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MG MGB Technical - Misfire/lights go out on lift off....

Bit of a copy of my post from MGOC so apologies if anyone has read it twice, but its quiet over there at the moment and i would like to get this sorted....

I have been having a problem where by my car has been backfiring on lift off. It would be fine on a more or less constant throttle (occasionally jittering but usually ok), but if i let off - especially immediately following a harsh throttle opening, the rev counter would go bonkers, occasionally the red light would go on and the car would electrically misfire/poor run/electrics cut out etc....

I am sure i have also noticed the overdrive coming out and back in again on occasion too....

On the advice of Paul Hunt i changed the points and condensor, but on a very small run out yesterday following these changes, i noticed on let off of the throttle, the lights both exterior and interior were cutting out. I did not notice the gauges though unfortuanately and i did not have the overdrive in to see if that was affected.

It almost seems like i am having a full electric *jitter* on lift off.....

Does anyone have any thoughts?? I'm pretty stumped to tell the truth. The small cloth covered earth wire in the dizzy (25D4) was slightly loose, and was pushed in its clip further to make better contact, but made little difference.

Perhaps its an earth thing...? Earth strap to engine appears tight though.... No sparks noticed jumping across from dizzy etc with the lights out last night....

TIA ~PHIL
Phil

Check the alternator and/or connections to it. Check also the engine earthing strap. You may find that it gets a tug if you drop the throttle quickly.

Just ideas?
Stuart Robson

Seen this with a loose battery cable at its connection to the starter motor. Worth checking.
Chris Cooper

Check the heavy brown wires coming from the starter to the alternator and the brown wire at the fuse box.
Chris Betson

Like Chris Cooper I say it has to be where the brown feeding the cars electrics joins the heavy battery cable and the alternator brown at the solenoid, and could easily be caused by the rocking of the engine as it changes from harsh acceleration to deceleration. Even though it was bolted up tight it happened on my roadster in the middle of an MOT, without any prior warning. Embarrasing - for Halfords as they couldn't diagnose what the problem was, which was probably why they allowed the car to remain on the lift while I drove down and fixed it, lent me there tools, let me use their washroom and gave me tea and biscuits (I'm joking about the tea and biscuits). Disconnect the battery ground cable then remove all the cables from the solenoid, clean them with emery to shiny bright, and reassemble with copper grease, then reconnect the battery ground cable. If you have the older type where the browns connect to spade terminals at the solenoid instead of the main battery cable stud make sure they are clean and tight.

Engine earth strap would affect cranking and ignition but not lights and things. Brown at the fusebox would affect interior light and horn but not much else. A problem in the alternator wire would affect charging but the battery would carry the load without you being aware of much except a faint flickering of lights. The red light indicates the alternator is still giving an output but you have lost the voltage on the white i.e. ignition. Electric gauges are slow-acting (unless a 64 or earlier) so would need a relatively long disconnection before you would notice anything.
Paul Hunt

Thank you!!

I will try tonight...

Paul i will copy this over to the MGOC page for their archives so you won't need to repeat it all (assuming you choose to answer there! ;-> )

~PHIL
Phil

Phil,
Not certain about home market cars, but North Americans(not all years) were fitted with a large
(@ 7/16") male spade connector at the starter B+ lug. Corrosion or a loose fit in its brown wire female spade connector can cause identical symptoms.
Mark Childers

Thanks a lot guys!

I *think* you were spot on with your analyses :->

Had a scrable under the car last night and the starter solenoid nut was tight and i could not move the ring connectors side to side.

Anyway i took them all apart and cleaned them up, but then noticed the thread of the solenoid lug was completely stripped where the nut was originally tightened (possibly done by me when fitting my headlamp relays... although that was some time ago...)

Luckily there were a few washers on the lug that i could remove and move the connectors back towards the body of the solenoid to gain purchase on the nut, which i did. Everything went back on tight, with me careful not to strip any threads this time.

The car is at a friends house at the moment while i am at work and he has taken it for a run this morning and said it has not been displaying the same symptoms as before, so hopefully we will have cracked it! :-> i have the confidence to use the car this weekend and should have covered a few hundred miles by sunday night, so I shall be able to confirm that the problem has either been solved or still exists then, but in the meantime it looks promising!

Thanks again! :->

~PHIL
Phil

Well, Phil made it about 100 miles down the road tonight and rang me to say his car had died. He thought it was the points gap had closed up. However, having reset them he then broke down a bit further on. Looked to be the same thing again. We changed the points and condenser the other night, but I thought I had tightened the plate up securely, but I didn't want to strip the thread. I gather he was planning to get a tow into Cardiff where he would get a fresh set of points and have it looked at by the local MG place. Otherwise, he said all the previous problems appear to have been resolved.
However, any comments would be appreciated.
I hope to see him at Donningtonon Sunday along with his mates in their GT and midget. I will be going down for the British Touring Car Championship penultimate round hoping to see MG bring home some more silverware. I will be parking my new MG along with about 40 other infield at Donnington Circuit.
TIA
Martin

Ah, the classic "don't think that when you have fixed one problem, you have fixed all the problems".

I'd want to have seen the tach drop to zero while the engine was still spinning, and everything else was as it should be; no flashing of a timing light when cranking; coil -ve with a permanent ground when cranking; and *still* look for something shorting out the points before I would believe points I had set 100 miles previously, let alone a few miles previously, had closed up.
Paul Hunt

Well it seems he has fitted new points but then discovered a dodgy earth on his petrol pump! So I await to see whether he makes it to Donnington tomorrow! Otherwise, all the previous faults related to the starter connection appear to be solved, now just to see whether his points go the distance.
Martin

paul

All fixed now. First off the previously erratic tach and misfire etc has completely gone, so i am well chuffed there...

With regards the points. I can't exactly remember the circumstances but the tach fell to zero with engine still initially turning (wierd sensation!) and the engine then grinding to a halt. No restart either.

First thought was points failure. Opened dizzy cap and they were closed, spinning the rotor made no change. I thought most likely the screw had slipped although it did seem tight.

I set them again (luckily having my feelers with me for once) but didn't get very much further, the car stopping with a misfire and a similar manner to before... When i set them again i couldn't get them to close properly and they wouldn't hold their gap.

The tow guy was a *proper* mechanic and had a fiddle (delighted to see a B and not that modern stuff! ;-> ) but could only get an intermittent contact at the points as well. So got a tow to my destination and picked up a new set the next morning. No troubles since.

As an aside once the points were changed i noticed the fuel pump not running and now am not sure whether this may have affected the previous nights poor running, but the symptoms didn't *feel* like running out of fuel - no kangarooing or loss of power under load. Think actually the tow guy may have knocked the earth wire (bit of a DPO bodge by all accounts by the looks of it) off the pump when he was unstrapping the car from the flat bed.

Have since done a further two hundred and something miles from friday nights problems and car now running fine, although i think the fuel pump minght need some TLC at some point....

Thanks for your help previous

~PHIL
Phil

This thread was discussed between 04/09/2003 and 07/09/2003

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