGB Technical - Help: Loss of power when hot

Hi guys,

I took my car out for her first proper run of the year, and things were great, for about 25 minutes. Then she started losing power and running lumpy below 2500rpm and then just stopped. When I tried to accelerate, this seemed to kill the power even more quickly than just leaving it to sort itself out. Fired up ok after sitting at the roadside for 25 minutes, but proceeded to run very hot after.

She was fine last year, going on 100 mile runs with no problems, but today 20 miles was too much.

I'm running a 2099cc stage 2 with a 276 magnum cam, twin HS6 carbs, K&N filters, oil cooler and a Kenlowe fan. The PCV valve is fitted and clean, and I have a brake servo fitted.

I was driving it quite conservatively today, as it is a torquey engine and I was driving through a lot if villages. Would it have helped to thrash it?

Any help would be much appreciated, as last year she was fine, but today she isn't :-(

Thanks in advance,

Grant :-)
G Hudson

Sounds like either the coil or the condenser is duff. In the interests of cheapness replace the condenser first and see what happens. After that beg borrow or steal a coil and see how it runs then.
Iain MacKintosh

Hi Iain,

Thanks. I've got an optronic lumenition ignition fitted and a Lucas sports coil. Looks like it'll be the coil, as there's no condenser.

Many thanks,

Grant :-)
G Hudson

Grant,
it could be lots of things other than coil (but it still might be the coil), loads of threads on this subject in the Archives

the running very hot after wouldn't be the coil

if this was your first real run of the year then lack of proper use can introduce niggles, regular year round use will help to prevent this, there are plenty of dry (and sunny) salt free driving opportunities through out late autumn and winter

I'd check the Optronic, make sure the rotor is sitting correctly and isn't damaged, check also the wires for snags and scuffs

as always a step by step diagnostics of the problem will be required to solve it and more information before and after each step to solve it over a forum

has any servicing or maintenance work been done since last year, any new parts fitted, do you do the regular driver's checks, etc.
Nigel Atkins

If the coil is faulty (unless it is the HT circuit which is rare), or anything else in the ignition primary circuit, the tach would be jumping all over the place.


Could also be fuel starvation, which could well make it run hot as it could be weak.

Do a delivery check by removing a delivery pipe from a carb (be aware that if the ignition has been on recently you will probably get a spurt of fuel), direct it into a container and turn on the ignition. You should get a minimum of one Imperial pint per minute and in practice at least double that, in a continual series of pulses with negligible bubbling.


Paul Hunt

I'd go for fuel more than ign. If you don't drive regularly (read every week at least) then you risk hoses going bad internally, corrosion in the tank etc. A little dirt goes a long way to blocking fuel up especially with the new 'ethanol menace' and even more so on a high tune engine. As Paul said once it's lean it will run hot but may seem ok otherwise. Have you checked your plugs afterwards? Colour says a lot.

A quick look at the ign wouldn't hurt of course!
Roadwarrior

Has anyone seen 95 E10 yet?
Paul Hunt

Hi guys,

Thanks for your responses. I think I've solved the problem, as the fuel filter was somewhat clogged, so fuel starvation looks like it was the cause of both the loss of power and the overheating, as it seems to be running ok now.

However, I have noticed that cylinders 3 and 4 seem to be running weaker than 1 and 2. The gunson colour tune was showing a nice blue for 1 and 2, but blueish white for 3 and pure white for 4.

Any thoughts on if this is an issue or how to overcome it if it is an issue?

Many tanks,

Grant :-)
G Hudson

The fact that the fuel filter was clogged may be of interest to us all. My local garage owner is having to deal with a number of classic owners who have problems with modern fuel. Apparently we are back to the "good old days" when fuel dies quickly in the tank - jellifies and blocks fuel lines (as well as all the the horrors associated with alcohol we have read about here). The answer is to leave your car as near as possible empty (and still to expect trouble on startup).
Grant - the differences between the cylinderss is probably your hardware but make sure you examine the engine on new fuel before you draw any conclusions and spend money
Roger W

"However, I have noticed that cylinders 3 and 4 seem to be running weaker than 1 and 2. The gunson colour tune was showing a nice blue for 1 and 2, but blueish white for 3 and pure white for 4."

Your carbs are probably unbalanced, although you could also have a vacuum leak on the rear carb or rear half of the manifold. Because both carbs sit on a tube that is common to all four cylinders, a little of the mixture from the front carb goes to cylinder 3, and even less to cylinder 4, and vice-versa i.e. rear carb to cylinders 1 and 2.

You need to do a carb setup for air-flow and mixture from scratch, and if you find that you have to alter one carb more than the other to get from the starting mixture position to the correct mixture, there is a problem on one or other carb, probably the one you have to adjust more. Even when you balance the carbs for correct mixture at idle, if you have a vacuum leak anywhere, even a small one, its effects will vary across the throttle range, and it could be weak or rich when running i.e. at wider throttle openings, depending on where the leak is.
Paul Hunt

I'm never sure what fuel these classic car owners are using that causes them so much trouble

I've left my car a couple of weeks unmoved a few times, sitting outside 24/7 (and 365/6) and not had the slightest of trouble

perhaps these classic car owners are leaving their cars static for much, much longer - you get problems and niggles from doing this anyway and if (and note I put if) regular full and proper servicing isn't carried out rather than I've not done many miles in it recently (or perhaps for decades) so it won't need any attention

you can get problems from fuel stations that operate poor fuel care procedures and those that can make an odd mistake
Nigel Atkins

A fellow mgb gt owner from the next village to me had the problem of over heating and subsequent misfiring. All the usual electrical checks proved useless and failed to resolve the problem. Float heights were investigated and one was found to be way out and adjusted - things improved but still the problem persisted. Fuel lines were flushed and some rubbish was found - things still not resolved.
For no good reason it was when the drain plug for the coolant on the engine block was removed the real problem was found. The water did not flow out as the water chambers were full of gunk and slime, a good flushing removed all the accumulated rubbish from block, radiator, heater etc. problem completely gone and over heating and misfiring a distant memory! Might be worth a try?
N T BOND

This thread was discussed between 14/06/2014 and 28/07/2014

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