1972 V12 pre ignition problems

My engine is beautifully tuned but has started to badly pre-ignite (keeps on running after I turn off the ignition). Any ideas on how to fix that? Someone told me I need to check the timing and the motor is running too lean. As that true?

sounds about right. If it runs too advanced it will likely be running hot . Also there is an Vacuum retard module on these engines, (not vacuum advance) check that is working too.

br Mike Clarke
S111 V12

Thanks for the advice, Mike. Much appreciated. I am familiar with the vacuum advance mounted under the distributor. The hose is difficult to fit and can sometimes pull out causing problems. I am not familiar with the retard module. Where is it situated?

What you refer to as the vacuum advance, is actually the retard for this engine and distributor.

Thank you Larry. Where is the vacuum advance located?

Bobby, I thought I understood from your earlier post that you knew where that was, and commented about the hose coming off occasionally. Unfortunately, other that answering your first question, which I could generically, I’m not familiar with the distributor on a V12.

There is only one advance retard vacuum device. At start the engine is retarded and as revs rise it goes to advance so ignition is in advance when driving around.

As to the original issue, the engine could be advanced to far so that the firing process is hot - can cause pinging and ignite any carbon in the engine which can cause overrun. So get the engine timed correctly with the correct static retard and that the distrubutor advance is operating correctly - eg vacuum advance and weights all working - a proper tun-up will sort this.

The other aspect as mentioned is fuel mixture - too lean and it will run hot with the same outcome as too much advance.

How many miles on the engine - carbon build up can also cause the above - it ignites or gets very hot and when the engine is turned off the glowing carbon ignites the fuel causing over run. An Italian tune up can help remove this.

So all related to tune up - a beautifully tuned engine will not ping or over run.


Hi, Aussieetype hit the nail on the head. Timing and mixture. I think that the only advance on a V12 is by the bob weights in the diststie. The vacuum unit just does retard I believe. So set static timing or dymanic with the hose plugged. I have not been on for 20 years but one of the racing pros at the time said you could run without the vac retard and just reduce the static advance a bit. I ran like that for a while but the tickover crept up when hot. Other wise it revved well. Probably contravened smog laws in the meantime.

Thank you AussieEtype. I often given it an Italian tune up. The V12 loves a good run. Engine has 75000miles on and runs beautifully. Starts well (for a V12 considering the long feul hoses). Lately started the engine over run.

Love to give you a call. Would you mind giving me your phone number? I live in the Sunshine Coast.

add to that which has already been said…check the heat range of the spark plugs…too hot a range can add to the issue…and…worse. (and so much for the “beautifully tuned” part…seems not quite…the running on is symptom of ill tuned) Nick

That is a great idea Nick. The problems started after recent service that included a spark plug change. I wonder if they used the correct spark plugs?

since I am not familiar with the V12, maybe you already know what others use and recommend…or they will chime in here for plug and gap. The NGK plugs have a wider range in a given plug, than do Champion…and some claim better quality as well. See what your V12 folks say…probably a lot in the archives already. Nick

Thank you Nick
Yes, the manual say to use a Champion N9Y. I currently have the equivelent NGK BP6EFS plugs in. If I can’t find the fault elsewhere I am considering using a cooler BP5ES plug. I am going to check my timing and retard vacuum unit this weekend. I have also learned that the centrafugal auto-advance bob weights handle the timing advance. It does appear that my timing is not retarding property, causing the engin overrun.

Bobby, have you considered a faulty ignition switch.
Dave Morrin S3 1973 v12

BP5 is hotter…in NGK lower number is hotter…BP 6 should be good. In Champion the higher number is hotter…many do use 9 should be good…but narrower range. Many in the 6 cyl have used the N11YC or N12YC…(the C is for copper and was an improvement after the N9Y so can use N9YC) Not sure if you meant BP6ES…(no F in it). NGK has now discontinued those…(still can be found on web)…and now all are resistor plugs BPR6ES…5,000 ohms resistance each plug, was 1,000. Your plug selection should be ok…it is rare…but sometimes you can have a bad plug…If not too hard on the 12, I’d change em out with new of the same…just to eliminate that. Of course it is easy peasy on the 6.

No, I haven’t Dave. One more thing on my list to do this weekend. Thank you.

The joys of owning an old car, and fixing it yourself! Thanks to everyone who chipped in advise. I found the problem: one end of the retard vacuum hose was dislodged. I learned a whole lot and enjoyed the experience. Should have looked that the vacuum hose first, but so you learn.

One comment: When testing, I could not get the vacuum retard to operate under vacuum applied with my mouth. I suppose it needs a greater vacuum than what I can apply with my mouth.

The V12 is running like a sewing machine again (and stopping too)

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That’s not a politically correct word any more. :kissing_smiling_eyes:

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it is always the last thing you check…well nice to re-look at it all anyway. good to hear it is purring nicely. Nick

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I have just become the latest patron of Jag Lovers.

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