Print

[KAS2A]

Not having heard a proper backfire since the 1980s, at the weekend my 1963 obliged with a proper backfire, like an unmoderated discharge from a 5.56 / .223. Why?

[Alan Drover]

Under what circumstances did the backfire occur?

[w3526602]

Under what circumstances did the backfire occur?

Hi Neil,

Fuel + oxygen + ignition .... as many people have found when they switched off their ignition, going down hill, but stiil in gear and clutch still engaged.

The engine still sucks fuel, but discharges it unburnt, into the exhaust. The next combustion (when the ignition is switched on again) will ignite the fuel in the exhaust pipe, often with enough force to spit an old pipe.

I suppose, strictly speaking, a BACK-fire, is when the detonation occurs in the induction, possibly suggesting that the spark has jumped to the wrong HT lead. The spark will take the line of least resistance, possibly via tracking inside the dizzy cap. I have heard it suggested that the "tail" on the rotor arm is to prevent cross-firing, but I cannot get my brain round that idea.

602
.

[Clifford Pope]

I suppose, strictly speaking, a BACK-fire, is when the detonation occurs in the induction, possibly suggesting that the spark has jumped to the wrong HT lead. The spark will take the line of least resistance, possibly via tracking inside the dizzy cap. I have heard it suggested that the "tail" on the rotor arm is to prevent cross-firing, but I cannot get my brain round that idea.


.

Proper backfiring, ie in the induction pipework rather than the exhaust, is a real risk in an engine running on LPG that uses a traditional mixer rather than injection. I think that's because of the ease with which too much gas can be drawn in, and exploded if anything is a bit out of adjustment in the timing or ignition system.

It can range from a harmless "pop" to a full-blown explosion that can blow the air inlet off, damage the air filter box, or even shatter the inlet manifold. One safeguard is to have a relief flap fitted in the inlet hose to release the pressure of any backfire.

I think the "tail" is to allow the largest possible margin for the point of opening of the points to coincide with a connection to the appropriate plug lead. Distributor rotors that have reduced tails, or simply a pointer like a pencil allow no margin of error for distribution of the spark. That is more of a problem on a multi-cylinder engine. An eight cylinder distributor for example has no room for a tail, so the correct alignment of gear teeth for example becomes more critical. A single tooth error on a 4-cylinder might not matter because the spark could still occur while the "tail" was within range of the terminal inside the distributor cap.

[ChrisJC]

Proper backfiring, ie in the induction pipework rather than the exhaust, is a real risk in an engine running on LPG that uses a traditional mixer rather than injection. I think that's because of the ease with which too much gas can be drawn in, and exploded if anything is a bit out of adjustment in the timing or ignition system.

The problem here is that you have an inlet manifold full of a stochiometric mix of fuel and air, and any spark leakage back past the inlet valve will set it off!

The usual trick is to leave at least one hose clip loose on the inlet side so it just blows a hose off rather than inverting the air cleaner.

Chris.

[Alan Drover]

I should clarify. What I meant was what was the engine doing when the backfire occurred.? Was it being driven,  on the overrun, ticking over, starting up, or being switched off?
A full backfire is very unusual even on older vehicles. It's the boy racers with their modern junk with drainpipe exhausts and a engine program that gives backfires which they revel in. It's painful to my mechanical brain.

[w3526602]

Hi,

I assume that many of our readers have driven with plug leads Nos 2 and 3 swapped? (Once heard, easily recognised). I first met this on my first "car", a 1934 BSA Trike, with a conventional 8HP, 4-pot water cooled engine, side draught carb, and no air cleaner. It struggled along on two cylinders. It weighed four pounds under eight hundredweight, so could take advantage of the motor cycle VED rate, and be driven on a motorcycle licence (I think). Due to the SUEZ fuek crisis concession, I was allowed to drive it without being accompanied by a qualified driver. There seemed to be nothing in the rules about carrying an unqualified passenger ... so i did.

With the bonnet lifted, you could see a spray of petrol droplets/mist being blown backwards out of the carb, presumably every other stroke. Later in life, I have wondered what would happen if that spray had ignited? Perhaps nothing? Perhaps the End of the World?

Once the problem was identified, and the 2 plug leads swapped, it went like a dose of salts.

602

[Larry S.]

Hi,

I assume that many of our readers have driven with plug leads Nos 2 and 3 swapped? (Once heard, easily recognised). I first met this on my first "car", a 1934 BSA Trike, with a conventional 8HP, 4-pot water cooled engine, side draught carb, and no air cleaner. It struggled along on two cylinders. It weighed four pounds under eight hundredweight, so could take advantage of the motor cycle VED rate, and be driven on a motorcycle licence (I think). Due to the SUEZ fuek crisis concession, I was allowed to drive it without being accompanied by a qualified driver. There seemed to be nothing in the rules about carrying an unqualified passenger ... so i did.

With the bonnet lifted, you could see a spray of petrol droplets/mist being blown backwards out of the carb, presumably every other stroke. Later in life, I have wondered what would happen if that spray had ignited? Perhaps nothing? Perhaps the End of the World?

Once the problem was identified, and the 2 plug leads swapped, it went like a dose of salts.

602

Within the first year of ownership my father-in-law, and his brother, got those two crossed - they blew up Grover's muffler.  I literally could not here for over an hour and I permanently lost a significant amount of hearing in my left ear.

[KAS2A]

Under what circumstances did the backfire occur?

I was driving slowly up a farm track, probably had just changed into second a few seconds earlier.

[Alan Drover]

Retarded ignition can cause backfiring but it's usually a mild "popping" on the overrun.
The only thing my pea size brain can come up with is a sticking exhaust valve as the engine was running relatively slowly.

[diffwhine]

Just as I was reading this post, a car drove past and did two enormous backfires. Nearly gave me a coronary!

And one of the previous posts is right - there seems to be a trend of having cars that do this deliberately these days as I've heard it a few times before. One day I'll work out what car it is, where he (or she) lives and perform a lobotomy on them on the side of the road.

[Alan Drover]

Just as I was reading this post, a car drove past and did two enormous backfires. Nearly gave me a coronary!

And one of the previous posts is right - there seems to be a trend of having cars that do this deliberately these days as I've heard it a few times before. One day I'll work out what car it is, where he (or she) lives and perform a lobotomy on them on the side of the road.

Without an anesthetic I hope.

[Sheepman]

My Harley backfires slightly when shutting the throttle down quickly - but this is the norm for these machines.........

[Alan Drover]

Trawling the depths of what little brain I have, another cause of the backfire could be if the ignition cut out briefly.

[diffwhine]

Without an anesthetic I hope.

I do my lobotomies with a baseball bat. No need for an anaesthetic.