Any old car experts here?

[davidimurray]

Hello

Just wondering if we had any old car experts on here - in particular MGB / B series engine ones.

I share my garage with a mate who has a 1970 MGBGT. About a month ago he started having an intermittent misfire. One evening he took it out for a run and it just died completely on him. He managed to get it going and limped back. It's running really rough but intermittently. So we went through checking the various bits. Result is it would seem that it has an intermittent weak spark. So changed coil, dizzy cap, rotor arm, plugs etc. It has a magnetronic point replacement system fitted so it shouldn't be a problem.No improvement. Checked the supply to coil and it seems to be getting a good feed and also checked the engine earth strap. Still no improvement.

As a long shot I got the battery from my donor sierra out. Connected the +ve to the coil and -ve to a stud on the block so only the ignition circuit was running on that battery. Car fired up instantly - better than it ever has and ran like a dream. Checked the MGs batteries over and they are all good - less than 2 years old and always kept on a maintenance charger. Checked the wiring on the ign switch for any dodgy contacts and all seems good. Even tried running a wire direct from battery to coil and that didn't help. Which is making me think it is a wiring fault somewhere - but god knows where! Got a set of points on order just to make sure it is no the Magnetronic unit.

Anybody got any thoughts on where the problem could lie? When the car does start it still runs rough so I don't think it is the starter. At the moment I am contemplating building a temp loom for it to bypass everything. My poor mate is starting to tear his hair out as he has missed lots of nice weather and now it has run out of MOT.

Any help would be much appreciated.

Cheers

Dave

[Twin]

Have you elliminated the ignition switch?

[twinturbo]

What if you replace the MG's battery with the sierra one.

TT

[deezee]

I'd agree about the wiring, so I'd start by checking the earth points to the chassis. If you've ruled out the +ve connection to the coil, that just leaves the earth from the sparkplugs, back to the battery.

[frankie boy]

Hello

Just wondering if we had any old car experts on here - in particular MGB / B series engine ones.

I share my garage with a mate who has a 1970 MGBGT. About a month ago he started having an intermittent misfire. One evening he took it out for a run and it just died completely on him. He managed to get it going and limped back. It's running really rough but intermittently. So we went through checking the various bits. Result is it would seem that it has an intermittent weak spark. So changed coil, dizzy cap, rotor arm, plugs etc. It has a magnetronic point replacement system fitted so it shouldn't be a problem.No improvement. Checked the supply to coil and it seems to be getting a good feed and also checked the engine earth strap. Still no improvement.

As a long shot I got the battery from my donor sierra out. Connected the +ve to the coil and -ve to a stud on the block so only the ignition circuit was running on that battery. Car fired up instantly - better than it ever has and ran like a dream. Checked the MGs batteries over and they are all good - less than 2 years old and always kept on a maintenance charger. Checked the wiring on the ign switch for any dodgy contacts and all seems good. Even tried running a wire direct from battery to coil and that didn't help. Which is making me think it is a wiring fault somewhere - but god knows where! Got a set of points on order just to make sure it is no the Magnetronic unit.

Anybody got any thoughts on where the problem could lie? When the car does start it still runs rough so I don't think it is the starter. At the moment I am contemplating building a temp loom for it to bypass everything. My poor mate is starting to tear his hair out as he has missed lots of nice weather and now it has run out of MOT.

Any help would be much appreciated.

Cheers

Dave

HI Have you checked if there is play in the in the rotor?

[twinturbo]

How was the sierra battery connected?

[Davey]

Double check your earth connections, both the engine earth lead and the battery earth leads, remove each one individually and clean the contact face with emery paper (both the lead end and the chassis surface, remake the connection then coat with vaseline.

I grew up working on A and B series BMC/BL motors:D . I'd replace the points sytem with an Optronic one to remove points wear/burning from future problems or even go over to megajolt with a trigger wheel.

D.

[twinturbo]

It's points free already.

TT

[Davey]

My apologies, I missed that detail:o .

D.

[davidimurray]

Hi Guys

Thanks for having a think about this.

Yes the car currently has electronic ignition.

We've run a wire direct from the MG battery to the coil, bypassing the ignition switch completely and still behaved the same.

We've also run an earth wire direct from battery to a stud on the engine

Not tried the Sierra battery on the whole car as the terminals are different and MG cables won't reach. Might have to get the jump leads out and try this.

When we tried the sierra battery we disconnected the car +ve supply to the coil, replaced the +ve feed with the one from the sierra battery. Then put a cable from the negative on the sierra battery to a stud on the engine. Ran like a dream.

Might have to get an oscilloscope on the coil +ve supply to see if it is smooth and steady.

[twinturbo]

I would definatly try the sierra battery in place of the MG ones , as you say use jump leads if need be.

Also check the voltage at the coil during crank ( disconect the HT lead ) and running.

TT

[flyerncle]

Does it have the old fuel pump with a set of points in it,if so they give trouble and stick.

[davidimurray]

Hi guys

Thanks for the ideas. Will try the sierra battery and get into looking into more voltages at the weekend. I stripped all the earth connections back (apart from the engine end of the engine strap as it requires you to lift the engine up off the mounts!) cleaned them back to bare metal and reasembled them all with some copperslip. Couldn't test run the car as my mate has got some of the dizzy bits at home.

Yes the car does have the original points fuel pump but I can hear it running and also we have put a set of the 'spark indicator' lights on all the plugs and when it runs badly you can see the lights going dim or not firing at all.

[K4KEV]

just my 2p worth.....it sounds like there is either a leakage of current on the +ve side of the coil or possible resistance (maybe corrosion) could be anywhere along that track, you say it ran perfect with a battery straight to the coil and earth to the stud, this sounds like the earth side of things is ok....things I would check would be internals of the ignition switch/barrel itself and its live feed. any relays, fuses for slight corrosion or water ingress to the loom. hope this is of any help.
rgds
Kevin