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MG Midget and Sprite Technical - Hesitation when pressing on

Chaps

Very recent annoying problem. Car has been running very well, but yesterday when accelerating through the gears, I noticed a hesitation in pick up - a slight stutter if you will. It feels like a fuel issue, almost as if the pump is about to die, but pump is a Facet cube thing and seems OK. I then thought of vapourization, but as I have never experienced that before then why now when nothing has changed? Coil breaking down perhaps? Dirty plugs...


Likely causes most appreciated.
Oggers

Does the rev needle flicker?
GuyW

Red rotor arm?
J Tickle

Not particularly and yes...
Oggers

Not particularly... as in no more than usual
Oggers

Oggers, what is the spec of your car. I guess modified if you are running a Facet pump, but perhaps you can outline the details.
What revs, what carb or carbs, what ignition. . .sorry if you have detailed this elsewhere on the pages.
P Bentley

You haven't tightened the valve clearances recently? If they are a tad right they will bind and you will lose power momentarily. Especially prevalent with a performance cam that needs greater clearance than the factory setting.
GuyW

Carb dash pot oil topped up?
Bernie Higginson

Tappets rattle a bit so loose if anything. Dashpot - yes worth a look......
Oggers

Bernie beat me to it...dash pot oil

Thats the easist to check

Prop
Prop and the Blackhole Midget

Hmmm I would check the timing advance weights in the distributor...at least look to make sure its functioning and if the vacuum advance is working.

Kind of a shot in the dark but easy to check and you can rule them out if you have to keep looking.
S

Any progress?
S

Same thing happened to me coming back from Donington a month ago.
Turned out to be the alternator on its last legs, just charging when the the engine was revving above 3K plus, followed a tractor for miles at low speed, battery discharged, not high enough voltage to generate a spark at higher rpm once I passed him.
Changed alternator all fine again, red light was on when I checked in the gloom of the garage, went out around 4K rpm.
Had all the trademarks of fuel starvation or coil failing which I checked first.
Tom Ward

Tom et al

Thanks for that. I will bear it in mind. Prime suspects included in line fuel filter - definitely dirty so cleaned it out, and one of the 3 wires attached to the +ve side of the coil - held in by a mere strand and came adrift as soon as I touched it - since rectified. As an aside, anyone know what those 3 wires are?

I will test the car first and yes, will then check alternator if symptoms persist. Hopefully I have cured it!
Mark Ogden

And to add - it has lumenition fitted - which I think is the (feed) red wire of the 3.....what are the other 2 though?
Mark Ogden

Dont qoute me

one wire is ground one goes into ignition system and the last goes to the positive side of the dissy to fire the points
Prop and the Blackhole Midget

Heres the luminition schematic...hope this helps.

Ogden after you fixed the wires to the coil..did that fix your problem?

Just wondering.


S

Yes, schematic shows the red feed as aforestated. This wasn't the problem wire, one of the other two was.

No idea if it fixed as I have not given it the definitive blast up the local hill just yet.
Oggers

I'm confused. Are Oggers Aberdeenshire and Mark Ogden Avon one and the same, or have two cars got the same fault? Otherwise, that's a long test drive😀
Bernie Higginson

They are indeed the same person. For some reason the BBS reverts to default settings now and again.
Oggers

Fault cured. Car runs better than ever. I suspect due to the loose wire on the coil rather than the filter. Must have been Guy and the subliminal suggestion of the coil - flickering rev counter - which incidentally has also been cured. I just though the slight flicker was normal!
Oggers

Thanks for the acknowledgement Mark, 'though others more accurately pinpointed the coil connections.


When an engine stutters there are 2 immediate simple checks to do without even getting out of the car that don't give answers but do point one in the right direction - apart from temperature and fuel gauge checks.

One is to pull the choke out a bit, richening the mixture. If running improves even momentarily it suggests a weak mixture/ fuel starvation problem. #

The other is look at what the rev counter is doing. If the needle is dancing then the fault is electrical - low tension at least. So could be ignition switch problems, coil, points or loose connections or shorting in any of the connecting wires.

If neither of the above then its time to open the bonnet and my next check, other than a quick look for anything obvious, is to pull the HT leads one by one and see if I can isolate the rough running to a lazy cylinder.

GuyW

Guy,

Pulling HT leads is not recommended if you have an electronic ignition fitted as it can result in damage to the ignition amplifier or outright killing it, with points you're OK.
David Billington

Hmm, I didn't know that David. My car has an ex - MG Metro electronic dizzy. Although pulling leads on a running engine isn't something I do that often I have certainly done it with this car! Seems I have been lucky and have got away with it then. Unless there is something different about the module it uses that sets it apart from the likes of Luminition, Accuspark etc.

Incidentally, the other thing I do whilst still sitting comfortably, is listen for a steady slow fuel pump tick - too fast tick, or no tick and there's trouble a't mill.















GuyW

I did it once and got the mother of all electric shocks, up one arm, across and down the other. Made my elbows go funny. Not doing that again!
Rob Armstrong

Yes, you do have to be careful and do it right. Wasn't it Willie in Aus that suffered some very considerable and long lasting effects from exactly that?

Perhaps I should rescind that particular recommendation!

GuyW

Yep Willie managed to hurt himself pretty badly pulling HT leads. Use insulated pliers or similar if you do it. I've never been badly zapped doing it, but that's more by blind luck than good judgment. Pls ppl take care of yourselves.
Greybeard

The problem is that as the spark can't jump the plug gap the energy isn't released caused by the collapsing magnetic field in the coil and so you can end up with much higher voltages in the coil primary which can exceed the switching semiconductor reverse voltage rating leading to failure or damage.
David Billington

So, if the plug lead in question was fitted onto a spare plug and earthed to the block then one could still do the same test safely? It would assume that one could stop and restart the engine on 3 cyls.

An safer way to detect a lazy cylinder then would be to use an infra red thermometer on each cylinder in turn.
GuyW

Guy

I think you prompted the chain of thought! The root cause was one of those very annoying symptoms where the solution was masked somewhat - by all wires appearing to be intact and the car performing reasonably well. I had initially thought fuel/mix hence cleaned out the filter and a good spray with carb cleaner, yet I must admit I had not formed a connection with my (slighly) dancing rev counter and the coil, but I had formed one with the coil and the hesitation in power delivery. Rev counter now as steady as a rock by the by...
Oggers

actually Mark, if I had thought a bit harder I might have pinpointed your problem rather more accurately! Some years ago I was having very similar problems. Misfiring, engine farting and general refusing to rev cleanly. The only variation was it only happened once the engine was warmed up a bit. I knew it was electrical because of the jiggling rev counter needle but still couldn't spot the problem.

Eventually it got bad enough to reveal itself, as is often the way! The wire from coil to electronic ignition module was fractured half way along its length, but only inside the otherwise intact insulation. The broken wire was making sufficient contact when cool, but as the engine bay warmed up the plastic insulation softened, sagged under its own weight and it would lose the tenuous wire contact inside. I eventually traced it using a meter and wiggle test.
GuyW

Guy

Sounds almost identical. Mine too only manifested itself when warm which also baffled me somewhat. My recalcitrant wire was not the red ignition module, rather one of whatever the other two are on the positive side of the coil. It had almost come adrift from the female spade connector, and very far from making a permanent connection.

However all is now well. Car pulls more cleanly and seems to rev more freely. Just had to test it out with a glorious run over the lower Cairngorms and through Strathdon yesterday just to make sure.....
Oggers

Just going back to plug lead pulling, I once knew a right-handed chief electrician who always walked round the factory with his left hand in his pocket, so that in the event of an electric shock, it earthed down his right side and not across his heart. It hurt just the same but not fatally!
M J Chapman

This thread was discussed between 05/09/2016 and 12/09/2016

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