OT Petrol for 2 stroke garden equipment?

On Thu, 23 Jul 2015 10:37:43 +0100, Jim GM4DHJ ... wrote:

Such as my younger brother's CB160 in the early seventies, despite the use of a '180 degree throw' crank, twin cylinder 4 stroke engine.
If you want your spark plugs to last 100 thousand miles(Km?), the only way to achieve this is by using the "Capacitor Discharge Ignition" (CDI) method. Using a 'transistorised' CB (as per my son's 600cc 4 cylinder Yamaha Bandit's twin coil 'wasted spark' ignition system) just doesn't 'cut it' as far as plug life goes[1].
Using a pair of double ended ignition coils on a 4 cylinder engine in a "Wasted Spark" system offers a relatively cheap way to avoid the expense and liability of a single coil/distributor configuration.
However, a coil fault could cause misfiring on one *or* two cylinders (often two if the 'coil fault' was really the CD module or transistorised CB driving the primary of said coil).
When diagnosing such misfiring problems, it was important to know which *two* cylinders were involved when using the classic 'detach each HT lead in turn' to detect the 'faulty HT circuit' since you had to make sure the 'waste spark end' had a low volt drop return to chassis (usually via a spare spark plug resting or wedged into contact with the engine cylinder head or other such metallic parts).
If you simply detached the suspect HT lead as 'in the good old days with coil/distributor' setups, you were likely to kill the spark on the opposite cylinder thus disabling two cylinders at a time with no means of identifying which of the two was the *one* failed HT circuit/sparkplug causing the symptoms of an obvious single cylinder misfire.
The higher performance engines would have one CDI module per cylinder which made diagnosis much easier (and the carrying of a single spare module and spark plug much less space consuming in the roadside repair kit that any well prepared driver would make sure to be carrying in their car at all times).
[1] When my son was trying (and failing) to diagnose a 'mystery' intermittent misfiring/loss of power fault on a 2nd hand Bandit he'd recently bought last year, I got the opportunity to see the ignition module circuit. I was rather unimpressed by the use of a high voltage switching transistor emulation of the humble (and very much deprecated) Contact Breaker, Kettering ignition system.
The use of a microprocessor to control the advance/retard (and dwell time) to eliminate the mechanical bob weights system of old was impressive but it seemed rather a shame that they'd stuck to the much deprecated 'interrupt the primary current' of old as the means to generate the 3 or 4 hundred volt pulse needed by the 100:1 step up ratio coil in order to generate the 30 to 40 KV HT pulse supply to each plug.
The reason why this antiquated (albeit 'transistorised') method is deprecated is simply on account the ignition coil not only has to act as a 'step up transformer' it also has to provide the necessary inductance to form a flyback voltage converter which function relies on there *not* being any shunt loading in either the primary or (and this is the most important bit) the secondary HT side of the coil circuit to 'suck this flyback pulse energy out of the voltage generating pulse before it has had a chance to reach a peak sufficient to jump the spark gap.
Leakage on the HT circuit, HT leads with cracks or other imperfections aggravated by moisture ingress or fouled plugs, is all the loading that's required to defeat the antiquated Kettering system (transistorised or not).
The huge benefit of CDI, aside from a well timed single *full* equipolar cycle hot spark at the plug points[2], was that such leakage had very little effect on the spark voltage pulse other than a simple bit of volt drop due to coil winding impedance. In a CDI setup, the coil primary was going to see that 3 or 4 hundred volt pulse come hell or high water.
What happened after that was purely a matter of coil impedance volt drop against whatever (up to a significantly larger amount of) leakage path loadings as may be present in the HT circuit (cable leakage or plug fouling leakage[2] or whatever).
This is important, especially in regard of allowing a harder grade of plug to be used. The spark plugs can be allowed to run much cooler than would otherwise be advisable in the old fashioned Kettering system if you don't want the inevitable effects of plug electrode insulator fouling to cause misfiring due to extensive city stop start driving conditions preventing the plug experiencing regular bouts of higher temperature operation to burn off such accumulations of fouling deposits during 'normal' cruising operation on motorways and open A and B roads.
In addition, the longer hotter spark allowed the rather inhomogeneous spotty idle mixture to be adjusted closer to the ideal than the over-rich setting normally required by the weaker spark of the Kettering system, which helped maintain a hotter, more economical leaner burning and less sooty mixture which mitigated the fouling effect of the cooler harder grade plugs that were being chosen.
[2] The optimum spark pulse, despite the claims made elsewhere, is the single full cycle (full fat, as it were) equipolar spark pulse on a 40 thou gapped plug. The single cycle equipolar pulse significantly reduces electrical erosion on the spark plugs' electrodes, the main limiting factor in a spark plug's service life.
The antiquated Kettering system ('transistorised' or not) generates a spark with a considerable unipolar bias on the spark current, so much so that this is why the LT connections on ignition coils have polarity markings in order to make sure the (adjustable) earth electrode is the one that erodes, leaving the centre electrode to grow an easily filed off 'pip'.
At the time when CDI was just starting to become available in the mid 70s as an upgrade kit to use the existing CB points and ignition coil, amongst the many real benefits, the extension of plug service life from a typical 6 to 10 thousand miles to something more like 20 to 30 thousand miles[3] was a real 'winner' (and the reduced current/voltage at the CB points from 4A/300 odd volts to a mere 50 to 100mA/12 to 15 volts, removing the major cause of timing drift made this aspect a "Win, win" situation. :-)
If you were going to "Transistorise" your ignition system, CDI left the crappy Transistorised CB circuit for dead. The 'benefit' of the simple transistorised CB was only a small fraction of what a CB points triggered CDI setup had to offer (later CDI 'upgrade kits' provided an optical triggering unit to replace the points assembly in the distributor itself by way of an extra 'embellishment').
[3] The 100 thousand miles rating for spark plug service life seems a little OTT to me but, in view of developments over the past 4 decades or so since the standard spark plugs of the day, back in the 70s, could be made to last 25 or so thousand miles in a CDI setup versus the 6 to 10 thousand miles in a 'conventional' Kettering setup, I'm willing to believe this actually might be true (although a 100 Km life is less stretching of credulity :-)
--
Johnny B Good

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Coil on plug, where there is one ignition coil per plug, built into the plug cap. There are no HT plug leads. The spark is timed by the ECU, which "knows" exactly where in the the Otto cycle the engine is by counting teeth on a timing wheel on the crankshaft. It fires the appropriate plug at the appropriate time. There is no distributor.
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That Otto, he's got a lot to answer for. So it is magic after all.
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On Thu, 23 Jul 2015 10:55:44 +0100, Tim Streater wrote:

All he's got to answer for is "Suck. Squeeze. Bang. Blow."
The rest is just fine detail.
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He surely does.
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On 22/07/2015 18:33, Tim Streater wrote:

Narrow power band, turbo lag, and carcinogenic smoke.
Andy
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nothing to do with "supermarket fuel" then - you put the wrong octane stuff in.
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I know Sainsbury's diesel has addatives. If you must use a cheap supermarket don't assume all others are the same.
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There are certainly 3 different fuels at the Sainsbury's that I use. One is diesel. so presumably the other two are different grades of petrol
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Pretty sure all the supermarkets around here to Premium fuel as well
--
Chris French


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On 22/07/2015 16:09, Charles Hope wrote:

Close. While Tesco part owns Greenergy and get their fuel from them, most probably still buy on the spot market, which means they go to the cheapest supplier, many of whom you quite possibly have never heard of. While that fuel will (usually*) comply with the statutory minimum standards, it won't have the additives that the main line fuel companies spend millions developing.
* A Google search throws up several hits on fuel contamination, from the extensive contamination of Tesco, Morrsion and Asda petrol with silicon in 2007, to a single garage where rainwater got into the fuel tanks. Most involve supermarkets.
--
Colin Bignell

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Says more about your Merc then i think.
I've only ever used standard fuel (petrol or diesel) supermarkets, oil company garages where ever, never had fuel gaugae problems on any of our cars
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Chris French


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On Wed, 22 Jul 2015 20:08:41 +0100, Chris French wrote:

I struggle to imagine how a fuel gauge sender could be affected by fuel, short of the float somehow dissolving, and that ain't going to fix itself.
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It could "gum up".
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On Wed, 22 Jul 2015 20:37:32 +0100, Charles Hope wrote:

A LOOOOOOOOOONG while after the pump, filter, injectors would gum up.
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On Wed, 22 Jul 2015 07:59:09 +0100, Broadback

It probably makes no difference, but I believe that most 95 octane regular unleaded petrol in the UK contains 5% (or is it 10%?) ethanol. I think most super unleadeds are ethanol free.
Ethanol has a lower energy than petrol, which probably makes no difference in a mower, and it can absorb a bit of water.
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On Wednesday, 22 July 2015 18:03:04 UTC+1, Caecilius wrote:

http://www.whatcar.com/car-news/new-e10-fuel-will-cost-uk-motorists/1229022
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On Wed, 22 Jul 2015 18:02:56 +0100, Caecilius wrote:

E5 - 5%. E10 95 is becoming available, but it'll have to be clearly labelled, and E5 95 will remain the default "protection grade".

Nope, they're all E5, too.
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On 7/22/2015 7:59 AM, Broadback wrote:

I have always used super in my tools and never had a problem... (never had a problem with stale petrol either)
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