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E: Engine Ignition Advance


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In these long long days of Covid isolation I have been mentally reviewing the preparation of my Comet in preparedness for when (if?) the restrictions on movement are lifted. For months now we have be confined to within 5km of our home .

So over the last 10 years having melted 3 or 4 pistons with the bike used only for touring I revisited the situation - in all cases there was no sign of lean running , actually in every case there were signs of being over rich!

So I now turned my mind to the ignition.

The ATD fitted as standard to ALL VIncent B and C series machines is a Lucas ATD model 47505A/D that provides an advance range at the magneto of 16º to 18º. This translates to an advance range of 32º to 36º at the crank. Depending on the individual ATD the actual advance can be anywhere within that range. And as the ‘fingers’ of the ATD wear, all be it slowly, then the advance range will increase.

It is generally acknowledged that the ignition sweet spot for easy and reliable starting is 4º BTDC (before top dead centre).

Reference to the Vincent Riders Handbook 10th edition advises ignition timing of 38/40º BTDC at full advance for twins but only 37/38º BTDC for singles. For twins this fits in exactly with an original unworn ATD and is close to the lower limit for singles. So why the need to fiddle with something that’s within the original specification? Compression ratio and fuel volatility.

Fuel Volatility: Vincent motors were designed over 70 years ago and were intended to operate on ‘pool’ fuel that had a very low octane rating, but more importantly burnt relatively slowly. Modern fuels have much higher octane ratings and burn much much faster. This faster burning means that combustion will happen much faster and with the ‘original’ ignition timing this results in peak combustion pressure inside the cylinder happening well BEFORE the piston reaches the top of its upward stroke. This has 2, both undesirable, effects. First it tries to force the piston back down the cylinder, before it has reached TDC putting a massive destructive load on the bottom end of the motor, especially the big end bearings. Another consequence of this is excessive heat generation that can lead to piston overheating and failure. Second effect of this faster burning is that almost, if not all, of the charge has been expended before the piston gets to TDC resulting in output power being diminished.

The remedy to the effect of increased fuel volatility is to lower the ignition timing at full advance. How much? On the basis of feedback in MPH across the years and suggested by Irving in “Tuning For Speed” and endorsed on the VOC Forum back in 2018, a reduction of around 4o is a reasonable starting point.

Compression Ratio: There is a tendency of motor rebuilders to use higher compression ratios (CR) that originally fitted at the works. Original for all B and C series, other than Shadows, was a CR of 6.8 to 1, Shadows were 7.3 to 1. It is now more common to find 8 to 1 or even slightly higher CR being used. Again, looking at the advice from Irving as the CR is increased the ignition advance should be reduced. Why? The increased compression ratio can result in an improvement in combustion efficiency which in itself results in an increase in the speed of burning of the fuel in the cylinder. Irving tells us that an increase of 3 in the CR should be matched by a reduction in ignition timing of 5o. So as the increase of the CR from 6.8 to 8 to 1 is around half that, it should be accompanied by a decrease in ignition advance of around 2.5º.

Conclusion: Allowing for both modern fuel and compression changes it appears that sensible maximum full advance to use on Vincent motors today is: For twins 38 less 4 less 2.5 gives 31.5º BTDC; for singles the result is 1º less thus 30.5º BTDC. You will recall that for starting 4º BTDC is optimal and advance at the ATD is half that at the crank.

Twins: 31.5º less 4 = 27.5, divided by 2 = 13.75º advance at the magneto

Singles 30.5º less 4 = 26.5, divided by 2 = 13.25º advance at the magneto .

Remember – the original ATD advance range at the magneto is 16 to 18º

Twin Spark Heads: All of the above relates to generally standard Vincent motor fitted with a single spark plug. If your motor is fitted with twin spark plug heads then based on experience in the field, its desirable to retard the crank shaft full advance by a further 4 degrees – or if you prefer retard the ATD advance by 2 more degrees – it’s the same thing. Result at the magneto is 11.75º for twins and 11.25º for singles.

OK - what have I got wrong??

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VOC Member
Re the reported failure of KLG plugs because of contamination of the ceramic the other VOC magazine Fishtail contains a letter saying the writer soaked the offending plug overnight in domestic oven cleaner and has used it ever since without a problem.
Now I havent tried it but it might be worth an experiment...


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What About sand blasting the plug?
I've been told no, don't do it, but I haven't had a problem. But I will also say that after blasting I look deep into the plug recess for any grit still trapped in there. Which is a good thing as I often find a few bits. Fortunately I am on friendly terms with a couple of dentists who have been kind enough to give me their surplus to requirement dental picks. So with those in hand I very carefully pick out the offending debris before it causes any problems. The picks also allow you to scrape the area of the central electrode that is shaded by the earth electrode.
The down side is the abrasive wears the glaze off of the ceramic and allows any fouling in the future a better grip. I used to work with a guy who swore that once a plug was fouled it was better to just trash it and get a new one. He wouldn't even wire brush them with a soft brass wire brush because he was of the opinion that even that destroyed the ceramic coating and made the plug useless. With that in mind I've tried cleaning fouled plugs with all sorts of solvents and blowing them out. I've tried solvents, wire brushes, and air. I've even tried burning off fouling with a propane torch. Nothing seems to work like blasting.
I've often wondered about oven cleaner. I sorting out a long running richness problem on my MG I've got a couple of fouled sets that are in need of a cleaning.
Once after blasting a set I also dropped them in my bench top ultra sonic cleaner with a strong mixture of simple green. Even after rinsing in fresh water and thoroughly drying with compressed air the metal developed that fuzzy white corrosion. After a quick going over with a brass wire brush, they still worked.
Spark plugs can take an amazing amount of abuse a still function.


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Spark plug sand blasting is done routinely with aircraft plugs when running them in avgas fed engines and their lead content. There you get lead fouling quickly when not caring for lean mixture and only full rich for take-off. You don´t bin aircraft plugs at 30 Pounds each easily. The Gunson type in the link below is quite allright for home use when you got an air compressor. But certainly it lacks a real tester with buzzer ht coil and pressure testing at max. 10 bar. Long time ago there were blasting and testing devices like from Champion, often found in car garages but these days you find them in Ebay at collectors prices mostly - or new from India, like in the youtube link.

Gunson blaster

Youtube tester:


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Wonderful how oven cleaner morphed into blasting:)
If I had an NGK crap plug I would sneak in the kitchen and try some cleaner
if it works its better than picking out bits of grit....


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But Robert. There is so much talk about plug failure on this forum rhat I get paranoid that my plugs will crap out on me when I am several thousand miles away the nearest plug shop. And plugs are cheap.


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VOC Member
Even cheaper if you buy a big case of fake NGKs on eBay.
The crazy thing is I ran a set for fifteen or twenty thousand miles before discovering that they were fakes. It never occurred to me that anyone would fake something as cheap as a sparkplug.


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VOC Member
If you sandblast with plastic grid you can soften the Surface. A corn of 50 Mikrometers is often used in dental laboratorys.
Aircraftspruce.com sells aviation approved blasting grit that does a great job on plugs. Champion part number 91893. It tends not to leave leftovers particles at the bottom of the insulator.

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