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MG Midget and Sprite Technical - Ignition (?) problem driving me to distraction ...

I have a misfiring problem with my 1970 1275.
Before Christmas it started misfiring on a journey. Nothing huge just a bit of a hiccup, as you accellerated. The car was then not used since the other week when I took it out and the same thing happened only much stronger jerks, but never at the same time, or under the same circumstances.
The tacho needle seemed to jerk so (as per archives) I suspected the LT ignition. I checked the wiring (remade a couple of connections), replaced the rotor arm, and plugs. Running Lumenition Optronic so no points or condenser. I checked the timing and reset at 13BTC
Took it out for a run the next fine day. Ran lovely not a hint of a misfire.
Came to go out in it last weekend though and almost immediately it was back, and seemingly worse than ever.
It does not favour hot or cold engine, accelerating or cruising. Compression test shows 150 across the board.
I had a good look over it in the garage and its definitely got worse to the point its a constant misfire on no4. Swapped the plug with no1 no change.
Checked the leads with a timing light. Nothing on 4, then nothing on 2 and 4 is fine!!!!
Tried a couple of different coils I had. No change. Tested the lumenition as per instructions I found on the internet, and a spark that can jump a 1/4" gap is produced at the coil lead.

Bought new leads, cap, rotor arm and coil. Just fitted them, and no change. Its still now constantly misfiring on 2. There is a spark (not great)at 2, tested by holding plug to earth.

I have also just noticed whilst pulling plug leads that the misfire on 2 stops just as you pull the plug lead off but before moving the lead right away from the plug. You can hear the spark jumping from lead to plug. If you put it back on it misfires again, pull it slightly and it fires. Whilst doing this 3 started missing and the same thing worked on that plug. Pull the lead just off and the misfire stops!!!

Help! This working whilst the lead is just disconnected makes no sense to me. But does suggest that it is indeed ignition related.

I should add it is running rich (well the plugs are black) but its been doing that for a year.

Sorry for the essay.
G Hawkins

Change the Lumenition unit for a hall effect one, old Optronics are nothing but trouble.
Brad (Sprite IV 1380)

Agreed. The LED that provides the light can go dim and pack up all together. Beats me why anyone buys elec ign which needs an external box (unless they want a rev limiter?)

A
Anthony Cutler



Sounds like your plugs (or wires) need more resistance? Misfire stops when there's a higher resistance (spark has to jump a large air gap).
JM Morris

Ive read in other online forums that dust and vibration is what really makes these things go nuts.

I wonder if those optics have some kind of damper to help with vibration...if so may check to see if the damper has hardened or deterioated in some way...just guessing!


one thing you might try that Ive found useful for misfires, is start the car in a super dark garage (cant see hand infront of face dark) and watch for the blue electric light show.

if its really bad, put on some pink floyd and invite the wife to a romantic evening..maybe a little exhaust manifold cooking...LOL

Prop
Prop

Well it would seem that an old Optical Lumenition external power moduled crappy electronic ignition system is just as good as a nice modern hall effect fits inside the cap one .... just fitted one(a couple of hours ago) and made bugger all difference! Grrrr.

But after 2 hours of farting about its running (smoothlyish) with new electronic ignition (couldnt be ar*ed to swap back), old cap and leads, new rotor arm & coil, and a mixture of old and new plugs !!!! So I dont hold out much hope that it's fixed for good, probably just a playing ball until I actually want to drive it...

Oh and when starting it on one occasion, guess who forgot the throttle was jammed wide open after doing yet another compression test ......
G Hawkins

As I suspected its not fixed.
This morning it starts and idles fine. Leave it idling whilst I reattach the air filters (standard & new) and it starts missing. A quick check with the timing light shows that today it's number 4 cylinder.
Again pulling the lead a fraction restores spark. But yesterday no 4 never missed a beat it was number 2.

I have tried, new plugs, 3 rotors, 2 caps, 2 sets of leads and now 2 electronic ignition setups.
Battery shows 13.9v whilst running.
Whilst running coil +ive shows the same. A light bulb from coil +ive to earth indicates no loss of power to coil. Direct feed to coil from battery did not nelp.
Timed at 13degreesBTC @ 1000rpm
25D dissy.

Someone mentioned lead resistance. That is fairly similar across the leads, even when removed and measured during misfiring.

I've also added an extra engine earth (jump lead)

Any more ideas? I'm probably missing something really obvious, but I can't keep throwing money at it :-(

Oh and no light show in a darkened room that I can see ...
G Hawkins

test compressions

it's too easy to blame electrics when it may be a sticking valve or duff tappet setting

sounds as if electrics have already had all the scrutiny they can bear, to me

after changing all those bits and still having the same problem I'd look elsewhere now
bill sdgpm

Maybe the voltage regulator sticks? (assuming old lucas box)

elektrickery is no fun.
Alex G Matla

Does your rev counter flicker when it is "misfiring" This is often an indicator of whether the fault is indeed electrical (ignition) or summat else (valves, fuel, compression) If the needle doesn't flicker, but just settles as the revs drop then it is likely that the fault is not electrical. It is an easy and instant check that should be the very first thing one looks for as an engine falters.

Guy
Guy Weller

Thanks for the suggestions.

I've tested compressions a number of times, although I failed to mention it in my last post.

Cira 150 across the board.

Tappets or valves etc. would not explain how the problem jumps from one cylinder to another would it?
Also would not explain how the spark reappears when the lead is pulled a fraction? Or how the same lead yesterday had no apparent problem?

Cant see how the regulator could affect things but I'm willing to learn.
G Hawkins

Agree with Bill... the electrics have been thoroughly experimented with.

What about the carbs? Do the pistons rise/fall smoothly? Do the jets return fully when you release the choke? can you weaken off the mixture a couple of flats to reduce the ambient richness? Does the engine speed up/slow down when you push the inlet manifold up/down? Carbs done up tight? All gaskets present?

A
Anthony Cutler

When the first lot of misfiring happened, the tacho did jump (I mentioned it in my first essay above), but after remaking the coil connections that has stopped. The needle now hold quite steady whilst its running on 3.

Carb pistons move freely and simultaneously. Dashpot oil is topped up. Jets do fully return.

I do know it has been running rich for a year, and to that end I weakened the mixture 2 flats when I first started looking into this problem.

Not tried pulling the carbs/manifold around though. Will give that a go.

Is the fact it seems to switch between cylinders 2 & 4 of any significance to anyone??
G Hawkins

It's erratic enough to be distributor. So I would pull the 25D and look at the integrity of the sliding baseplate. When the baseplate rivet wears it can allow the contact breaker plate to tip and oscillate,leading to random firing. 4 and 2 are next to each other in firing terms. Also worth checking the distributor shaft for bearing play and end float, as well as the fit of the rotor arm.
F Pollock

Just to update.

This morniing it was still missing on 4, as I left it yesterday.

Pulling manifolds and carbs about made no difference to idling.

No 4 was the only cylinder running a new plug now, so I swapped it for one of the original ones. Immediately started firing again!

Left it idling whilst I further investigated the carbs. Adjusted the choke linkages as I noticed that the front jet started moving a while after the rear one. Checked carb balance, still spot on. Refitted air filters.

All this time the car was idling fine, about an hour in total, on and off choke.

So maybe its sorted ??? Although I'm pesimistic (sp?)

Might have to put that plug back in to see if the misfire comes back, but if it doesn't I'm likely to cry!!!

Hopefully when I get a chance for a road test the sun will still be shining.
G Hawkins

Out of interest I've just done a test.

Using the apparently problematic No.4 cylinder as a test.

Old plug still in, starts and runs fine.

Swap the no.4 plug with each of the "new" plugs in turn. 3 fail to fire, 1 fires intermittantly. All tested using the timing light.

Old one back in and it's firing again.

So dodgy batch of plugs or an enormous coincidence.

Old and new plugs are both NGK.
G Hawkins

wow. What a frustrating episode, but I am glad that you figured it out.

Bad plug, I wonder what the odds are on getting one of those? I bet small, but (evidently) possible.

Maybe it had been dropped at some point (manufacturing, shipping, handling, etc.) so that the insulator cracked or something, leading to the intermittent failure.


Systematic problem solving wins the day!


Norm
Norm Kerr

Sounds like breakdown internally in the plug?

Is the plug gap larger than the one that works?

A
Anthony Cutler

Just to update this, as I find nothing more perplexing than searching in the archives and not finding whether a fix worked or not.

The weather and my free time finally coincided at the weekend and I drove for a very pleasant 40 miles or so with no hiccups, misfiring or any of the previous symptoms. So hopefully problem sorted.

And just to reiterate, 3 out of 4 of a brand new set of plugs would not fire (or fired intermittantly) when tested. Gaps were the same. Sounds very odd to me but there you go. I wonder if the box had been dropped in store or something, damaging them??

So basically I believe I had 2 different problems. Some bad wiring causing the initial problem, then a bad set of plugs which introduced a different problem which I incorrectly assumed to be the same one .. Next time I will stick to doing one thing at a time and resist fitting new plugs as "it cannot hurt"....
G Hawkins

Glad to hear you've solved it! I agree, that one was a real head-scratcher. I remember having a similar issue with my condenser as you have with spark plugs... meaning that it was the last thing I checked, because what could go wrong with a condenser? Ha... now I know. And apparently plugs are similarly susceptible to gremlins.

Happy motoring,

-:G:-
Gryf Ketcherside

All at one time in the chored of G

"Thank you china"

Prop
Prop

Hi

I had a similar problem to yours with a 1966 Humber Hawk.

Upon pulling ht leads whilst the engine was running, the misfire was changing from cylinder to cylinder, after changing plugs, leads, rotor and condensor all that was left was the distributor cap, i went and bought one, fitted it- the problem vanished...

under inspection with a magnifying glass i found a hairline crack running across from the lug where the clip fits to very nearly the centre carbon brush....

John :-)
j b biggs

It would be interesting to put one of the dud plugs back in and run it in the dark just out of interest to see if the spark is tracking down the outside of the insulator or wether it is a breakdown inside the plug itself. You mention that the pluggaps were the same, but what are you setting them at ---- Willy
WilliamRevit

This thread was discussed between 23/04/2010 and 29/05/2010

MG Midget and Sprite Technical index

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