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MG MGB Technical - Car sputters and backfires under load 71 MGB

Just a question 71 mgb roadster starts fine idles great but under load sputters and back fires. I have changed and replaced every thing but the wiring harness. The only way I can get the car to run correctly is to run a wire direct from the battery to the coil. I have switched it on the console so I can shut the car off. My question is will I be doing any harm running this way the tach sure does not like it. I can turn the switch off once i am at crusing speeds but must engage it again when I come to a stop and run through the gears. Thanks Joe
Jh Clark

Try a wire from the white on the fusebox to the coil +ve (the one with the single white wire). If that fixes it as well then it looks like the circuit from the ignition switch through the tach (RVI I assume) is faulty in some way, maybe high resistance.

If just the same then try from the brown at the fusebox to the coil +ve, and if that works it looks like the ignition switch, which would be affecting all sorts of things.

It won't hurt anything, but it bypasses the tach so it won't register.
paulh4

Joe
Hi----You say you've replaced everything---
Just by chance, has the car been fitted with an external ballast resistor on the coil-
William Revit

Or maybe even a coil intended for a ballasted system without a ballast. That will generate a higher voltage which might be breaking down the HT.

The coil for a 71 should measure about 3 ohms between the spades and is used without a ballast resistance, a coil for a ballasted system about 1.5 ohms with the ballast resistance (contained within the harness) about the same.

That exclude North America cars with the 45DM distributor which use a very low resistance coil and no ballast.

Unfortunately the 'potential' for confusion is rife.
paulh4

I have checked the coil it checks out correct to specs
changed it anyways still same problem. I put in a new ignition switch not really to solve the problem but because I lost my keys some where between Ontario and Arizona over the winter still no change. Has a new dizzy, no change. Changed the tach no change except I do not have to tap the glass on the tac at start up to wake it up. I sent a new wire from the coil side of the tac to the coil no change. The only thing that worked was running a new wire direct from battery to the coil leaving everything else connected with a switch in between to break the connection in the wire. I just was not sure if I would be doing any harm to the system by hitting it with the higher voltage direct from the positive battery post to the coil.
Thanks Joe
Jh Clark

My thinking was that it sounds like it has a normal 3ohm coil on a balasted system---feeding it 12v to correct it backs that up
I ask again has it got an external ballast fitted-

"With the engine running" what voltage is there at the power in side of the coil
Non ballasted should be approx 12v----3 ohm coil
ballasted----around 7v-------1.5 ohm coil

Same as Paul says --but you need to know what you've got for a supply voltage with the engine running
William Revit

There is no ballast fitted and the coil is 3 Ohms I have not checked the voltage at the coil while running but will do that tomorrow. will check back when I have this completed. Thanks
Jh Clark

Is this points or electronic ignition of some kind? If electronic ignition where is the module picking up its 12v supply? If the coil +ve/white then move it to the white at the fusebox, although that is more usually done to correct tach problems not ignition problems.

As far as voltage goes on a CB you should have 12v minimum and in practice more like 14v on the coil +ve i.e. the white wire. If you see less than that then there is additional resistance in circuit somewhere, and as running a wire from the battery to the coil stops the problem then run one from the white and then the brown at the fusebox as suggested previously and that will help narrow down where voltage is being lost, if that is the case.

Whilst measuring at the white measure the other spade terminal as well. Should be around 8 volts, what does it show when putting some load on the engine to cause the misfire? Ideally use an analogue meter, digitals can give various results depending on their internal construction.
paulh4

Paul, it is points I tried your suggestion white on fuse panel no difference brown on fuse box works fine but that is pretty much the same as running a wire from the battery as it is always hot. This is not this cars maiden voyage I have owned it for 30 + years it only started acting up last fall and this spring. I never thought twice about jumping in and going a long distance but since the troubles I have been gun shy about going anywhere but in and around town. I just want to know if anyone can tell me if I will burn out my coil or do any other damage feeding it direct from the battery. Thanks
Jh Clark

It won't damage it as the most you can poke into it by doing what you're doing is battery voltage which is what it should be getting anyway----
The idea of measuring the voltage at the coil + terminal is to see if you have low voltage there-
If it is low (less than battery voltage with the engine running) the next step would be go back to the tacho and measure voltage in and out to check there--if it's still low there then back to the ignition switch----process of elimination
Paul knows all about this stuff so I'll leave you two to it----too many cooks
William Revit

William Thanks for your comment I was only worried about feeding the coil with charging voltage of close to 14 volts from the alternator. I carry a spare coil just in case something like that happens. I have started what you have described only in the opposite direction starting at the ignition which is new, the White wire that fed the coil was so brittle that when I removed it from the coil I ended up with the wire breaking off just where it entered the taped part of the harness . I have removed the tape on the whole harness under the hood and replaced what wires I could that I found brittle but am hesitating removing the dash to get at the main harness. I will press on. Thanks
Jh Clark

Joe--The charging voltage(close to 14v)or thereabouts, is what there should be present at the coil with the engine running-most everything in the car runs at that voltage, lights, horn , basically everything except the fuel gauge circuit-------------your coil is safe to run with the jumper wire, but there must be an issue there somewhere, That crumbly wiring doesn't sound too good-
If you have good voltage at the ign. switch maybe run a new wire from the ignition to the tacho and then tacho to coil+
cheers
willy
William Revit

If you don't get the problem when connected to the brown at the fusebox but do when it is connected to the white then the problem is between the brown and the white at the fusebox.

There are separate whites coming off the ignition switch to feed the coil via the tach and the fusebox so if both cause the problem then the cause must be before those connections at the ignition switch.

In a 71 the brown comes off the starter solenoid to a sealed junction behind the dash with three branches to the ignition switch, lighting switch and fusebox. If you don't get the problem with the coil connected to the brown at the fusebox then the fault must be after that sealed junction, i.e. the brown feed to the ignition switch or the switch itself.

Incidentally with the original instrument voltage stabiliser the fuel gauge _does_ run at charging voltage i.e. 14v. That stabiliser is a thermal device switching system voltage on and off so that the average is about 10v and is why the gauge is the slow-acting type. It's only the after-market electronic stabilisers that output a steady 10v or so.

paulh4

Does sound like lost voltage. In addition to sources already mentioned, and depending on how the connections are arranged, it could be the fusebox itself. The terminals are only riveted on/together and with age frequently develop resistance between them.

IIRC there is also an internal brown junction in the harness, if so that could be another poor contact point.

Volt meter is the key. You can also check with the engine off if the points are closed. 2V or more less at the coil + than battery voltage will be enough to give problems. If that is what you find then work back through the circuit ... Tacho whire wires out and in, ignition sw out and in, etc.
Paul Walbran

Power to the coil prior to 1973 model i.e. with the RVI tach doesn't come via the fusebox but via the tach direct off the ignition switch.

Because powering the coil from the white at the fusebox has the same problem as being powered via the tach, but not the brown, indicates to me that both white supplies i.e. tach/coil and fusebox are being affected by the problem, which puts it at the ignition switch, or in the supply to that switch from that sealed junction as mentioned previously. Having dismantled one of those sealed junction for interest it's very unlikely that would have failed, anything's possible, but I tend to think it's the switch itself or its connections.
paulh4

Thanks all for your comments / suggestions I will let you know if OI come up with a solution one wire at a time.
Jh Clark

This thread was discussed between 19/06/2023 and 27/06/2023

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