Just in time

[Bones]

About a month ago I fitted a 'pop off valve' in my 1978 SC's airbox . I think it's just as well I did , a couple of days ago I 'bagged up ' my engine and degreased and pressure washed it . Upon restarting the engine a loud backfire occured , which has never happened before , the car then fired up as normal . I haven't checked the condition of the valve yet as the car performed normally so I suspect all is well , but if I hadn't fitted the valve then I don't think the air box would have stood up to such a backfire .
On reflection I think the bag I placed over the air box intake allowed petrol vapour to build up in there and ignite on restart .
So I think the lesson is if you don't have a pop off valve fitted and you block off the air intake to clean the engine remove the air filter for a few minutes to allow the air box to expell the extra vapours that have built up. (even if you do it's still a good idea to remove the filter as the backfire was quite severe and could potentially loosen the valve).

[Jim Sims]

I would propose that one or more of your car's spark plug wires were wet and misfired (fired at the wrong time) and thereby caused the backfire. Anyway, you were wise to install the "pop off" valve. Cheers, Jim

[Bones]

Yeh that's a possibility too Jim . I came to the conclusion it was unburnt vapour because it was logical to have a build up there after I had run the car to a garage and the car started immeadiately after the first misfire without a splutter? Do you think its's possible for a misfire to be caused by the spark plug supressor leads being wet (I have the original Porsche braided steel type wires) they took a dowsing .