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MG Midget and Sprite Technical - Engine fade in traffic

I'm running a 1500 midget with K&Ns, LCB manifold (etc.). It goes like the clappers (thank you Peter Burgess) but fades in traffic when heat builds up in the engine bay.

I have a Kenlowe fan with Revotec thermostat which is in full working order.

I've moved the front number plate to the front bumper and relocated the horns to maximise airflow, but this only really helps when the car is moving forward.

I was assuming the problem was due to fuel evaporation and was intending to fit a small inline fan and ducting to blow cold air onto the carbs, but there isn't really enough room... and then I thought:

Is the problem due to warm air in the engine bay, - which is thinner than cold air - being drawn into the carbs and affecting the mixture, making it weak? If correct this potential problem is probably being exacerbated by the K&Ns, which are able to draw in hot air rising from the LCB (which is ceramic coated btw). Presumably this problem, if it really is the cause of the fade, could be fixed by using a bit of choke when stuck in traffic?

Any thoughts gratefully received. Thank you.

Yes... id say your spot on across the board

but normally we dont see this issue untill its mufh hotter outside

make sure your heat shild is in place... its not there for looks

cermic coated LBC ceramic exhaust manifold...nice BUT for thst to work effectively... it must be ceramic coated on the inside... if its on the outside the ceramic is preventing extra heat from soaking into the exhauste manifold ....think of the cermic tiles on the space shuttle coming back to earth...

cbeck out the thread wing vents


have you got a heat shield?

Rob Armstrong

Could be fuel vaporising in the fuel delivery pipe between fuel pump and carbs. The pipe runs across the back of the engine bay beneath the heater box and is quite close to the rear of the head - which radiates heat. Try fabricating a heat shield / deflector to protect that horizontal length of pipe. Worked for me.

You may be running slightly weak. Never a good thing really and sometimes difficult to tell from the plugs. Enrich the mix very slightly - I assume CW about half a turn but check - and see what happens.
M Ogden

if it's been Rolling roaded the fuelling will be right.

Look at heat shields; dad's car would die in traffic but an extra 3 inches on the heat shield fixed it

Rob Armstrong

Rob...Not sure about that. Fuelling may be right to achieve maximum performance, but may not be right at idle.
M Ogden

The fuel was fine everywhere as was the timing or we would not have let the car go or we would have said 'get this sorted and come back for a rerun'. I am thinking the engine bay is suffering from the effects of too much heat in traffic conditions. A good cold air supply to the side of the rad helps, a heat shield is a must and a small cooling fan may help too.
Some cars can be real heat soaks! 100/4 Healey, Rubber Bumper Bs, TR4, TR6, Spitfires and Rubber Bumper 1500 Midgets come to mind. Oh yes, remember the MG Maestros with downdraft Webers? I think the S series sported tiny carb cooling fans as standard? The Maestro carbs got so hot they would missfire under full throttle conditions and try and catch fire under the bonnet.

Peter Burgess Tuning

I used to suffer from this terribly.
I did quite a bit of investigation involving putting temp sensors at various places under the bonnet.

I found that with the car moving, the under bonnet temp will be not many degrees above the ambient, however when stopped the temp rises to around 80 degc, which is also the boiling temp of petrol.

The main culprit for this is the fan sucking the energy out of the coolant and dumping it into the engine bay.

The best fix for this is to have the fan working backwards so that it sucks air from inside the engine bay, through the radiator and deposits the hot air outside the car. Though obviously not while the car is moving.

I ran some experiments with an electric fan wired to relays and a microswitch on the carb throttle shaft to change the polarity so that it worked backwards whenever the accelerator pedal was not pressed.

It worked very well except when in stop/go traffic with the fan constantly reversing rotation, i was worried it would burn out the fan.

If you did the same thing but had another fan , maybe on the front of the radiator then i think it would work very well.
Simple relay switched on and off by a microswitch on the throttle shaft or even manually via a rocker switch which changes which fan works, the puller for when car on the move and the pusher for when stationary.

I dropped the idea once i had fitted a large vent/bulge over the top of the carb which was nescessary also for clearance for the air filter.

Now i'm remembering all this i might resurrect this myself as i now have two small fans on my radiator...
Andy Phillips (frankenfrog)

What about one of these to measure where the heat is most extreme (obviously not while in traffic!) and then decide on appropriate action.
Jeremy Tickle

Those cheap thermometers work pretty well. you just need to hold it at a consistent distance and avoid using it on shiny/glossy objects.
Trevor Jessie

I think you would be better off with a thermocouple sensor as then you don't need to know what the emissivity of the surface is in order to get an accurate reading. Cheap DVMs often come with K thermocouple sensors.
David Billington

I used a datalogger with 4 thermocouples on flying leads . it uploads temps from each sensor every 2 secs to a text file on a laptop.
You can then use this in Excell to make a graph.
its very interesting to record the differing temps during a half hour drive....

See my post from about 4 years ago,

I seemed to have much more time on my hands back then...
Andy Phillips (frankenfrog)

My son has bought me an Arduino thingy and told me to get stuck in, I notice it can run is a link, I fell off my chair as it looks complicated to a pedestrian like myself!

Peter Burgess Tuning

Re. 1500 Midget hot engine bay. Has plagued me for decades (I've owned the car since 1978).

(i) Terry Hurrell (Triumphtune) once suggested raise the bonnet by 2cm at the hinge end by using spacers, which will create better airflow out of the engine bay. Would probably help, but aesthetically I never liked that idea (likewise vent panels fitted in wings etc).

(ii) I have resorted to popping the 'bonnet release handle' in hot weather, that does help cooling with it being raised an inch, but potentially dangerous when driving at speed.

(iii) Using a 74C (or lower) thermostat in summer helps.

(iv) Using 'water wetter' helps. Info here:

(v) Also, I permanently refitted the water-pump-driven cooling fanblade (managed to buy the BL 'hot climate fan blade' optional version in 1979). Strangely, on a rolling road test with the pump-driven fanblade fitted, the BHP increased marginally (compared to Kenlow fan only). I put this down to better fuel temperature control.

(vi) I use the permanent water-pump-driven fanblade, supplemented with the Kenlowe (cuts in at water temp circa 95C).

(vii) Under-bonnet heat got so bad on one summer afternoon that I lost clutch. I think the dreaded 'red clutch hose' was ballooning as I depressed the clutch pedal. Had to wait an hour for temperature to drop before clutch returned LOL.

(viii) The 1500 Midget bay is a pig of a design - that 1500 lump should never have been shoe-horned into the cramped Midget engine bay in the first place LOL. Not helped if an oil cooler is fitted under the water radiator - more hot air being dumped into the engine bay.

(ix) I experimented using (old vacuum cleaner hose) pipes from the heater matrix outlet flaps, to air (roof down). With the (heater matrix) blower fan on full speed, it's surprising how much heat is taken from the coolant circuit. Did help keep bay temps down, but I got pi**ed off having lengths of hoover pipe in the cabin LOL.

(x) Honestly, I try to avoid using the Midget if ambient air temperatures are forecast to be >25C. Autumn is a much lovelier time of year for driving with the roof down . . . .
Andy Hock

Oh dear, what a tale of woes!

I had my 1500 for 12 years and only once had running problems caused by overheating. That was (as earlier mentioned) fuel vapourising in the delivery pipe where it runs across below the heater box, and close to the hot cylinder head. Easily and permanently cured with a bit of reflective lagging/ heat shield over that length of the pipe. Other than that it never faltered with overheating problems, despite the raised compression head, fast road cam, tubular steel LCB manifold and "enthusiastic" driving style.

Maybe the overdrive gearbox meant less frenetic engine revs? Actually the main difference is probably geographical. Ambient temperatures over 25 deg C are few and far between in Cumbria! And the other factor is rarely having to crawl in traffic at less than 25mph, below which engine bay cooling is pretty non-existant.

Andy the cure is actually very simple - move away from the overheated and congested South East!

I have no idea what thermo couples, k or otherwise are or do and can only look on in gob smacked awe at the link you posted Peter - is it some form of magic?
Otherwise louvres (as in a recent post)or swap rubber bumpers to chrome or swap to an A series LOL?
Jeremy Tickle

As Andy has remarked, when the car is stationary an electric fan will drag the heat out of the radiator and dump it in the engine room with the obvious results.

My 1500 has the sound deadening kit fitted to the underside of the bonnet (by a previous owner) which must have a detrimental effect in conducting heat away through the metal bonnet as well as being fitted with K&N filters which draw air from directly above the exhaust manifold.

Late model 1500,s came with two convoluted hoses attached to the standard air filter case and drew cool air from a point beside the front grille. Although I have this set-up in my possession I have never fitted it in the belief that the K&Ns were God,s gift to performance.

Has anyone had experience of ditching the K&Ns and returning to the standard arrangement?


I would say keep the original airbox/convoluted pipes but use K&N elements rather than the original paper type.
Andy Phillips (frankenfrog)

Jeremy, I haven't a clue as yet, I'll have to get brave and open the book and bag of parts!

The problem isn't cool air entering carbs it is one of keeping heat away from float chambers by virtue of ensuring a barrier exists twixt ex manifold/systrem and carbs. This can be aided by trunking/piping cooler air from in front of the rad to the carb area. Check a previous owner hasn't ditched the insulation blocks carbs to manifold.

Peter Burgess Tuning

Gday All, Dave I posted an upgrade to the original "two hose convoluted" air box some time back. I increased the hose size to 50mm (ID)works very well and improves breathing above 4000RPM. The engine will now pull easily above this but I don't go over 5000 (not game), also modified the air intake and added a fan see pic. The standard exhaust manifold is wrapped with fibreglass tape which was then all sprayed with silver Hi-Temp paint to reinforce it. Stainless steel heat shields are fitted to the carbs.
Even so I must limit my driving to days under 28 degrees Celsius.

R W Bowers

Pretty neat Rod

Peter Burgess Tuning

if you aren't running with heat shields, I have some new hs4 heat shields if you are interested?

Malcolm Le Chevalier

I fit a home made stainless steel heat shield and have not had any problems yet runs all year round all temps.

mark 1500 on the road Preston Lancs

Hey Mark, is that ss shield as well as the original with a spacer between them? It sort of looks that way in the pic.
The only time I have had a prob is on a very hot day stood still on the A1 in a traffic shunt q.

Dave Squire

no its all stainless with a air gap between two plates
mark 1500 on the road Preston Lancs


nice... i like it... and thank you for pointing out the need for an air gap...without an air gap nothing is achived


This thread was discussed between 26/04/2016 and 29/04/2016

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