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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.

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When you removed the spark plugs were they covered in soot or damp with fuel? both conditions could cause a weak spark and would point to running too rich on your carbs. The loud bang could be unburnt fuel hitting a hot exhaust pipe from a cylinder which is running ok. –  Mauro May 2 '12 at 8:21
    
Thanks Mauro. I've taken the plugs out again and they are black but not wet. I'll look at adjusting the fuel/air mix. What's a good way to clean the plugs? –  Tim May 3 '12 at 21:33
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1 Answer

I suspect the bang was, as you suggest, unburnt fuel going up.

I take it you now have a good spark, if it has been running well since changing the coil?

As Mauro's comment says, what do the plugs look like? Are they clean, or do they show signs of overfuelling?

Does it turn over at normal speed on the starter, or does it spin a lot faster, slower or lumpier than usual? I assume you have tried pouring a little fuel into the carb again, and/or a quick squirt of easy-start ("startyabastard" over there I believe? ) in the intake?

Do the exhause gases smell of unburn fuel when you turn the engine over? (You'll need an assistant for that, obviously!)

It might be worth doing a compression test, that's usually a good way of spotting any signs of internal damage - a tester can be picked up for very little from most car parts suppliers.

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Thanks. I will recheck spark and fuel supply and perform a compression test today and post the results. –  Tim May 2 '12 at 20:55
    
I've had a look at the plugs again and they are black but not wet. Seems like carbon fouling, probably over-fueling so I will try and adjust the fuel/air mix. I might also consider getting a new air filter just in case. The starter turns at normal speed and adding a bit of petrol to the carb doesn't help. I've got someone to help me today and will try and do a compression test. What's the best way to clean the plugs? –  Tim May 3 '12 at 21:31
    
If they are only newly fouled then a light brushing with a soft wire brush or stiff plastic brush might remove all the carbon build up, failing that, if you get the engine running right (fueling wise) then a good wide open throttle run should clear them up nicely - be prepared for soot coming out the exhaust (it did on mine) –  Mauro May 3 '12 at 21:56
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