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+--- Thread: Cause of backfire or popping back. (/showthread.php?tid=9967)
Cause of backfire or popping back. - Peter Naulls - 25-04-2025
Two years ago I started a thread about an occasional misfire on my Ruby. To cut a long story short I spent countless weekends trying to solve it without success. The problem gradually worsened over the course of a couple of hundred miles and it became evident it was popping back through the carb (or possibly backfiring but I don't think so).
The story was interrupted by a garage build and it became evident when we got the car back home that the block was cracked. It is now all back together with another block and pistons and ran well for the first 20 miles or so. Now the popping is back, albeit only occasionally.
Before the block was replaced I swapped the following parts (one at a time):
coil;
condensor;
points;
rotor arm;
distributor cap and leads;
plugs;
carb;
I also added an earth wire to the distributor base, cleaned out the pump, blew back through the fuel line to the tank and cleaned out the tank.
Before the block was changed it was evident that one cylinder occasionally had low compression so I reamed the valve guides and re-ground the valves. It made no difference. The new block has new guides, one suspect cam follower (and the valves) changed and the other followers visually checked for cracks. The compression is fairly even (110, 110, 106, 110) and is consistent over several checks.
I'm running out of things to check, as well as patience. Any ideas?
RE: Cause of backfire or popping back. - Ruairidh Dunford - 25-04-2025
Many years ago I had a misfire in the Ulster that drove me demented - it turned out to be a small flap of rust that covered the exit hole of the fuel tank, every so often, and restricted the fuel. Took me years to resolve...
Have you changed the the baseplate on the distributor for a known (good) one - I can send you one if you don't have a spare. They can breakdown internally...
RE: Cause of backfire or popping back. - Peter Naulls - 25-04-2025
Ah, now that's one part I haven't changed! I've been meaning to because it's a repro one and a standard cap doesn't fit it.
I have a good one on another distributor I can try but thank you for the offer.
RE: Cause of backfire or popping back. - AustinWood - 26-04-2025
Backfire on the over-run can be caused by and exhaust leak close to the manifold.
The Nippy I had used to do this. I'd change down into 2nd to turn into a minor road and half way round the corner there would be a very loud backfire. This caused panic among prdestrians.
Our fabric saloon is now poping on the overrun. I plan to check the exhaust.
RE: Cause of backfire or popping back. - Peter Naulls - 26-04-2025
Very interesting, it sounds very much like mine. I did find a tiny crack in the manifold, so that has also been changed, with no improvement.
I've swapped the distributor base plate (and cap) as suggested by Ruairidh. Will road test once today's social committments are out of the way!
RE: Cause of backfire or popping back. - AustinWood - 26-04-2025
On the DK4 distributer I fit an earth wire to one of the screws holding fixed points contacts and take it out of the distributer through a small slot filed in the base and earth it on the engine. I use one of the dynamo throigh bolts. This bypasses the bad earths that these distributers have. They depend on brass strips embedded in the Bakerlite and then connecting to the ditributer body by one of the screws holding base plate down. On manual advance distributers the body has no firm connection to the engine to allow it to rotate either.
RE: Cause of backfire or popping back. - Peter Naulls - 27-04-2025
Thank you Jim, I had already done exactly as described to my DK4A.
I can now be confident it's not the HT side of things, having changed each component in turn again today (including the base plate), always substituting known good parts. It runs well for 5 or 6 miles then begins to backfire. Once it begins, it keeps doing it until it's left for half an hour or so. The good news is I covered about 25 miles - on drive it day - and the engine is feeling much free-er.
So, my next question... I want to rule out the ignition switch, even though it seems a highly unlikely culprit. Can I simply connect a wire from the battery to the coil? if so, do I still turn the ignition key on or isn't it neccesary? - I'm think about possible implications for the charging system which I don't really understand.
Thanks in advance.
RE: Cause of backfire or popping back. - AustinWood - 27-04-2025
We did the Borders Vintage Automobile Club's Drive It Day outing today from St Boswells via a circuitous route to the Polish Map of Scotland near Peebles,
The car was running very well but was backfiring on the overrun all the way, gradually getting louder. It happens especially at about 45-40 mph on the overrun in top.
I'm pretty sure it's caused by the exhaust leaking where the downpipe goes into to silencer. I shall investigate tomorrow.