Cause of backfire or popping back.

[AustinWood]

We checked our manifold and downpipe joints but they are tight and there is no sign of leaks.
The new silencers from Seven Workshop have the inlet the same diameter as the downpipe. I have fitted a sleeve over the joint but it is not very satisfactory and I shall try to improve it today.

[David Stepney]

Jamie told me of this problem and supplied a suitable sleeve. I used exhaust paste (as I always do) when assembling the system.

[AustinWood]

I have remade the joint between downpipe and silencer and had a short test run. The popping back has gone but it did manage one backfire on the overrun. That is probably a leak close to the engine though I can see no signs of one.

[Peter Naulls]

I spent a while last night cleaning and re-cementing the join between the downpipe and silencer. There was no significant evidence of a leak but it'll be another possible cause ruled out. I'll test it this evening; I have an old school friend visiting who has just bought (but not yet collected) his first Seven. I've a feeling he's going to expect a driving lesson.

[AustinWood]

The new joint between downpipe and silencer is a big improvement. The car is going much better and is quieter. It tends to backfire still on the overrun. Usually when slowing between 45 and 40mph. I didn't expect a performance improvement but it is now much better when the engine is cold. It always starts easily but wouldn't stay ticking over when closing the garage doors with the engine cold. Now ticks over well with a slight throttle opening. I also found I was doing 50mph through the 40 speed limit section approaching Melrose.

["Slack Alice" Simon]

It probably has no relevance at all ....

I find that if I want to attract the attention of an acquaintance whilst driving, an alternative to a beep on the horn is to lift off the accelerator and put on full retard, simultaneously.

If the starting engine revs are high enough, an amusing series of exhaust pops can be produced.

Well, they amuse me, anyway .... 

[Peter Naulls]

Mine is still troublesome, if anything getting worse. I'm really struggling to know what to try next but thinking I should probably disconnect the petrol pipe at the pump and see what sort of flow rate I'm getting from the tank.

On the plus side my friend enjoyed his first ever drive of an Austin Seven and adapted to it very quickly. The poor brakes were the only thing that really phased him.

[AustinWood]

I had 1948 Daimler DB 18 that had been owned by the foreman of the model shop at Shelvoke and Drewry who used to built dustbin trucks. He's made a formidable exhaust of immense strength. If the ignition was switched of while driving then turned back on again an impressive jet of flame shot out of the exhaust from all the unburnt fuel. Very effective once when being tailgated by an E-Type Jag. He kept a safe distance after that.

[Ian Williams]

Have you checked Ignition timing and advance curve? If so how? Retarded ignition can cause symptoms described especially if you have unreliable advance mechanism or inaccurate static timing brought about through general wear in the system.

[AustinWood]

The distributer is a newish Accuspark fitted with Powerspark electronics and works well. I use the manual adv/retard to set it by ear which is very easily done using the engine note. The car runs very well and revs freely under power up to 40mph in the high Andes 2nd gear, so I very much doubt it's retarded.
I check the advance is working with a timing light powered from an external 12V battery on a mark on the fan pulley. It advances and retards smoothly with revs.