Can anyone please tell me all the reasons why my engine backfires. MGB 1800, HC. Was idling badly, tuned carburettors, Timing and ignition etc.
Runs fine untill one revs up. Sharp back fire in the exhaus system and sometimes out of the caburettors.
Help would be appreciated.

Hi Hugo.

I'm sure you know all the usual reasons for backfiring - weak mixture and too-advanced timing. It can also be caused by the valve timing being badly out.

But, you've checked all these again and again?!

Might be worth having a look inside the distributer cap for unwanted 'tracks'. It's possible for the spark to travel to the wrong lead point if moisture has gotten into the cap.

If it ticks over fine and only back-fires when you accelerate, could it be something to do with the advance mechanism - you didn't knock one of the 'bob'-weight springs off making it advance too quickly? Or has the vacuum pipe been refitted properly?

Chust thinking aloud!

Good luck.

Sounds to have all the trade marks of an air leak / weak mixture, check all the breather connections and vacuum connections around the carbs, recheck the needle jet heights.
HTH
Mark

Hugo,

I agree with Mark - most likey weak mixture when off idle due to air leaks. Having done all the checks you have done, i would head for the inlet manifold and check all the take offs (servo and advance/retard)and the plugs of unused ports for leaks. Don't overlook the push in plugs on the ends of the manifold too.

Happy Hunting,

Dave

Thanks all, for your advice. It is great to be a member of the "club"
I took the vacuum advance tube of thinking the mechanism might be stuck ( the car has not been run properly for many years) so I applied some sucktion, which changed the running of the engine and now she seems to run OK. I will see, she is officially on the road from tomorrow, so I will take her for a run.
Thank for your help.
A great team

Hugo