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 - mgb backfiring problem

Having correctly set the timing on my 1974 mgb roadster over the weekend, I set out for work this morning. All was well until after about 5 miles when the engine had warmed up nicely, the car started to backfire regularly and persistently.
When the backfiring happens under light load, the tacho also drops right down to zero. This leads me to believe that it is an ignition problem but not one that happens all of the time.
With the tach also dropping off could this be a faulty coil?

Has anyone else had this happen?
Thanks in advance for your replies
Dave
dave

When you set the timing, did you alter anything? if you adjusted the dizzy, did you tighten the pinch bolt back up? it may have slipped as your journey progressed.
Tatty

Yes I checked the dizzy and even had a quick look at the timing again when I got to work. Luckily I still had the timing light in the car. Also I had a quick check of the leads dizzy cap etc and all seemed fine. An hour ago I tried the car again and had the same problem. Runs fine to start then when warm 'bang bang'and funny looks! The car runs great from startup and idles and revs very well.

Dave
dave

Did you put the light on when it was backfiring, did it show a good spark still, did you change the setting of the timing back to what it was last week if you changed it.
Tatty

I have tried changing the timing back although it wasn't changed more than a degree or two. This is to no avail and the symptoms do not occur when the vehicle is stationary. Personally I have a feeling that it is coincidental that this happened following timing changes. I think that the fact that the rev counter drops right off leads me to believe that power out of the coil may be intermittently stopping?
dave

If its only happening under load, then check the condensor, and its attachment within the dizzy, see if it is touching ground.
Tatty

Thanks, I will have a look this afternoon and let you know.
Dave
dave

Dave. In addition to the ideas of Tatty, also check the wire which grounds the movable base plate, the thing the points and condensor are attached to, to the side of the distributor. The dropping of the tach indicates some form on intermitant circuit with the tach dropping when the circuit is broken. If the distributor is secured to the engine by the clamp properly, it is mostly wires and connections which could cause problems (at least ones that would cause the tach to drop). Since you say it only happens when you are driving, this might narrow it down to the ground wire, points, condensor and the wire running from the coil to the points. All of these shift position slightly when you are driving because of the vacuum advance, which moves the base plate. (Centrifugal advance moves the point cam in relation to the base plate, while the vacuum advance moves the base plate in relation to the points cam. A quick test to see if the problem is coming from movement of the base plate might be to disconnect and plug the line to the vacuum advance, then drive the car to see what happens. However, do that after checking to see all the connections are tight.) Please let us know what you find. Les
Les Bengtson

My money is on the wires leading to the coil - this problem is common and generally caused by a broken wire near one of the spade terminals.
Chris Betson

Anything in the ignition LT circuit will cause the tach to drop off and the engine to cut. The first thing I'd do is check the dwell, very closed-up points can cause backfiring particularly when accelerating. Other than that it is out with the voltmeter, and if you can't reproduce the problem at a standstill position it where you can see it when driving, and connect it to coil +ve and -ve and that should tell you whether you whether you are losing the ground (open circuit to the distributor), getting a permanent ground (short to/in the distributor), losing the 12v to the coil (open circuit ignition feed) or through the coil (bad coil).
Paul Hunt

thanks for the replies, I had a look again last night and re-made the spade connectors to the coil as per your suggestions and that appears to have fixed the problem.

Until next time, thanks
Dave
dave

Dave. Thank you for taking the time to tell us what you found to be the problem. Many do not. When this happens, people searching the archives find a lot of suggestions, but do not get any idea of what was done to correct the problem. If everyone were so kind as to post what they found, we could begin to get a better idea of what the most common problems are and could focus on checking that first before going on. Your kindness is appreciated. Good luck. Les
Les Bengtson

As is posting the answer to the original thread and not creating a new one ...
Paul Hunt

This thread was discussed between 20/01/2003 and 23/01/2003

MG MGB Technical index

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