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MG Midget and Sprite Technical - Mysterious ailment

Setting out for a Spridget meeting earlier this week, the car (Midget 1500) was running well until I got caught up in a lengthy traffic Queue. After a while the tick-over became lumpy and when the traffic began to move, the engine failed to pick-up and although it continued to run, it would tend to stall when put under load. Pulled over to try and find out what the problem was and let things cool down a bit although the engine hadn't overheated. After a while the engine would start and run for a while before the problem returned.

I managed to get home in stages, conking-out and re-starting. I spent most of yesterday investigating.
Good level of fuel in both float chambers.
Mechanical pump delivering fuel.
Spark plugs very sooty.
Starts and runs well from cold but soon starts to cut out.

Like most 1500 models it suffers from a bit of fuel vapourization in traffic but always picks-up when moving.

I'm baffled.


Are you running points or electronic ignition?
Dave O'Neill 2

Sounds like an ignition problem. Someone had a similar issue recently (it was probably actually years ago!) but I can't remember the details of the fix.

Malcolm Le Chevalier

If the rev counter flickered crazily, it's ignition problem.

But my guess on a 1500 is that it was fuel vapourising in the pipe between pump and carbs, where it runs close behind the hot cylinder head. Little air movement and lots of heat. That bit is particularly vulnerable when in slow traffic, but it can be easily cured with a home made heat shield from a bit if shiny aluminium.

Coil can break down when hot and give this effect.
David Smith

To add to the above...on the 1500s the dissy has one those (heat sinks) for lack of a better word for the actual part name... ive read they can over heat and partially work

Prop and the Blackhole Midget

Ditto the heat shield, I expect you have one but some people take them off and wonder why the car runs badly when hot!
JB Anderson

The heat shield l am suggesting is a modification in addition to the standard heat shield that should be there anyway. I got the impression that the standard ones are already fitted on Dave's car.

Yes. I assumed that too, but you never know.

I havn't found the need for extra heat shields on my 1500 over the last 20 years, probably beacause of colder weather and less traffic here, but I felt it pertinent to make sure that there is one normal one fitted.

As I said, there have been people on here who have bought cars without a heat shield fitted and just assumed that all Midgets were badly behaved when the underbonnet heat built up, until someone asked if they had a heat shield,
to which the answer was no.
JB Anderson

Need some vents :-)
Malcolm Le Chevalier

Thanks for your comments folks.

Dave O'Neil - I'm running an electronic ignition (Britpart's cheap and reliable system for the Lucas D45 distributor) it's never given any trouble over many years.

Guy - The rev. counter is steady, just fluctuating with engine speed, so no problem on the low tension side. Fuel vapourization was my first thought but the problem sets in just a few minutes after starting from stone cold and before operating temperature is reached. Standard heat shield is fitted. Will insulate fuel pipe in due course.

David Smith - I changed the coil for a known good one this afternoon but still suffering the same symptoms. The engine starts on the first turn of the key and runs faultlessly for a few minutes before it starts to misbehave.

All of this suggests that the fault is probably not heat related.

I'll have another bash at it tomorrow, but running out of ideas.



I had a few running problems stalling and the odd miss found that the connections on the coil were a bit loose a quick squeeze with the pliers and been fine since.
mark 1500 on the road Preston Lancs

Possibly rotor arm.
Dave O'Neill 2

What about plugs? Are you using the correct temperature ones. ( Can't remember what that would be for a stock

What about you commited a sin agianst jesus and be is punishing you for it

just saying

Prop and the Blackhole Midget

Guy - regarding the spark plugs, I recently changed from the recommended NGK BP5ES to the colder BP6ES as the engine has had a few tweaks and might produce more heat than standard. It could be that they are too cold and not burning off the deposits. I'll go back to the standard plugs and see what happens.


I know it doesn't help to just throw random ideas around, other than to produce a long list of possibles. But l had a not dissimilar symptom on my 1500 many years ago.

It would start from cold and initially rev quite nicely, but then after a few minutes under load the spluttering and lack of power started. It turned out to be a split in the mechanical fuel pump diaphragm. It still pumped fuel, but not as well as it should so that with the engine under load the pump wasn't keeping up. By the time l pulled over and opened the bonnet the pump had caught up and everything was ok again! Then 400 yds up the road it would do it again.

I remember clearly where l was - going up the long hill from Moffat early on during an extended tour of Scotland. But l did work it out and did a roadside repair with a plastic carrier bag (free in those days!) Which lasted the remainder of the 2000 mile trip.

"Spark plugs very sooty"
Hmmm maybe too much fuel?? if it was fuel starvation wouldn't they be showing signs of lean? jmtcw
But I think Guy might have it.
R W Bowers

good shout guy! man you are a spridgeting genius!

bp5es are standard. I did exactly the same, engine has a few mods, I will try 6s. it ran like utter crud, I thought the head gasket had blown or something. I went back to 5s and it was fine.

Malcolm Le Chevalier

Situation so far-

Guy - I thought your suggestion of a split in the fuel pump diaphragm fitted the symptoms exactly.Apart from removing the gauze filter I was unable to access the diaphragm as the pump appears to be a sealed unit. By a stroke of luck I had a brand spanking new fuel pump in the garage. Having swapped them over, I started the car this morning and it fired up and ran perfectly for about three minutes before stalling again. Hey ho.

Rod- The blackened condition of the plugs also made me think that far from fuel starvation the problem could be over fuelling. It would not be confined to one carb. as all four plugs were in the same condition. I have already checked the float needles are shutting off. Just going to try some different plugs now.


I had something very similar on my1500 last month and had very similar thought processes until a bell rang saying ' it's the points it's always the bluming points'

Like you I'd don't have points I have cheap and cheerful electronic, but I followed the rule anyway and after really close inspection found that the braided wire that gives the points/module the earth was broken - giving an intermittent earth, it's hidden within a cloth insulation.

I replaced it and everything went away, I must admit I hadn't been clear that the unit needed earthing but when I fitted the module it didn't seem to do any harm and after that experience my first thoughts are still 'it's always the points'

Btw my tachometer was rock solid throughout which had led me away from the LT circuit, maybe that diagnostic isn't as reliable with electronic points?


Try with the standard BP5ES. As I said, my car ran so differently, i.e. like utter guff, on BP6ES and when I pulled the plugs they were sooty as Santa on Christmas eve.

BP5ES are the only way to go for a road going 1500.

Malcolm Le Chevalier

Dave, the top on the original 1500 fuel pumps is located with screws, giving access to the diaphragm. But most replacements seem to have the top crimped on so it cannot be dismantled. Anyway seems you have eliminated that bright idea!

Tim, l guess the jiggery tach diagnosis is one of those things whereby if it jiggers there definitely is an LT circuit fault, but the converse isn't necessarily true!


""Jiggers"" that copyrighted .. because i want to use that word.


Prop and the Blackhole Midget

;-) !

Had another go today.
Changed the plugs for a used set of Champion N12Y,s.
Engine started and continued to run.
There is an in-line glass fuel filter on the suction side of the fuel pump and I noticed that bubble were forming in it which suggested that there might be an air leak somewhere between the tank and the filter. On the other hand, the bubbles could well be the fuel vapourizing ,as the filter is situated in the hot spot behind the cylinder head.
So in an act of bravado I took the car for a run, It pulled well and gave no trouble although it wasn't tested in slow traffic conditions.
It looks as though the change of plugs might have done the trick but as an added precaution I'm also going to fit the mechanical fan back on to work in conjunction with the Kenlowe if only to get more airflow into the engine room.
Thanks all for the advice so far.


This thread was discussed between 04/08/2016 and 07/08/2016

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