V12 engine Backfiring badly, occasionally

My V12 has been running exceptionally well lately… I know I never should have said that!!

I had a mild pre-ignition problem that I put down to using 98 octane fuel. I was told the timing has to be advanced as the fuel burns too quickly. I run standard timing.

Since than I have been using 95 octane and the pre-igniting stopped but I have now developed a nasty backfire every now and then. So bad it makes the motor cut out. When the motor stops it starts up again no problem and runs well until the next violent backfire. I can’t figure it out?

I also have to mention that I, by accident, put in a valve saver lubrication in the petrol that actualy belongs to my VW diesel. How stupid can you be. On becoming aware of my mistake I drained the feul tank and cleaned the feul pump. Took it for a drive and it backfired again with new petrol. Was it a tiny bit of petrol between the pump and carburettors or was the diesel lubricant (I only put a tiny bit in) not the problem. I have now confused myself even more. Can anyone help? What can be the cause of a bad backfire inbetween the motor running beautifully be?

As far as I know, backfiring is unburned fuel that “explodes” in the exhaust.
You might have advanced the timing too much and now the fuel does not have enough time to burn completely.
You should maybe check if your centrifugal advance is stuck. This would give too much advance when the engine does not want it and would explain your pre-ignition issue.

As Aristides said. Make 100% sure everything about your timing is correct. Base, centrifugal, etc. Then if you still have backfiring look for other issues with ignition.

That sounds like good advice. Thank you Aristides. I will oil the bobweights and check the timing

Thank you Bob. What octane petrol do you use for your V12? I have a 72 E type.

First of all, higher octane fuel protects you from pre-ignition. Octane number is a proxy measure of resistance to pre-ignition. It allows you to run a higher compression or more advanced ignition with less risk of pinking. If you use the higher octane fuel without further advance there is no disadvantage compared with lower octane fuel. High octane fuel does not burn more quickly

The reverse holds- the higher octane fuel is usually denser, so you get a few percent more fuel in each litre. Furthermore, Paul Ireland in his book “ Classic Engines, Modern Fuel” explains that some products have better performance in other measurable ways

As for the dead cut/ backfire- i am experiencing this too - clearly a brief but total ignition failure. Almost certainly low tension electrics. It is not absolutely random - in my case occurring in association with a little more throttle than level cruise, but not at all predictable

I suspect that there might be a fault associated with the Crane distributor pickup or its wiring, perhaps moving as the vacuum retard effect is dropped under those conditions

Timing seems ok- note that if the issue were due to incorrect timing then the fault would be much more likely to be consistent rather than “now and agsin”

My plan - Crane out, Pertronix in

Our 89 XJS and 71 eType are both happy on 92 octane. The eType has a Pertronix set up.

Bobby, I think you may be approaching this problem backwards. A backfire in the exhaust is caused by unburned fuel getting into the exhaust system. A backfire in the exhaust will NOT cause the engine to die. However, if the engine dies because of a lack of ignition then the unburned fuel will be pumped into the exhaust, causing a backfire. The backfire is the result of your problem and not the problem itself. I suggest that you may have an intermittent ignition switch, like I did. When the switch opens up and the engine dies, you then turn the switch off and then on again, wiping the contacts and re-establishing the connection. Try jiggling the switch some time when you’re driving.

It’s easier to change the switch at home in your garage than it is to stand on your head under the dash, take it out and use clip leads, in 95 degree weather by the side of the road 300 miles form home. Don’t ask me how I know. It would have been much worse if I hadn’t had the clip leads.

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I had wondered about a dodgy switch, preferring to discount the idea for three (possibly flaky) reasons. Firstly, the whole assembly is very robust and a dangling key is unlikely to affect the switch (as it might in a TR5 or Morris Minor). Secondly, i have noted that there is no “ignition” warning light illumination associated with the fault . Finally, the dead cut only occurs for a second and ops normal return without any intervention- so a wipe of the switch contacts doesn’t sound like it

Unfortunately, the second reason holds no water- there is no ignition light in the jag after a dead cut using the ignition key until the revs drop (lesser cars do show an ignition warning in this circumstance)

I might wire up a test light to the ignition switch to observe whether there is a problem there before swapping out the Crane