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MG Midget and Sprite Technical - 1098 runs fine for 15 minutes

I have followed some older threads but am not sure that they have covered this issue, so here goes. I have a 1098 mark 2 midget. Recent professional engine rebuild and she has not run right since. She is great for 15 minutes or so then feels like she is about to give up. Foot to floor causes a serious stutter and I limp back home. It appears to be a WOT issue as the problem is not/less apparent if I pootle along at low speed/revs.

She did give up altogether recently at a roundabout and after about 15 minutes started again and I managed to limp home.

What do I know?...fuel is getting to carbs. Have changed dist cap and all HT leads and condenser. Nothing odd with tacho before she runs rough. Recent service and tune up means she runs great...for 15 minutes! Any idea where to look next?


Andrew Grant
A Grant

Does it have a vented fuel filler cap. when it stops running properly, take of the fuel cap and listen to hear if you can hear/feel the air release of a vacuum.
frogeye Gary


And do not expect new parts to be fine.

To be sure you can go for the cheap, simple but good electrinic conversions from simonbbc (google that)
Get a coil to match than that should not be the problem

Should make the ignition almost service free
Onno K

Cheers Gary, I will give it a go! Having put almost the full value of the car in repairs in 1 year, it would be nice to actually be able to use it for the 3 days the sun is shining this year!
A Grant

Have changed the coil, but if it does not appear to be fuel related, I suspected the coil would be next in line. Will check simonbbc out

A Grant

Use it as mutch as you can than it will get very reliable.
This is always the point with the reputation of unreliability of classics.
Cars that are used little or have been laid up often wear worse than cars used daily.
Onno K

It could be a faulty rotor arm. This was a known fault for a while a few years ago with items which failed as they warmed up and then later recovered when they had cooled down again. Red replacement rotors from the Distributor Doctor were the way to go to solve this particular fault.

The problem with suggesting this sort of known fault is that the tale hangs around in the collective memory long after most of the faulty items are out of circulation. So whether this applies in your case I have no idea. But your symptoms seem to fit so it may be worth checking.

But do Gary's fuel cap check first! Its quick, easy and costs nothing!
Guy W

Carefully check the LT leads inside the dist cap for any chaffed insulation. Also you say you dont think its fuel related and you are getting fuel but it could be a vapourisation problem (is the heat shield in place?).

T Mason

Thanks for all the rapid help guys; really appreciated!

Trev - I did check pipe flow and ensure the heat shield is in place. It may be fuel...I guess it was only really be fuel or electrical! fuel pump appears to work OK and it does get to carbs, but I will check for pressur ebuild up.

Guy - the rotor arm is definitely black so I will search out a red one.


A Grant

>>Recent professional engine rebuild and she has not run right since.<<
so could be related to this or work or parts changed since, as already put don't assume new parts can't be faulty or that the existing or previous parts fitted were the original type of fully correctly worked

have you changed from running-in oil and had head retoqued and tappets reset then cb points and plugs checked followed by timing and mixture

checking the fuel cap is a good idea as this appears to happen once the car is warmed up - still could be fuel or electrics

those nasty points can play up, check the fitting order (the Driver's Handbook is your friend here) also lube cam and oil dissy if not already done

certainly iffy condensors and rotor arms have been about so worth checking and replaced with know working items same as coil

personally I found the 123 fully electronic dissy improved the running and performance of my car

very unlikely I know but have you checked the engine breathing system and check battery and all connections, leads and earths, HT and LT wires and connections are clean, sercure, and full protected - wires have been know to part when warmed up

I think possibly the WOT only hightlights the problem as it's less apparent at lower revs

er, carb pistons are matched and correct oil in

let us know how you get on

personally I'd run through a check of the 12,000 mile service list (DH again) as that would provide an ordered system tick list rather than the scatter approach we've all listed here and cover almost every suggestion here
Nigel Atkins

ETA: I was going to add first check that fuel cap as it's easy and quick then the 12,000 list
Nigel Atkins

" the scatter approach we've all listed here"

Nigel, I think you are the only one using the scatter approach! Half of the suggestions you make simply don't relate to the symptoms described!
Guy W

yeah mine are the widest (usual are, possibly highlighting the point, you decide) but collectively it's a scattering of ideas

without more information, being there, driving or passengering in the car then who really knows which will be the source of the problem(s)

you could start with the most potentially obvious solution given the limits mentioned above and then move on, that might be another approach

not all of my suggestions may be directly related to the symptoms as diredctly described but some cover stuff that will need to be done/have been done already or could be related

Andrew has probably seen the recent similar threads and suggestions (and solutions?)

if it turns out to be the rotor or fuel cap as you've mentioned both I'll sing your praises even more than I do now and not mention the good book for a month :)

but - if it turns out to be something that would be picked up on a DH 12k service check tick list will you praise the good book for a month :)

just remember the 'catch-all' scope of those sevices :)
Nigel Atkins

It might be an obvious one but how old is the fuel in the tank?
Ed H

The word on rotor arms has certainly not reached all quarters and they are still causing failures on the road.Only a couple of weeks ago, in France, I had to deal with a failed rotor arm. In this case it was a good quality item but the dizzy spindle had a massive build up of rust under the rotor that I assume it to have caused a hairine crack.Its the first time that I have had to take a rotor off with hammer & chisel as it was bonded on by the rust. I quickly diagnosed the fault by holding the coil HT lead to the rotor contact and witnissing a spark to earth when the engine was turned over with the ignition on. The car was not mine but someone on tour with me
Alan Anstead

Two things to check:
Dirty air filters

Sorry if these have been mentioned already but sometimes difficult to sort the wheat from the chaff :)
Neil T

I would also check the points gap as I've had a set of points that rapidly wore the heal and closed the gap and gave the same symptoms when hot and after cooling started and ran fine for awhile.
David Billington

I agree, Fuel cap then

Fuel filter/ Blocked petrol lines

Beware the varying quality of the electrical components. some new items can fail almost immediatly!
Bob Beaumont

If the rebuild included head work, and especially bronze guides, the guide clearances may be too tight. This leads to valves hanging open when the engine is loaded enough to get things good and hot. Bronze guide clearances typically need to be .001-.002 over standard, depending on the actual guide alloy.

All this can be expected to be even worse if very effective modern stem seals were fitted.

FR Millmore

Sometimes a fuel filter will get blocked under full acceleration because of drawing a higher rate of fuel through it. The blockage can fall off after time, and fix itself till the next time you need a high rate of fuel. This would fit your description of it runs OK at low throttle. Good Luck finding your problem.
FWB Frank Bernett

valve clearence maybe (to tight)
awe rijt

Wot's WOT?

I'm another vote for coil. Had it happen a couple of times with different cars.
Max max-at-midgetmax-dot-com

The curious point is that in 50 years of working on British cars, including about 20 total in large scale British only service operations, I have NEVER found a bad OE Lucas coil that was not obviously physically damaged. I have replaced a bunch of aftermarket crap (recently including allegedly "Lucas" "sports coils" which ain't) that people installed when incorrect diagnosis led them to throw away perfectly good Lucas ones, some of which I have been able to retrieve, and which are still in use years later.

And Mr Grant has not responded to my implicit question re valve guides.

FR Millmore

Weeeelllllll...speak as you find. I've had Midget coils fail and recently OE Porsche/Bosch ones fail. It's a cheap and easy check, eliminates one problem from a list.
Max max-at-midgetmax-dot-com

Indeed, check and decide. No question there, and I have done a million times, but not found any bad OE or old manufacture Lucas coils. My 50's & 60's real Sports Coils are treasure; it started downhill when they began riveting the LT terminals. And you frighten me, as Bosch Super Blue are my preferred substitute!

FR Millmore

So...after a busy week the sun shone so off we went. After about 20 minutes, the same thing. Stuttering engine, very little power. Limped home. I checked the fuel filler and there was no indication of pressure build up. I filled the car up with fuel during the journey.

When the car got home I took off each plug lead in turn and one appears to make no difference when removed. The engine is very lumpy when hot (sweet when cold), so I guess that could point to over tight valves on that pot, perhaps from the rebuild? Next on my list is to get it in the dark and see if there is any arc-ing.

The cap looks clean with no obvious signs of trouble, but I am no expert.

The coil is hot, but am I right to assume that as 1 particular pot seems to be impacted, the coil is not likely to be guilty?



A Grant

I was told a tip from the guy who recovered my car on Saturday.
A way to see if you have a slight crack in the dizzy cap is to wipe it with talcumpowder, as it should highlight any cracks in the plastic. Don't know if it actually works or not.
frogeye Gary

This thread was discussed between 24/07/2012 and 30/07/2012

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