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MG Midget and Sprite Technical - Stumped


Mostly standard 1275 but with Piranha electronic ignition, servo, Maniflow LCB.

It started with a slight hiccup when changing down to 3rd, slowing down and then accelerating.
Occasionally the engine died and it was only the momentum (it seems) which got it to run again.

It stalled (my fault, ahem) when appraching a hump backed bridge and wouldn't restart. Two drivers very kindly pushed me over the bridge and dropping the clutch in 2nd, it restarted.

On a 6 mile trip it ran well but after a 30 minute break, wouldn't restart.

No ticking from the recently replaced SU fuel pump so several taps later, still nothing. Fuel in the filter and nothing obviously missing/unattached on the ignition side either.
Rang the breakdown people and settled in for a wait. Decided to pull the car forward so the brakdown truck had an easier approach, so cranked the starter and let in the clutch. As it moved forward it fired!
Cancelled the breakdown callout and got home.

The fuel filter looked grim so was replaced. One tag on the coil was ever so slightly loose so was tightened. Otherwise all seems ok.

Fuel is getting to the arbs so I'm now thinking electrical?

It starts well from cold and will restart once it's cooled down for about 30 minutes but I can't rely on it which is frustrating.

Grateful for your thoughts/suggestions.
Jeremy Tickle

I had these symptoms many years ago and it turned out to be a faulty coil.

Car ran for a while - coil got very hot - car conked-out - coil coiled down - car ran ok again for a bit, cycle repeated itself.


Simon
SA Wood

Does the Pirahna use a rotor arm? If it does, try swapping it for a "Red" rotor. Poor quality rotor arms can cause similar symptoms caused when hot.
Graeme W

My vote is for rotor arm as well!
john payne

"Fuel is getting to the arbs"
My vote is something is very wrong if you're trying to get fuels to the Anti Roll Bars.... I'll get my coat.
Hope you get it sorted soon
G Lazarus

Hi Jeremy,
First thing to establish is is the fault electrical or fuel.
Does the rev counter needle flicker when you have the issue. If it does it is electrical, look at the rotor arm or coil. Get a red rotor arm from the dizzy doctor and a new coil - both cheap.
If not, look at the fuel.
You say that the car would not restart after a 6 mile trip and the fuel pump was not ticking (working). Have you replaced the fuel pump? It stopped working and you had to tap it to get it to start. If it stops working while you are overtaking, will you be able to tap it to start it or are you going to be left in a dangerous place ready for a head on collision?
A replacement, modern, reliable pump is not expensive.
I have fitted one of these -
http://www.fuelflow.co.nz/FF_cms_03/eshop?page=shop.product_details&flypage=flypage.tpl&product_id=66&category_id=28
Has worked a treat so far and no ticking.
You may need a fuel pressure regulator too.

I have found the su pumps unreliable, even the electronic replacement su type pumps, in my experience, are not up to the job (cannot keep 2psi at full throttle). You have replaced the mechanical points in your ignition system, so get rid of the mechanical points in the fuel pump.

Good luck
Dave
Dave Brown

Having had an SU fuel pump on a Minor for 11 years of everyday, only car motoring, I can say that you can make them more reliable. Mine was in a poor unreliable condition until.... With the cover off, thoroughly clean the points with very fine emery cloth and ensure the whole mechanism is absolutely clean and free of dust and grime. Soak the mechanism with lots of WD40 spray or similar and replace everything. Do it twice a year. 11 years, many tens of thousands of miles and only one pump.
Nick Nakorn

Thanks for the responses, including fuel filled anti roll bars!! LOL
Didn't check the temp of the coil so next on the list. It does use a rotor arm and I just happen to have an unused red one bought from the NEC show last year so will now swap.
Dave, funnily enough I did have an electronic pump but it died after 6 years and I couldn't get a direct replacement for a reasonable price (£29.50) and didn't want a Facet cube type (noisy little b#ggers) so replaced it with a s/h SU which has been working fine until now possibly.
Jeremy Tickle

Sounds very similar to my problems earlier this year - faulty red rotor arm.

I didn't know how to test the Ignitor - so sent it all to Aldon who confirmed rotor arm failure.

Now note how to test Ignitor featured in recent Mascot.

R.
richard boobier

Yeah ive heard that the electronic SU pumps are not worth writing to grandma about

Im not familiar with your ignition system... I had a pertonix do the exact same thing... rotor bug was GOOD, but the computer chip had gone bad

Back to Points and condensors now and very happy

Prop
Prop and the Blackhole Midget

Jeremy,

Do switch out the Piranha ignition for a simpler simonbbc unit.
I've had a lot of trouble with mine when I first got my car.
Switched to the simple unit and never had any trouble since.

Coil rotor or pump might cause this problem but Piranha unit might be it next time
Onno K

Nothing wrong with SU electro mechanical fuel pumps. And you don't need to service them either, -- as long as they are installed the right way. I've had two on my Sprite in over 35 years. The first one lasted from 1977 -- when I got the car -- until around 1998 when it started acting up, and rather than replace the points, I bought a new one. The new one is still on my Sprite. Never missed a beat since. The original is now a spare.

My '73 midget has the same type. I drove it back from london with only one side of the points working. One of the contacts was loose. I soldered it back onto the blade, and it worked perfectly. But I've just bought a replacement set of points for it - since I intend to flog the car.

I've no reason to think that the SU electronic versions wouldn't be just as reliable too.

Still on points ignition too. Never had trouble with that either. But Rotor arms are a bit crap these days if you buy the really cheap ones.
Lawrence Slater

All the above are worth checking, we've had coil, rotor, pump all cause this sort of problem. It could also be the condensor.

Dave Brown's point about determining first whether it's fuel or electrical is a good one, but note that scondary circuit issues (rotor, coil) don't show up on the tacho. We find a timing light to determine whether there is any spark or not a useful quick check, though this isn't always infallible.

On the fuel side, once the car has done its thing and stopped, leave the ignition on (turning it off then on can kick start a marginal pump) then carefully start to work one of the fuel hoses off its carb. If there is fuel pressure there then fuel will start to bleed out. (Don't just yank it off the car or you'll get a flood of fuel which isn't handy with the exhaust hanging around :-() If fuel starts bleeding out, you've established the point and can push the hose back on immediately.

SU pumps are definitiely not unreliable if serviced properly. I still run them on my rally car (2 in parallel as a single doen't quite have capacity for 100mph+ on WOT) and never have had issues with them for 25+ years.

Their only failing is that they keep soldiering on in a state in which other pumps would die. In this condition most people just give them a hit, then do nothing else about it other than mutter about them being unreliable.

It's the equivalent to having a bit of a misfire in a spark plug and not doing anything about it. If they are kept serviced (doesn't need to be done that frequently, every 2-3 years is plenty) they go for ever without a problem. Correct adjustment of both the tangs which determine points clearances, and the throw of the points via the diaphragm spindle position, is important - but simple to do, and is covered amply in the manuals.

We are now using bi-directional diodes for spark suppression. Shunts everything in excess of the nominated voltage (25V) leaving no spark at the points and so very little if any points erosion/burning.
Paul Walbran

Sorry for the delay in responding but my lame excuse is I've been giving it a thorough shakedown, ahem.

Looks like it was the rotor arm, see attached photo (note horrible black 'burn'). To my shame I'd changed the plugs and leads but hadn't checked the rotor arm (a quick rub on the tyre sidewall would have been enough).

There's a red rotor arm now in place and it's running much better. I'll clean this one up to keep as a 'just in case' spare as it's done 6 year's service.

The coil was checked and it didn't become any more than lukewarm. Maybe Father Christmas will bring a Simonbbc ignition system to have in, in case the Piranha gives up the ghost.

Thanks for your help and advice, all much appreciated as the car is reliable (and fun to use) again. Cheers.



Jeremy Tickle

If you've got that much build up on the rotor arm, have you checked the distributor cap segments for crud and or burn back too?
Lawrence Slater

I'd second checking dissy cap too

as with rotor arms, and beware of poor quality copies of Red rotors, beware of poor quality dissy caps if you can't clean yours up

electronic units do seem to have more spark to them than the (IMO nasty) CB points so some can mark up the rotor and dissy cap

to take advantage of the electronic units that do have more spark to them they need good quality and condition rotor, cap, leads and plugs and perhaps to widen the plug gap
Nigel Atkins

Yes thanks I did check the distributor cap and it was rather surprisingly, ok. I did expect it to have some build up as well as the arm.
Lawrence and Nigel agreeing, that's a refreshing change :}
Jeremy Tickle

I doubt the black on the outer edge of your rotor arm was the issue, the more likely cause of your problem was an internal fault allowing the spark to leak to earth via the rivet, this is a know failure mode of these rotor arms with rivets. see link below


http://www.distributordoctor.com/rotor_arms.html
R.A Davis

we do sometimes, depends on the subject though :)
Nigel Atkins

True. :).
Lawrence Slater

Stop it !!! lawerance and nigel, ....

if you keep agreeing with each other then a royally pissed off jesus will show up for the 2nd time.

Its just not natural


Prop
Prop and the Blackhole Midget

I heartily agree prop!
Steven Devine

It was only once. lol.
Lawrence Slater

Aha ha ha! Ok you have a pass this time....but remember we are keeping an eye on you Lawrence...LOL!
Steven Devine

Probably not the same but a few years ago I had a VX Carlton GLi that would start in the morning when it had been stood or 30mins after a jouney but as soon as the engine warmed, I knew that if I switched the igition off then I would have to bump it!!

I fitted another starter and it wizzed over twice as fast as when it was cold with the old one!!

Turned out that (I think anyhow) the lacquer on the windings was melting when ho so causing shorts, when cooled the lacquer set and did it's job of shieling the wirse from each other!!!


John :)
j b biggs

This thread was discussed between 21/11/2014 and 02/12/2014

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