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MG Midget and Sprite Technical - Losing power

First, the engine spec:

10.5:1 CR
Big valve Metro head
Kent 276 cam
A+ distributor, 5 BTDC
pertronix ignition
Lucas DB105 coil
Plugs NGK BPR6ES (renewed within last 1000 miles)
Maniflow medium bore LCB
Running on super unleaded.

Suddenly I am losing power at anything over 2/3 throttle. Doesn't seem to be related to revs, just throttle opening. I suspected the coil so checked the resistances. They seem fine - LT 3.5 ohms, HT 9.4 Kohms. The plugs look an odd colour. The pic is from no 2 cylinder. The end cylinders are slightly lighter. The porcelain should be clean but the whole thing is light orange. I cleaned the plugs but they go back to this colour.

The carb was stripped recently and all seals are in good order. Engine ran fine after that, until just a couple of weeks ago. I would check the dizzy advance but my timing light is broken! Advance mechanism was also stripped and lubricated not long ago. Vac advance unit is new. What have I missed?


L B Rose

What sort of fuel pump are you running?
Dave O'Neill 2

Ordinary Lucas electric, rear mounted, electronic conversion from Burlen.
L B Rose

Easy stuff first

Fuel pump on its way out
Blocked fuel filter
Crap in the carb
Poor seal at carb/inlet manifold
Poor LT connections
Check carb is pulling vacuum for the advance
Leaky air hose somewhere

Mixture may be weak - but if it ran fine beforehand.....have you fiddled with it recently?

I thought about the fuel pump, but I don't think that would produce an instant effect as the engine will have to lower the float chamber level first. Same for fuel filter, but I'll check both. The pump should be fine as Burlen fitted a new coil a few years go. Dirty fuel? Possibly. Vac advance unlikely as this is more for economy on the overrun. I've had manifold air leaks before and these usually make the engine run rough, which it doesn't, and/or idle fast, which it doesn't. I agree it feels like a weak mixture, but the plugs don't show that. I always switch off at high revs before doing a plug test (lovely backfire!). I haven't changed the carb needle.

Reminds me of 40-odd years ago when some bastard put sugar in my petrol tank. Merry time stripping carbs by the side of the road in France.
L B Rose

Mechanical or electronic rev counter. If the latter, does that continue to read ok?

Nothing has fallen apart inside the dizzy?
Graeme Williams

Just looking over the specs of your engine... im wondering abiut fuel quality... maybe its suffering a bit

Whwn i built my engine orginally i built it for 11.5 CR, luckly bill just a few mes down the road got to me and cinvinced me other wise and we talked shop and he gave me pleanty on insight on gasoline, compression ratios, and how things work like dissy timming,

The car probanly would have ran at that time but were the best fuel i can now get is around 89 octane and the common stuff at 86 and now newee lables at rje pump are showing 81 octane with more consentration of corn syrup in the gas and once in a while cross paths with 93 octane, but rare now.

Im wondering if your car needs high quility fuel as in a higher octan rating as maybe your fuel quality is getting bad like ours

Prop... just a thought
1 Paper

Graeme, it's a mechanical tach. I'll check the dizzy but it was serviced not long ago.

Prop, I've been running this setup for several years. On regular fuel mostly, but I am trying super unleaded at the moment. Our octane ratings here in UK are higher than US ones (although the units are different).

I've just pulled off the fuel filter. Doesn't seem to be blocked, but I'll try a run without it.

L B Rose

When you say loosing power, do you mean at 2/3 throttle it stays steady and won't accelerate even if you floor it, or does it start to stutter and run rough until you ease off?
Graeme Williams

I see you put the carb was stripped and cleaned recently and that the engine ran fine after that until a few weeks ago but I still wonder if there could be a problem there - needle and seat type and wear, dashpot oil type and level, reassembled correctly everything aligned, seals and gaskets not squashed or pinched, springs, etc..
Nigel Atkins

Graeme, it runs rough and falters/fluffs, then picks up when throttle closed a bit.

Nigel, I am using the correct dashpot oil from Burlen. Actually for some 45 years I have only ever used 3 in 1 or ordinary engine oil, only recently started with the correct oil when I had that trouble with idling (which turned out to be a failed vac advance). The puzzle is that I haven't actually done anything to any engine system immediately prior to this fault appearing. This appeared out of the blue.
L B Rose

The plug colour is perfect - should no be clean!

That indicates it isn't a fuel problem.

Check dizzy points gap, earth wire inside dizzy and try changing the rotor arm & condensor.
Chris at Octarine Services

I discounted the photo of the plug colour as colours on monitors can't be trusted, on my monitor Les has some skin on his hand totally absent of colour and on my monitor the plug's certainly not the odd orange Les describes.

Having wondered about the carb after being stripped I do actually go along with the idea that these problems are much more often from the electric ignition than fuel side.

If the Pertronix is of a more recent vintage I'd give it a closer look and regardless of its vintage I'd inspect and clean up the rotor arm and dissy cap as their combination can throw up various problems - even a problem HT lead can cause problem in the dissy cap though I'm not sure it's the case here.
Nigel Atkins

"The puzzle is that I haven't actually done anything to any engine system immediately prior to this fault appearing" - faults don't always let themselves be known straight away.

Having put that, " This appeared out of the blue." - wear and tear problems can appear to come out of the blue when it's more that they've reached a critical point, such as an example rotor and dissy cap wear.

I also forgot to put to check the delicate wires on the igniter head, make sure they're not nicked or squashed, have play to allow for movement of the dissy plate.
Nigel Atkins

You haven't changed your fuel cap or blocked the vent?

For the sake of 15 mins I would check internals of dizzy, or even better get a timing light and check you are getting the advance you need.

The king lead between coil and dizzy is ok?

Graeme Williams

Sticking my neck out here
Two possibles
Seized distributor shaft stopping it from advancing
or my favourite
Blockage/restriction between the foat bowl and the jet tube-piece of O ring etc
William Revit

Les, you say you havent changed anything, but I notice in your first post you say plugs changed about 1000 miles ago. Maybe a plug or plugs are breaking down under certain conditions. Its certainly not unusual to have duff new plugs these days.

T Mason

Thanks guys. Yesterday I took the fuel filter off and reverse flushed it with petrol. Got nothing out of it. Tested pump - fuel flowed strongly (into a container!). Took dizzy off and checked advance mechanism - all OK. Checked fuel pump hoses and seals - all OK. Put fuel supply back without filter. Went out today and seems to be running fine. Revving cleanly to over 6500 again.

Still not sure what the problem was. It reminded me of my early days with a 2-stroke bike - plug whiskering. Very similar effect - open throttle and engine falters, close it and engine fires evenly again. Poor fuel was usually blamed.

BTW there are no points or condenser to check - the Pertronix kit is Hall effect diode and magnet. Of course I cleaned the rotor arm so maybe it was that.

No I haven't changed the fuel cap - been there, done that!

I'll put the filter back on and try again. Come to think of it, the last tank of fuel was from a different station. Today's run was after filling up with my usual Tesco Super Unleaded.

L B Rose

Be very interesting to see if putting the fuel filter back on makes any difference, I'm assuming (dangerous I know) that it might be a permanent type rather than disposable as I've very recently changed my disposable fuel filter as part of the "bigger" service and I didn't seem to be able to fully empty it out. I was trying to stop the petrol flushing the engine bay and instead save it in a jar to use for cleaning but no matter which way I turned it or blew from some petrol seemed to remain and there was next to nothing in the jar from the filter or either ends of the fuel pipe.

I abrasive clean my rotor and dissy cap at least every 12 months now as the electronic spark really scars and burns the dissy cap posts particularly and when I was late doing so last year I got a slight feel of misfire which stopped immediately I was able to clean up the rotor and dissy cap posts.

So even if it was a duff batch of petrol still worth keeping an eye on rotor and dissy cap in future.

I've gone back to using Tesco Super Unleaded (Momentum 99) and Shell V-Power (now with a more sensible name) and I feel happier more for the cleaning packages of both than any extra octane, I'm sure I might be feeling a bit of placebo in both but perhaps 100% placebo -hang the cost. :)
Nigel Atkins

I missed the edit again -

should have read >> ... I'm sure I might be feeling a bit of placebo in both but perhaps *NOT* 100% placebo.<<
Nigel Atkins

Have you checked the valve clearances? If one or more has closed up this could cause it to falter.

Don't forget the standard diagnostic check on the behaviour of the rev counter as the engine misbehaves - it will help to clarify if this is electrical or fuel related.

If it's going ok now and the only thing different is that the fuel filter isn't fitted ,it sort of speaks for itself
It would be interesting to chop it up and see what the filter element has to show
Possibly impregnated with water and causing a flow restriction
William Revit

I'm very interested in the fuel filter too but bear in mind if the filter comes up "clean" there's also been a change of fuel and cleaned rotor arm to consider.
Nigel Atkins

Oh, l missed that message about the fuel filter and clean running. I thought you were still floundering with a misfire problem. Ignore my deviation onto valve clearances (l was heading towards a sticking valve theory).


Les mentioned earlier that it's a mechanical tacho.
David Billington

Yes,I agree it also shows all the symptoms of fuel that had gone off
I don't quite think cleaning the rotor would make that much difference, but---
William Revit

as normal I'm more thinking the opposite. :)

I've not seen mention (though I might have missed it as I often do) that the petrol was old and it's not that often we get duff petrol from stations, though it does happen. As you put though the filter might reveal all if this was the case.

I (more recently) favour perhaps rotor especially if combined with an as yet undiscovered item that's a bit faulty or passed its optimum like dissy cap, HT lead, igniter head, etc. (or just about any modern made part recently fitted).

There that should guarantee it was a fuel problem. :)

Nigel Atkins

Dont know what you got till its gone....haha

Yeah fuel only takes about 3 weeks to die, so use a busy station

Id certianly cut the filter open for a look...
And dont put it back on if there is any suspison if there is crap in there it may have gotten loose and will go straight into your carbs and then you loose 2 days cleaning and working that mess out... just get a new filter and be done with it

In fact id get 2 filters keeping carbs clean is essential anything that gets past the 1st wont get past the 2nd

I think it would be a huge mistake to put the old filter back on even for a few moments

1 Paper

Fuel and air filters can become clogged with fine particles invisible to the naked eye and this residue can also become so tightly packed as to withstand backflushing. The definitive test is to remove it and see what happens. Replace anew if required.

Surely if you have done this - and only this - and the car behaves, then you have discovered your problem n'est pas?

I have never bothered with using a fuel filter on my car and have only once had a fuel blockage problem. Though it did give very similar symptoms to Les' car.


I'd still be tempted to use a fuel filter simply because if the tank starts to run dry, there is a risk of pulling incompressible tank-metal corrosion residue into the engine.

My fuel gauge is decidedly dodgy, and it is all too easy to run out of fuel up here in the empty wilderness and thus risk sucking in metal residue from the tank....and yes, I do carry a jerry can at all times!

Les also refueled with different petrol and also cleaned the rotor arm (which I've now committed to to possibly be proven wrong for the umpteenth time) so the filter isn't the only possibility.

I like the idea of a filter mostly to see that the petrol has got that far than for filtering plus if the MGBs had them then the Spridgets need them more. :)
Nigel Atkins

One thing I forgot to mention. Many years ago, just after the rebuild, I found that the pump would not pick up the last quarter of the tank. I guessed that the pickup pipe inside was broken or cracked. Impossible to reach without cutting into the tank. My son who is a medic suggested catheterising the pipe. I found a piece of Bowden cable that would just slide inside the pipe, cut it to length, and fitted a neoprene seal at the outer end. I then screwed the union back on. Amazing - it worked! So this time round I pulled out the catheter in case it had failed, but it was perfect. The plastic sheath had not degraded at all. So I put it back. In any case the symptoms back then were not quite the same - the misfire was at any throttle opening.

So I'll just see what happens as the tank level goes down - and buy a new filter.
L B Rose

Of course if the filter was blocked, then it shows it is doing its job!

Had something similar in my 79 MGB. Turned out the intake/exhaust manifold nuts had loosened just enough to create a vacuum leak..

Try tightening them and see what happens.


79 MGB
gary hansen

This thread was discussed between 20/08/2017 and 25/08/2017

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