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MG TD TF 1500 - engine backfire

I know I have brought this up before, but I still have a problem.
When running downhill with the throttle closed I get a series of crackles and backfires in the exhaust.
The carb have been rebuilt, (rebushed, new spindles,throttle discs in the right way round and centred) inlet manifold vac tested for air leaks.
The car runs perfectly and gives good fuel consumption, having driven it for 35 years I thought I knew all it's faults, but apparently not.
Compression is around 150 psi all cylinders and and an air line check shows no valve problems.The ignition
timing is at 3 deg btdc static where it has been since I started using unleaded 12 years ago.
Any ideas

Ray Lee TF 2884
Ray Lee

I believe it is from running rich,,, and it is the unburned fuel is burning in the exhaust system,,,

Steve Wincze

That to me is perfectly it all the time... If you 'coast' down a hill, fuel is still pulled into the cylinders and mine 'pops' and 'crackles' in that situation.... We have a large hill outside of town and at 40 or 45 I coast down and love the sound! Car runs just fine on the flat or going up the hills....!

My TD started doing the exact same thing some years ago- it actually sounded pretty good at first. Turned out to be the coil wire had loosened and burned in the center of the distributor cap. George
George Butz

Hi Ray,I have found that this is usually caused by a leak somewhere in the exhaust system, probably where it joins onto the exhaust manifold. On a trailing throttle, air is sucked into the exhaust which then mixes with unburned fuel vapour to form a combustible mixture in the exhaust system which ignites and causes the popping.

Hope this helps,

Paul van Gool


I have a motorcycle that makes the same popping in the exhaust on deceloration. A cycle mechanic friend said it was caused by exactly what Paul from Queensland has described. If you can check the electrical condition as described by George Butz above, and find all is well, you might look for the air leak in the exhaust pipe. Apparently it does not have to be large to cause the popping or back firing.

George Raham
TD 4224

G. L. Raham

I have tried all new ignition parts and tested the exhaust system for leaks. The mixture is spot on going by my fuel consumption.My next trick is to switch off the ingnition while it is doing it and see what happens!
If nobody tells me I am going to wreck my engine, I will just carry on enjoying it.

Ray TF 2884
Ray Lee

....ummm... if you mean backing off, then turning your ignition off, then on, you, ah-umm, might want to hold off...although we did it as kids all the time!!!! Prepare for some 'loud' backfires cause all that fuel mixture you hear popping, saves up until you give it a spark...then... BANG!

Perfectly normal....noticed my Mazda did it yesterday going down a hill (have a tiny hole in my muffler)...!

Ah well, here's dating myself even better -- one of the greatest fun things with Model A Fords was the ability to readily control the spark advance at the steering wheel hub. Man! could you make those suckers backfire. Set the floorboards on fire once.
Bud Krueger

This thread was discussed between 03/07/2009 and 04/07/2009

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