E: Engine Ignition Advance

MarBl

Well Known and Active Forum User
VOC Member
I once got pinging on my Velo due to a weak magnet.
I assumed, the correct timed, but weak regular ignition gave the mixture enough time for a second, irregular ignition.
Remagnetization cured the problem entirely. No other settings had to be changed.
It may be worth checking, if the remagnetization of your rebuild magneto has been done properly and the shoe gaps are not too wide.
 

Nigel Spaxman

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VOC Member
I think the speed of burning has a lot more to do with the turbulence in the combustion chamber than the burn speed of the gas. Apparently without that turbulence the fuel wouldn't even burn fast enough for our engines to work at higher RPM. The mixture in our cylinders if it was static would burn really slow, not even fast enough to be complete in one cycle. Over many decades there has been a change in ideas from engine tuners. It used to be people would set their ignition at the maximum advance the engine could stand without pinging. Now a lot of people set their ignition at the lowest figure that still allows the engine to rev our and produce maximum power. I have a programmable ignition with a vacuum switch so what I do is at high vacuum the engine runs at 38 or 39 degrees advance and I knock it back to 34 when there is less vacuum. My bike has 8:1 pistons and doesn't ping even if set at 38 degrees all the time. Probably I could just set the ignition at 38 degrees. It may be that when I am revving the engine hard and the throttles are wide open that the bike has a bit more power at 34 than 38 degrees. Apparently some Norton engines when run on the Dyno need less advance at high RPM for maximum power than at lower RPM. Probably this is because of the better turbulence at high engine speeds.
 

Magnetoman

Well Known and Active Forum User
VOC Member
Everyone,

The following list is probably missing something but there are at least nine factors that determine how well an engine runs: degrees at full advance, rate of advance, mJoules in the spark, shunt resistance of the plug, combustion chamber shape, compression ratio, inlet tract geometry, chemical composition of the fuel, and octane rating.

Although some of these variables are fixed when comparing two otherwise mechanically-identical engines (e.g. two Comets), even then there are enough "variable variables" left to make it very difficult to arrive at definite conclusions. That's why I asked if anyone is aware of something published by the SAE where experiments were conducted with enough of the variables controlled that conclusions can be drawn about what, if anything, we need to adjust to compensate for modern fuel.

The latest Forum update seems to have made it impossible to quote the relevant snippets of text when composing replies off line, which is a step backwards in my view. Be that as it may, Albervin wrote "The other additive I bought is a fuel stabiliser. It supposedly stops our modern excuse for petrol from going stale and is quite cheap compared to the octane booster, about $2.50 for a 236ml bottle. Ideal for when you don’t use the car very often."

I did a year-long "scientific" study of a popular brand of stabilizer back in the '00s. Making a long story short, I estimated the total area in a Monobloc's bowl for fuel vapor to escape from it, then made containers with holes of that size. One container held gasoline from the pump, and another held the same gasoline plus the highest recommended dose of stabilizer. The initial volume of both containers decreased rapidly at first as the most volatile compounds evaporated (which is why your bike is hard to start after sitting for a few weeks -- assuming the pilot jet didn't get blocked, which is another likely problem), then continuously slowed. I'd have to look up the data to be precise, but at the end of something like a year there was only a tiny amount of tar-like residue remaining in each. I used a precision scale for this experiment and found that the fuel stabilizer made no difference whatever in either the rate of evaporation, or the quantity of residue left after a year.

For what it's worth, if I'm not going to ride a bike for more than a week I drain the carburetor. My main reason for this is I've found within a couple of weeks a thin membrane of "varnish" forms to block the pilot jet, which makes starting a lot more difficult than if only some of the volatiles are missing from the fuel in the float bowl.
 

LoneStar

Well Known and Active Forum User
VOC Member
Perhaps that was the "Sta-Bil" brand, which many of us in the US use? I thought its claim to benefit was that it prevented chemical decomposition of the gasoline, rather than making it less volatile.
 

Roslyn

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Non-VOC Member
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??

Martynretard the ingition
 

Magnetoman

Well Known and Active Forum User
VOC Member
Perhaps that was the "Sta-Bil" brand, which many of us in the US use? I thought its claim to benefit was that it prevented chemical decomposition of the gasoline, rather than making it less volatile.
Sta-bil claims to keep fuel "fresh" longer, which they define as receiving a higher number on an accelerated oxidation test than fuel from a refinery. That test requires fuel to meet some standard for a minimum of 240 min. at 100 oC, which is roughly equivalent to 42 days at 20 oC. To the extent the accelerated test is valid, which is another issue, it means fuel would remain "fresh" for 4 months rather than ~1½ months. Which, if it were kept in a sealed container (i.e. not your bike's tank or carburetor), wouldn't be a bad thing.
 

timetraveller

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VOC Member
It is all going to depend upon more than just whether one has twin plugs or not. I know of one case where twin plugs, high CR and squish bands, at one stage, required about 18 degrees of advance.
 
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