MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Fires but won't run

My 74.5 B with Weber side draft ran prior to an engine rebuild in 2011. When trying to start the engine after the rebuild it ran fine but would not start the following day and has never run since. After setting it aside for about 2 years I tried it again today and it acted the same. It fires every time but then immediately shuts off. I assume it is getting starved for gas once it starts but I'm little more than a novice so would like some ideas where to look. I've had the carb apart looking for stuck needles, dirt or anything else I could see but have not seen anything suspicious.
Tom Gillett

First thing check the pump is delivering. Take the fuel pipe off the carb, stick it in a container and get someone to turn the key. After a prolonged period of inactivity SU pumps often need waking up.
Allan Reeling

Hi Tom,

It should run on the fuel in the float chamber for a while at least.

Is this a rubber bumper car with the cold start system on it?


Are you sure the the coil feed is ok when you stop cranking or is the spark disappearing? In which case it sounds like the ballast resistor wire in the wiring harness has failed.


Try running a wire from the battery to the + side of the coil and see if it fires up then.


Regards steve
SR Smith 1

Firing-up but cutting out as soon as you release the key is a symptom of faulty ignition wiring of the rubber bumper (i.e. 74.5 on) ballasted ignition system.

In that case the main feed to the coil through the ballast resistance is missing, it only gets the 'bypass' feed direct from the solenoid while cranking. As soon as the key is released that is lost. Check for voltage on the coil +ve with the ignition on but not cranking. It depends on what ignition system you have - the 74.5 originally had points, electronic ignition was only fitted during 1975 - you should see 12v with the points open or 6v with them closed.
Paul Hunt

Snap. But other than doing a quick test to see if that is the problem don't leave it with a wire from a 12v source (like the white at the fusebox) or the coil will overheat.
Paul Hunt

The quickest way to get you out of trouble is to connect a lead from the white fuse as Paul suggested, & install a 12 Volt coil. You can then decide at leisure whether to reinstall the ballast system or not. Barrie E
B Egerton

All,

Thanks for the help, you solved my problem but not all my questions! Turns out, there was no wire from the fusebox to the coil+ through the ballast resistor. Although I do have white wires to the 3rd fuse and also white/Green stripe wires coming from the coil+ they weren't connected. From other postings I was able to determine that I actually have a 12V coil so I added a temporary jumper from fuse 3 to coil+ and she started right up and ran.

However, I think I did originally have a 6V-Ballasted system becuase of the presense of the white/Green stripe wires. There are two. One goes to the starter area as shown in the 1975 schematic. Here is the question. The schematic also shows the circuit with the white wire from fuse 3 connected to the White/Green stripe wire through the resistor. In my case though, the 2nd white/Green stripe wire goes into the wiring harness, passes the passenger's headlight and comes out near the driver's headlight from the harness. The end of this wire has the end of a spot-welded wire attached to it but nothing else. I'm guessing this once connected to the ballast wire but there is no sign of it. Can anyone tell my why this wire would route to the driver's headlight area and what is the actual physical makeup of the resistor? Was it just a regular reistor or a resistive wire or what?
Tom Gillett

It was a resistive wire, pink in colour, probably steel cored which was routed with the front cross loom just to gain length and therefore add the required resistance. Would be surprised if it had been removed. The white with green was the 12v feed to coil from the started solenoid , only during cranking. The resistive wire ended in white and fed 6/7v to the coil for running. The second fuse UP is the "live with ignition on" fuse, fed from the ignition switch.
Allan Reeling

OK, thanks again. I swear, I would give up this hobby if it weren't for this website! So, is there any harm in just not replacing the resistor and continuing to use my 12V coil?
Tom Gillett

No harm at all, but if you suffer a lot from arctic vortexes. bear in mind the following! It was a mod done for cold climate exports, which just got used on everything. The idea is that when battery voltage is down due to low temps, (can be as low as 7v) and the starter is working hard to churn the engine, the coil would still get it's full whack when cranking, then when running continue on 6v.
Allan Reeling

As the end of the green/white is visible by the headlight it seems that someone has dug it out, maybe in an abortive attempt to fix an earlier failure in the ballast circuit.

It would have been originally a ballasted system, hence using a 6v 1.5 ohms coil, but as you seem to have a 12v coil (which should measure about 2.5 to 3 ohms) it also seems that was the fix decided upon, which would have needed a wire from the white at the fusebox to the coil +ve, but that got lost during the rebuild.
Paul Hunt

This thread was discussed between 09/02/2014 and 14/02/2014

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.