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DB18-1 big backfire, now sounds like a tractor!

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Frankie&Ollie
Posts: 7
Joined: Mon Mar 25, 2019 7:21 am
Location: Poole, Dorset. UK

DB18-1 big backfire, now sounds like a tractor!

Post by Frankie&Ollie » Sun Jul 28, 2019 7:51 pm

Hi all,
we had issues with the fuel reserve cut-over which has now been resolved.

Drove car about 10 miles yesterday, ran perfectly OK.
Today, started car and immediately didn't feel right, engine hesitating etc. After about 1/4 mile really loud backfire.
Turned off ignition and hopped out (just in case!). Then tried to restart car, but as if no fuel (again).
Left for about 10 minutes then tried again, started but sounds as though exhaust has blown!

Any ideas?
(Another vintage car owner we saw today suggested it might be the condenser)

Thanks as always

Sydsmith
Posts: 854
Joined: Sun Feb 14, 2016 11:15 pm
Location: Aberystwyth Wales

Re: DB18-1 big backfire, now sounds like a tractor!

Post by Sydsmith » Sun Jul 28, 2019 9:52 pm

Frankie&Ollie, most back fires are caused by raw fuel entering the exhaust system and being ignited by heat, usually as an exhaust valve opens and the hot gasses ignite raw unspent fuel in the manifold silencer or exhaust pipe.

Unspent fuel usually finds it way into the exhaust system because a plug (or plugs) has not fired when it should have done and the unspent fuel is then forced into the exhaust.

In your case the back fire (explosion) you heard was almost certainly somewhere in the exhaust and probably the silencer baffles have been blown out of place or a hole has been blown in the exhaust system somewhere.

As to the cause, in my view it is very unlikely to be the condenser, it could be the contacts, coil or an intermittent connection in the 12 volt feed to the coil and ignition system.

Backfires can also often be caused by a timing issue, (too far advanced), or fuel starvation caused by carburettor problems.

You will need to repair the exhaust system, but before you do make sure you find the fault or you will have another exhaust destroyed.

Best of luck Syd

thomas@prags.co.uk
Posts: 24
Joined: Tue Sep 11, 2018 11:37 am
Location: Inverness

Re: DB18-1 big backfire, now sounds like a tractor!

Post by thomas@prags.co.uk » Sun Jul 28, 2019 10:27 pm

Had a similar issue - and got some good advice on checking every bit of ignition etc, carb and air intake... It looks as if in my case it was a fuel feed problem caused by (and thanks to Derek Sleigh who suggested checking this) ... the fuel reserve 'tap' which rarely gets used - so the corks can dry out or stick a bit. I pulled it out and it ran fine - then exercised it a bit and the problem seems to be cured! Weird.

grahamemmett
Posts: 722
Joined: Mon Feb 08, 2016 8:12 pm
Location: Northwich, Cheshire
Contact:

Re: DB18-1 big backfire, now sounds like a tractor!

Post by grahamemmett » Sun Jul 28, 2019 11:21 pm

Check the plugs and see if one isn’t firing, maybe oiled up. Then this is the culprit allowing unburnt fuel into the exhaust. Ive been suffering from number 2 oiling up on idle and it was cured by enriching the mixture. Odd but like Tom above sometimes it’s jut not obvious.
My old exhaust was made from a patchwork of bits and bobs at the local exhaust centre but now fitted a stainless steel one fro PD Gough of Nottingham. .
Graham Emmett
Northwich, Cheshire
Joint DB18 Registrar (with Marcel Renshaw)
DB18 1949 LCV522 (Yes that one with the P100s)
https://www.db18.org

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