Classics Monthly - - Q & As -

I have re­cently had a strange oc­cur­rence whilst out in my 1979 Tri­umph TR7. I was in fourth gear whilst de­scend­ing a fairly steep gra­di­ent on one of our lo­cal roads when I was shocked to hear a loud bang, fol­lowed by the sound of a badly blow­ing ex­haust.

I had the ex­haust re­placed and I’m now con­cerned in case this prob­lem re- oc­curs caus­ing me to have to pay out again to re­place a per­fectly ser­vice­able ex­haust sys­tem. Andy Turner The symp­tom you have de­scribed is that of the com­bus­tion gases ig­nit­ing in the ex­haust sys­tem. This nor­mally oc­curs when the fuel air mix­ture has not been ig­nited within the en­gine cylin­der and has been al­lowed to flow out on the ex­haust stroke and into the ex­haust pipe.

The cause can be a fail­ure in the ig­ni­tion sys­tem, such as the point or con­denser but on the TR7 one of the more com­mon prob­lems was the ig­ni­tion switch fail­ing. The con­tacts can make and break caus­ing the loss of ig­ni­tion for a short while. As you were de­scend­ing a gra­di­ent, it is likely that you wouldn’t have no­ticed the prob­lem un­til the con­tact re-made re­sult­ing in the un­burnt mix­ture in the ex­haust be­ing ig­nited by the hot ex­haust gases from the re-ac­ti­vated en­gine. I would first check the ig­ni­tion switch and test to see if wig­gling the ig­ni­tion key will cause the en­gine to cut out. If you can find no de­fects with the ig­ni­tion sys­tem I would rec­om­mend you check through the con­nec­tions to and from the coil and con­tact points.

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