Cause of backfire or popping back.

[Peter Naulls]

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?

[Ruairidh Dunford]

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...

[Peter Naulls]

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.

[AustinWood]

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.

[Peter Naulls]

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!

[AustinWood]

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.