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MG TD TF 1500 - Engine Backfire

This year the car ran fine but developed uneven firing? or lurching. I changed the plugs but no difference. Then it started to backfire when trying ti accelerate. Some days it idles fine, other days it idles at half the speed and continuosly backfires. In both cases the back fire continues regardless of engine speed. Next I changed the distributor cap and installed the red rotor. No condenser, this is electronic ignition. I swapped a new coil and still the problem persists. Looking at the old plugs #1 was lite colored but #2 was good! #3 was also fine but #4 had an orange tint on the back side and was light colored. I also leaned the carburators but still backfires. Finally the timing is now set to around 5 degrees BTDC. Still backfiring. Ideas? The distributor does have a little play, you can twist it 2_3 degrees either way. It just amazes me on day it'll idle fine and the next badly. Oh I did check the float bowls, plenty of gas there, I just replaced the needle float valves and rebuilt the pump last year.
RJD Dombrowswki

Plug wires?
(Your description is consistent with resistance wires failing)
J Stone

When you say backfires. Is that out the carbs or out the exhaust?
W A Chasser

Check that the carburettor spindle clamps are tight and that the carbs open together.
Ray TF 2884
Ray Lee

When you say you can twist the dist a couple or degrees either way do you mean the shaft or the dist. body itself. If it's the body of the dist., maybe it's not getting a good earth
William Revit

At one point when the engine was idling smoothly, I placed my timing light on each spark plug wire and the light flashed consistently, nothing unusual. It's the distributor shaft that has a slight wiggle, the housing is secure.
RJD Dombrowswki

Interesting comment about plug wires. One time when it decided to idle smoothly, I pressed on each of the plug wires at the plug and on the distributor. I also did the coil wire and while pressing on one of the distributor wires, wish I could remember but I think coil, I got a shock. Could there be arcing?
RJD Dombrowswki

Check that your carb floats are floating not treading water. I had an intermittent rough running that led back to a float that was about 1/4 full of fuel.

Stuart
S Grimm

RJD
Warning--If you're going to poke around with the plug leads with the engine running get yourself some plastic pliers to use--The sparks hiding in there can cause you serious permanent damage----------
William Revit

If you pull the choke out and it still backfires, that would seem to indicate ignition rather than fuel. However, Check for induction air leaks. I have heard many of the manifold gaskets now fail. Start car, and spray carb cleaner around the manifold to head junctions, as well as carb to manifold. There should be no change to engine speed or smoothness if no leaks. Check the float level and floats as above. While doing that, fish the floats out and look for water in the bottom of the float bowls. This can happen with modern ethanol containing fuels. If all fuel seems to be good, suspect the plug wires or the EFI. Also, if not done already, try new plugs, forget about cleaning them. George
George Butz III

Today I replaced the cobalt plug wires and coil wire with the old set. It stated right up but idled poorly, tach was jumping 600_800 and stumbling. As the shaft had some slack, I'm going to order new bushings but it seems the TDs could have different distributors. So tomorrow I'll pull it to see which I have. A wobbling shaft might explain why the first distributor cap showed signs of wear and a black mark on the terminal for plug #1 , #3 no black but signs of pitting, #4 no sign of any wear and #2 seemed ok.
RJD Dombrowswki

"No condenser, this is electronic ignition."

If your electronic ignition if pertronix or some similar type, there are different magnetic collars to fit the different TD distributor shafts: Hi-lift, asymmetrical and the other one. When I tried to install a Pertronix for the wrong shaft I cracked the plastic. If you can, you might try temporarily going back to points and condenser to see if the problem is with the electronic trigger.

Unrelated to the foregoing, my stock distributor self destructed and I replaced it with one of Len's CSI unites. So far it's been doing great.

Jud
J K Chapin

Many decades ago my TD developed a miss and engine stumble at around 2800-3200 RPM. Idle was OK. It was caused by a wobbling distributor cam so would not hold a dwell. The shaft and bushes in the distributor body had been redone but still had the wobble. Folks at the time said just convert to an electronic ignition so I did, fitting a Crane Cams unit as I recall. It has a slotted disk in the distributor and and the slots trigger the firing as I understand it. Been running fine for 30+ years since then.
John Quilter (TD8986)

The dizzy could be worn, but most likely not the cause of your problem. Check for intake leaks first. And as suggested above, switch back to points to rule out a defective electronic ignition (which I mistakenly called EFI above). George
George Butz III

Well, the saga is over. I must have had a senior moment when I changed out the distributor cap as plug wires for #2 and #3 were reversed. I did take the car out for a short ride today and it ran really well. It definitely needed new plugs distributor cap and rotor. 1 3 4 2 and counter clockwise distributor rotation are forever embedded in my mind. Thanks to all those who took the time to respond.
RJD Dombrowswki

You're not the first or the only one to make that mistake. Ask me how I know. Jud
J K Chapin

This thread was discussed between 24/05/2022 and 05/06/2022

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