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Series 3 back fire


OldMaster
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I have a Series 3 2.25 petrol which has recently taken to squirting flame out of the exhaust, backfiring and stalling..now refuses to start at all!

Given the weather I thought carb icing but now have my doubts. OK, tests and symptoms...have a timing light and Gunson analyzer for dwell, co, rpm etc...

It is fitted with a weber..only around 20,000 miles old and a new fuel pump two months back...dwell reads correct and timing as low compression is stable at 6 degrees BTDC. Compression check shows fairly even compression on all bores and I have fitted new plugs/leads...

Cleaned the carb and idle jet as best I can. Usually starts pretty promptly.....like this morning....runs for about 10-15 minutes reasonably smoothly and then it all goes horribly wrong!! Let my foot off the pedal and don't stand near the exhaust as it shoots a flame out and then starts missing and stalling...I intend to change the points again and fit a new condenser but my experience is telling me carb somehow.. when it does first run from cold she is smooth and picks up well.

So something is changing dramatically in the warm up period....any clues anybody...Help!!!

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Guest otchie1

Back firing is usually an ignition fault but flames shooting out the exhaust is a bit excessive - you'd need a massive flame front for it to travel all the way down the pipe. Seen it with engines run on just downpipes.

Maybe valve clearances too tight? As it warms up you could end up with valves not closing properly....

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I'd say get yourself down the local Maccy D's with all the kids and start firing flames, they'll think "it's wikked that" and you should be able to sell it for a decent price and pick up another truck that runs fine :)

Sorry couldn't help myself! It does sound like a timing fault, maybe not compltely burning the fuel, or moving timing etc.

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I have a Series 3 2.25 petrol which has recently taken to squirting flame out of the exhaust, backfiring and stalling..now refuses to start at all!

Given the weather I thought carb icing but now have my doubts. OK, tests and symptoms...have a timing light and Gunson analyzer for dwell, co, rpm etc...

It is fitted with a weber..only around 20,000 miles old and a new fuel pump two months back...dwell reads correct and timing as low compression is stable at 6 degrees BTDC. Compression check shows fairly even compression on all bores and I have fitted new plugs/leads...

Cleaned the carb and idle jet as best I can. Usually starts pretty promptly.....like this morning....runs for about 10-15 minutes reasonably smoothly and then it all goes horribly wrong!! Let my foot off the pedal and don't stand near the exhaust as it shoots a flame out and then starts missing and stalling...I intend to change the points again and fit a new condenser but my experience is telling me carb somehow.. when it does first run from cold she is smooth and picks up well.

So something is changing dramatically in the warm up period....any clues anybody...Help!!!

Condenser first choice.

Ignition coil second choice.

Regards,

Diff

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The only thing flammable in the exhaust would be petrol. I'd be really surprised if you could get enough of a flame front to travel that far and also make it though the silencer. So that leads me to believe that it is running too rich, or actually dumping so much petrol into the cylinders that it's not burning and being pushed down the exhaust, possibly building up in the silencer.

For it to get ignited it would need a good ignition source, which might be something that would happen when the engine is really coked up, bits flake off and get expelled out the exhaust while red hot/burning. You let your foot off the pedal and enough air gets sucked in the tailpipe to get the air/fuel mixture right and Kaboom!

This is all theory, though I did have a silence split open because of backfires on a Series years ago. Unfortunately it's been so long I have no memory of what I did to sort it out.

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The only thing flammable in the exhaust would be petrol. I'd be really surprised if you could get enough of a flame front to travel that far and also make it though the silencer. So that leads me to believe that it is running too rich, or actually dumping so much petrol into the cylinders that it's not burning and being pushed down the exhaust, possibly building up in the silencer.

For it to get ignited it would need a good ignition source, which might be something that would happen when the engine is really coked up, bits flake off and get expelled out the exhaust while red hot/burning. You let your foot off the pedal and enough air gets sucked in the tailpipe to get the air/fuel mixture right and Kaboom!

This is all theory, though I did have a silence split open because of backfires on a Series years ago. Unfortunately it's been so long I have no memory of what I did to sort it out.

Agreed its petrol... But could be caused by ignition, or lack of it... As a young man, driving Land Rovers for the RAF, we used to get the Landie upto 40mph turn the ignition off and keep the throttle pedal down. Turn the ignition back on and 'bingo' - Misfire and flame out the back upto 2'. Regularly used to hold competitions at nightfall on the way back to the MT yard. :ph34r:

Downside as you stated is the occasional split rear box... :o

Not a real help but puts it back at Fuel or ignition or culmination of both...

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Hi,

Have you checked your ignition switched lead from the dash to the coil? An intermittent fault could stop the ignition long enough to let unburned petrol into the silencer which will backfire from hot carbon or a return of the ignition power.

The carburettor bowl may be letting excess fuel past the gasket when it gets warm, check the body is not warped.

The RAF landrover reminded me that you can do it even better with a 3 tonner going down hill, I remember two poodles on leads going vertical! The REME were p****d offf when I returned with a blown silencer sounding like a Centurion Tank. Must have been a missfire corporal :rolleyes:

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Thanks guys..this is really useful..."rebuilding" distributor today and will check connections to coil again...strange phenomena this..left it Tuesday pm just refusing to start..occasional fire with the choke pulled open or throttled depressed but otherwise nothing...

Go out Wednesday am, first turn and she starts and runs!! Been building engines since I was a kid..got me baffled this time..but still suspicious of the carb!!?

As for you RAF guys..and on tax payers money!!!!!

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Agreed its petrol... But could be caused by ignition, or lack of it... As a young man, driving Land Rovers for the RAF, we used to get the Landie upto 40mph turn the ignition off and keep the throttle pedal down. Turn the ignition back on and 'bingo' - Misfire and flame out the back upto 2'. Regularly used to hold competitions at nightfall on the way back to the MT yard. :ph34r:

Downside as you stated is the occasional split rear box... :o

Not a real help but puts it back at Fuel or ignition or culmination of both...

Sorry, didn't mean to imply that was the answer, and to have petrol in the silencer would mean fuel or ignition (or both) problems to start with.
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Yes I am fairly sure it is unburnt fuel..I can smell it but I am now at my wits end!!

I have replaced the coil, I have replaced the condenser, points and leads. First off would not start at all..OK, the timing might have changed although the distributor was replaced exactly so it should have been good enough to at least make an effort until I could get the dwell meter on it..

Because the Weber has been mucked around it was set to default...mixture 2 turns out and throttle stop around 1.5 turns of throttle opening...

Cough, splutter, bang....attempted restart several times..eventually just about managed to tick over long enough for me to read the timing light...

OK, jumping around of course but still about 6 degrees BTDC (low compression) and the way it always ran really well..but something odd..I painted the notch on the crank pulley silver to make reading easy....yes it was jumping around but also disappearing because the light was going out and then switching back in....so, intermittent spark then??? cap? rotor arm?...all look good and not that old..timimg chain? valve clearances? ..grabbing at straws now!

Up until a week ago..at least when cold the timing light reading was rock steady!!

Baffled now?

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OK, thanks all!!

I have finally got there. It is true that it was running on a faulty fuel pump for what must have been some time..and that was replaced end of last year.

It was difficult to solve it just by process of elimination. The problem was certainly unburnt fuel, lack of power, stalling and back firing when warm...

Anyway, replaced the points which were very badly pitted..this was odd in itself because they were not that old. Changed the coil and the condenser as well.

Set the points with the Ducellier out of the lump and returned it to a marked position...

Turned the key and "Bingo"..she started and ran. Using the Gunson analyzer and a timing light I was able to set the dwell, CO and timing and took her for a run..transformed..even when warmed up!!

Will still do the tappet clearances and change the oil because I guess the last lot has had a hard time!!

Thanks again!

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Guest otchie1
OK, thanks all!!

I have finally got there. It is true that it was running on a faulty fuel pump for what must have been some time..and that was replaced end of last year.

It was difficult to solve it just by process of elimination. The problem was certainly unburnt fuel, lack of power, stalling and back firing when warm...

Anyway, replaced the points which were very badly pitted..this was odd in itself because they were not that old. Changed the coil and the condenser as well.

Set the points with the Ducellier out of the lump and returned it to a marked position...

Turned the key and "Bingo"..she started and ran. Using the Gunson analyzer and a timing light I was able to set the dwell, CO and timing and took her for a run..transformed..even when warmed up!!

Will still do the tappet clearances and change the oil because I guess the last lot has had a hard time!!

Thanks again!

Good tp hear - out of interest, what CO figure did you get?

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