I have an old 1993 Triton ute. It has a 4G54 carby engine. It's run as well as can be expected of an engine like that with nearly 400k on the clock.
Just recently one day I jumped in to go for a drive down the road and it wouldn't start. The engine would turn over but just wouldn't ignite.
I went through the usual test. Pour a small amount of fuel into the carby directly and see if it will ignite that way. I figured that should be able to tell me if it's a fuel supply issue or an electrical issue. Nothing.
Assuming then that it was electrical I pulled one of the plugs out and shorted it to the body and had someone try to start it. I got a weak yellow spark from the plug. I had expected more of a blue spark and I assumed that perhaps there was a fault in the ignition system. I put the plug back in and reconnected it, then unplugged the lead from the coil to the distributor. This I attached to a spare plug and shorted it and turned the engine, this resulted in an even weaker spark. I also tested the ballast resistor just to make sure it was okay.
The leads and plugs weren't that old and in good condition so I decided I would have to start with replacing the coil. A few days later, when I was ready to do so I thought I'd check. It started and ran fine, multiple times. Up to yesterday in fact.
I drove it yesterday and it seemed normal. Later that night I wanted to drive down the road but it wouldn't start. This morning I thought I'd try and start it.
The starter had barely turned when there was an extremely loud bang. Louder than a normal backfire. I checked everything and could not find any sign of external damage, no smoke, no fire, no evidence of explosion. The starter motor is intact. I've turned the engine by hand to check the valves were still opening and closing, they are so it shouldn't be the timing chain. After checking what I could think of I tried starting it again.
It turns over but doesn't start. I'm lost as to what the bang was... the only thing I can think is that somehow there was fuel in one or multiple cylinders since trying to start it last night. It's never happened before though.
I'd appreciate any advice anyone can offer.