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backfire

thomas@prags.co.uk
Posts: 22
Joined: Tue Sep 11, 2018 11:37 am
Location: Inverness

backfire

Post by thomas@prags.co.uk » Sat Jun 08, 2019 11:27 am

having had the rebuilt gearbox refitted a l was well - until my DB18 started scary big backfire noises! Mostly in top or 3rd while under a bit of pressure (inclines etc) it runs very smoothly otherwise and behaves in low gear at high revs .
- a friend checked the distributor timing and there dont seem to be any air inlet leaks. suggestions ? My usual helpful near neighbour is too busy to help just now and I have had to miss the peeblles rallly this weekend as a result....
thomas in Inverness

Phillmore
Posts: 847
Joined: Sat Feb 13, 2016 1:25 pm
Location: Worcestershire Herefordshire border

Re: backfire

Post by Phillmore » Sat Jun 08, 2019 12:15 pm

It could be down to many things. Backfires usually occur due to the fuel/air mixture igniting in the exhaust manifold. Causes could be insufficient spark allowing incomplete combustion in the combustion chamber - check all ignition components.

Or a sticking valve allowing mixture out - Do a compression check.

It could also be igniting the mixture as it comes in due to a sticking inlet valve - again a compression check should find this.

As you say ignition timing is crucial and apart from static settings the bob weights and vacuum advance will also influence timing - Check for sticking weights, weak or broken return springs, damaged vacuum pipes or diaphragm.

Has the valve timing been disturbed? Has the timing chain been off? Has the distributor been off?

Could it be down to intermittent fuel supply? - Check pump, filters, jets, crud in float bowl etc.

It sounds like a long check list so I would start with the easiest (plugs and points, and compression test) first.
Andy

1954 Conquest Mk1, 1956 Conquest Mk2, 1957 Conquest Century Mk2, 1955 Austin A90 Westminster

Chris_R
Posts: 442
Joined: Wed Feb 10, 2016 12:48 pm
Location: Twickenham

Re: backfire

Post by Chris_R » Sat Jun 08, 2019 11:06 pm

Go for the simplest thing first, replace the condenser.

thomas@prags.co.uk
Posts: 22
Joined: Tue Sep 11, 2018 11:37 am
Location: Inverness

Re: backfire

Post by thomas@prags.co.uk » Sun Jun 09, 2019 9:10 am

thanks for the advice. pretty sure the condenser is relatively new. I was wondering if there might be a connection between the gearbox surgery and something else. Probably not.
I do have a plug or two that oil up a bit and compression could be an issue.

t

Stan Thomas
Posts: 513
Joined: Sat Feb 13, 2016 2:14 pm
Location: Penkridge. Staffs.

Re: backfire

Post by Stan Thomas » Mon Jun 10, 2019 8:02 am

Hi Thomas (strange - a Welsh name in Scotland - although I did once know a man named Paddy McThomas. .......... but I digress).

Your problem is invariably the ignition system, with the spark breaking down under load.

Check all connections first.

Check the points are O.K and correctly gapped (better still, check with a tach dwell meter if you can borrow one)

Are the spark plugs relatively new? They can break down under load but its unlikely.

It could be the capacitor as suggested by Chris_R (proper name for the now defunct term "condenser"), but I doubt it. To check, remove it and connect it to a 12 volt supply with a volt meter across the terminals. Disconnect the power source and it should take a few seconds for the volt meter to return to zero as the capacitor discharges.

Next, check all the ignition leads. If you have copper cored ignition leads as original, they will be O.K. - but if you have later suppressor ignition leads (the central "wire" looks like black string) - these ultmately go high-resistance - and directly cause the problem you have. Check the run of the leads for possibility of chaffing which can lead to arching to earth under load.

If you have suppressor plug caps - suspect these as they too finally give up the ghost and cause misfires.

Check the coil itself by substituting a replacement. Coils do ultimately "go down" and loose their ability to produce sufficient H.T. discharge under load. A spark in "free air" (holding the disconnected H.T. lead near to earth whilst you crank the engine) is not a conclusive test.

Finally, although very very very unlikely, the rotor could be tracking to earth.
Last edited by Stan Thomas on Mon Jun 10, 2019 8:32 am, edited 1 time in total.

Phillmore
Posts: 847
Joined: Sat Feb 13, 2016 1:25 pm
Location: Worcestershire Herefordshire border

Re: backfire

Post by Phillmore » Mon Jun 10, 2019 8:13 am

There are certainly some duff rotor arms on the market. Get a decent one from the Distributor Doctor and rule that out.
Andy

1954 Conquest Mk1, 1956 Conquest Mk2, 1957 Conquest Century Mk2, 1955 Austin A90 Westminster

Stan Thomas
Posts: 513
Joined: Sat Feb 13, 2016 2:14 pm
Location: Penkridge. Staffs.

Re: backfire

Post by Stan Thomas » Mon Jun 10, 2019 8:29 am

Question:

Do you have to wait three weeks to see the Distributor Doctor - like you do if you ring our surgery in Penkridge?

Sydsmith
Posts: 790
Joined: Sun Feb 14, 2016 11:15 pm
Location: Aberystwyth Wales

Re: backfire

Post by Sydsmith » Mon Jun 10, 2019 8:52 am

Nice one Stan, just about covers it, except no one has mentioned weak fuel mixture, can also cause the problem especially if the timing is slightly too far advanced.

BTW, the Welsh way to spell Thomas is Tomos :)

thomas@prags.co.uk
Posts: 22
Joined: Tue Sep 11, 2018 11:37 am
Location: Inverness

Re: backfire

Post by thomas@prags.co.uk » Mon Jun 10, 2019 12:57 pm

thanks all - a few things to try. dont think its the coil - tested last year, the rotor arm was replaced via Derek Sleigh last year too.

Thomas not Scottish (except by adoption) or Welsh (though I spent much time there in my youth and one of our neighbouring farmers was Tom Thomas) - but German by extraction . Hence Thomas Prag.

grahamemmett
Posts: 609
Joined: Mon Feb 08, 2016 8:12 pm
Location: Northwich, Cheshire
Contact:

Re: backfire

Post by grahamemmett » Mon Jun 10, 2019 6:32 pm

Mine backfires when the ignition is too far advanced.
I'd also try a new rotor arm and condenser - for the small cost it's worth putting them in - but use Distributor Doctor's items only as there are just too many Chinese rubbish ones around.
I switched to electronic ignition and it's brilliant. All contained with the distributor cap - from Jolly Engineering. https://www.classicheads.com/Electronic-Ignition/
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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