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Backfiring through carb
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Martin A



Joined: 06 Oct 2014
Posts: 17

PostPosted: Wed Nov 05, 2014 11:02 am    Post subject: Backfiring through carb Reply with quote

The problem: Car backfires through carb when throttle is opened under load.

I have a '54 Mk1 Ford Zephyr, bought recently from Sweden. Little known of its history - apparently well restored around 20 years ago and little used recently. Last Swedish road test dated 2010 so possibly not used since then (and not impossible the fuel in the tank is that old). There are one or two obvious problems that will need to be dealt with (eg the clutch slips under heavy load).

At first the car ran with lots of black smoke when idling, together with the backfiring problem. Replacing the knackered economiser diaphram in the carb cured that. Correct size jets (according to the manual) in the carb. No blocked passages in carb. Accelerator pump works correctly.

I've replaced nearly all ignition components (coil, condenser, rotor arm, plugs) and a strobe says the timing is right. Only the leads and the distributor cap remain to be changed.

The air cleaner (wire gauze/oil bath type) is clean.

The car starts easily and revs freely. At moderate speed under load (accelerator perhaps 50% open) the car will misfire, with noisy backfiring through the carb into the air cleaner.

I'd be very grateful for suggestions what I should check or try changing.
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kevin2306



Joined: 01 Jul 2013
Posts: 1359
Location: nr Llangollen, north wales

PostPosted: Wed Nov 05, 2014 11:31 am    Post subject: Reply with quote

Sounds to me like an over fueling issue, if its been standing I would be checking the carb operation..float heights etc.
Also, I would check the ignition side of things..My singer Vogue came with a Lucas electronic ignition system already fitted..had loads of apparent fuel related issues during the initial running after recommissioning, turned out to be the ignition system breaking down both at idle and under load, the cure for me was a replacement electronic distributor set up from accuspark, £58 delivered and work every penny.

Good Luck

kevin
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Bitumen Boy



Joined: 26 Jan 2012
Posts: 1733
Location: Above the snow line in old Monmouthshire

PostPosted: Wed Nov 05, 2014 12:21 pm    Post subject: Reply with quote

Could the spark be "tracking" on the inside of the distributor cap, perhaps? Sometimes you can see what's been going on under close examination, but worth trying another one anyway - and if it's not the source of your trouble a spare is always handy to have in stock.

Also could be worth checking over the vacuum advance unit on the diz (assuming it has one), they can give rise to odd symptoms sometimes. If it's been standing a while remove it from the diz and shake it to make sure there's nothing inside, and check it moves when you suck it - if it passes those tests it's probably OK.

Keep at it - these things can be frustrating but you'll get there in the end Smile
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PAUL BEAUMONT



Joined: 27 Nov 2007
Posts: 1281
Location: Barnsley S. Yorks

PostPosted: Wed Nov 05, 2014 1:43 pm    Post subject: Reply with quote

Worth checking the valves and valve action, weak valve springs etc. I have known everything to look ok on compression test but on higher revs the valves do not keep up fr some reason.
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47Jag



Joined: 26 Jun 2008
Posts: 1480
Location: Bothwell, Scotland

PostPosted: Wed Nov 05, 2014 2:13 pm    Post subject: Reply with quote

Martin,

Where are you checking the timing with the strobe? If it's on the front pulley/vibration damper then it is possible the rubber in the damper has sheared and the effect then is the timing marks are actually retarded. I use a technique that I call power timing where you hold the revs at about 2000 rpm and slowly advance the distributor, If the timing is retarded there will be an increase in the revs. When the 'improvement' stops, back off a bit and then road test to see if you get pinking (the rattle sound you get under load). Another cause could be an air leak in the carb/inlet manifold. If you have access to a propane torch, turn the gas on a little bit (unlit) and low it over all the intake joints. If the revs increase you've found a leak.

Art
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Peter_L



Joined: 10 Apr 2008
Posts: 2680
Location: New Brunswick. Canada.

PostPosted: Wed Nov 05, 2014 3:42 pm    Post subject: Reply with quote

Similar backfire can be caused by intake manifold leak.

We used to have a 1962 Ford Consul, that misfired and then, later, would backfire when switched off. There was a break in the inlet manifold gasket that had got larger over time.

20 years later my MkV1 Cortina, developed a crowd scaring backfire when switched off, cause was broken gasket on the inlet manifold.
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Phil - Nottingham



Joined: 01 Jan 2008
Posts: 1252
Location: Nottingham

PostPosted: Wed Nov 05, 2014 7:39 pm    Post subject: Reply with quote

Whilst its likely to be a fueling issue a bad dizzy earth or condenser can cause this. New condensers can be faulty too
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Martin A



Joined: 06 Oct 2014
Posts: 17

PostPosted: Wed Nov 05, 2014 8:37 pm    Post subject: Reply with quote

Many thanks for all the suggestions so far. A number of things to look into there.

Martin
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Martin A



Joined: 06 Oct 2014
Posts: 17

PostPosted: Thu Nov 06, 2014 3:20 pm    Post subject: Reply with quote

On reviewng the replies I noticed my comment that the age of the fuel in the tank was unknown. I knew that fuel is said to deteriorate in the tank but I had more or less taken that as an urban myth, having never found any hard data on precisely how fuel deteriorates and what are the effects.

The state of the fuel gauge was unknown but I assumed that there was not much left in the tank. I added 10 litres of fresh 98 octane and tried the car. After five minutes or so (the time to use the fuel already in the line and the fuel pump?) the misfiring became noticeably reduced.

Next day, armed with a 20 litre fuel can, plastic tube and a hand-squeeze fuel pump, I emptied the tank. Then 10 litres of fresh 98 went in.

After the engine had warmed up, the misfiring had gone. So the problem was the fuel.

Judging from what I found in the boot of the car when I bought it, a previous owner had the habit of adding some lead replacement compound to the fuel. I don't know if this can produce misfiring. If not, then it has to be put down to the age of the fuel.

My thanks for all the comments. A good number of things there to keep in mind as I continue sorting out the car.
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47Jag



Joined: 26 Jun 2008
Posts: 1480
Location: Bothwell, Scotland

PostPosted: Thu Nov 06, 2014 5:59 pm    Post subject: Reply with quote

Martin,

I suppose what could have happened is that the petrol evaporated increasing the lead additive/petrol ratio thereby causing plug fouling or you've had a sticky valve and it has now freed off once the oil got to the stem. Either way I'm glad it's sorted.

Art
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