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 Pinking on 2nd Choke 
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Firing on two.
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Joined: January 5th, 2009, 5:48 am
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Post Re: Pinking on 2nd Choke
Thank-you Sean and Ken, some small advancements in understanding are being made at this end. I read the wiki article Sean linked to and it does explain in some detail. I'm now clear about the difference between pinking and pre-ignition, and what we're talking about here for the moment is pinking. However although it lists some of the cures and therefore hints at the causes, it still doesn't explain all the links from the various possible causes. I going to try working my way through the references it lists.

In the meantime just to clarify, the slower burning of a leaner mixture gives more time for detonation to occur ahead of the flame front combined with the higher temps creating 'preferable' conditions.

But to go back to pretty much where we came in, now I know exactly what pinking is, I'm still not seeing how overly advanced ignition timing causes it. I can see that too much advance would be a problem in itself, but I would have thought it would cause symptoms similar to pre-ignition. Is the link an indirect one through heat again?

The same with too hot a grade of spark plug, I can see how it would cause pre-ignition but not how it contributes to detonation.

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November 21st, 2010, 10:04 pm
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Firing on two.

Joined: April 22nd, 2009, 11:06 pm
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Location: Ecosse
Post Re: Pinking on 2nd Choke
different plugs soak the heat away at diffrent rates you want the plug tip to be cool enough that it doesnt melt ( or change its electrical properties) but hot enough to burn off any carbon that would cause tracking ot current to leak rather than jump the gap. it may have an overall effect of the overall temps in the head
Richer means more fuel evaporating it cools the charge and all its in contact with
leaner burns hotter the richer mix the unburnt fuel still there after the oxygen used up helps cool? im guessing

timing is measured BTDC B being "before" so you are counting backwards

so if its timed at 40* you may have the burn too soon before the piston reaches tdc so you have the burn increasing the temp and pressure in the rising piston and so detonation

"retard" the ignition to 32* fires nearer to TDC and the flame front, the squish and all that works as intended
( too late and its still buring in the exhaust and so the car feels really hot and smelly)

my brain stops at 7pm and the painkillers are kicking in so im probably not making sense now

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"Any advice of a technical nature is given on the understanding that I've actually done this shit, not just read about it in D*lly club mag some time ago.


November 21st, 2010, 10:41 pm
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Agony Aunt - You have a car problem? Speak to Ken

Joined: March 6th, 2009, 1:40 am
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Post Re: Pinking on 2nd Choke
Hi Joolz,
this should help fill in any gaps, I hope...
< http://www.flickr.com/photos/30132857@N ... otostream/ >

Too much advance on the timing means that the cylinder pressure 'peak' arrives earlier than it should be and is much higher, increasing the risk of spontaneous ignition of any unburnt mixture still in the combustion chamber.

Too lean a mixture means that the rate of flame spread is too slow, so the rising cylinder pressure causes auto ignition of those pockets of mixture which haven't already been consumed by the flame front.

Sounds complicated maybe, but what it boils down to is that if the timing's set correctly, plus the mixture is correct or is 'slightly' rich, then the engine will have more chance of running well for longer. ;)


That recommendation to use colder spark plugs was to reduce the risk of pre-ignition.
Our engines do run hotter on unleaded fuel and I think it's safer to go at least one step colder in a manufacturer's range of suitable plugs than the original advice.
I've run all of my cars and my customers' cars on NGK B7HS or B8HS for many years, without problems.
On the other hand, I've had cars arrive with 'no name' plugs which will 'run on' for several seconds when switched off after a fast run...

ken

Joolz wrote:

But to go back to pretty much where we came in, now I know exactly what pinking is, I'm still not seeing how overly advanced ignition timing causes it. I can see that too much advance would be a problem in itself, but I would have thought it would cause symptoms similar to pre-ignition. Is the link an indirect one through heat again?

The same with too hot a grade of spark plug, I can see how it would cause pre-ignition but not how it contributes to detonation.

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November 21st, 2010, 10:57 pm
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Firing on two.

Joined: February 17th, 2009, 8:43 pm
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Post Re: Pinking on 2nd Choke
Ok. The talk about changing jets from the standard ones made me want to see some list of things that might be good to differ from the original setup from Citroën.

I've experienced that my car likes the floats in the carburetor to be adjusted so that is gives a bit higher fuel-level in the chamber, than recommended by the workshop manual. The manual says the clearance between float centerline and joint face with gasket should be 18mm. But my car runs totally smooth at low revs with 16mm, and works well in general. With 18mm it doesnt. It might be because of worn carburettor, or it might be the same reason why Ken suggest new jet's to compensate for modern fuel? Thoughts? Should I try to order some jet's and have a play with that when spring-time comes?

Have read about different things here in this forum, but cant remember them all....
What I can remember seen mentioned to be changed from standard is valve clearances and fuel jets.

What are the "new values" for these things, and is there more? The ignition is kind a covered in this thread ;)

Everything based on a stock engine (compression 8,5 or 9) with correct ignition(points or electronic) and stock exhaust and airfilterbox.

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November 21st, 2010, 11:35 pm
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Firing on two.

Joined: April 22nd, 2009, 11:06 pm
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Location: Ecosse
Post Re: Pinking on 2nd Choke
in the UK we seem to find that :-
2cv 18- 26 carb running CR 8.5:1 or 9:1

primary main jet 107.5 ( 1.075mm)
ngk b 7s ( or 8 if you drive hard)
valves 0.2-0.25 inlet; exhaust 0.25 - 0,3
float at standard
standard filter and exhaust
timing set at 32* max advance as a nice safe max for 9:1 you may find that a 8.5:1 may be able to go up to nearly 35* but that depends on the petrol you use and other factors above.
some variations due to personal preferances are alowed!

it is also worth checking that the 2nd choke butterfly actually closes fully ( but not so that it binds when the carb cools so ajust it in a cold carb) as any "false air" passing that exagerates the flat spot at 3/4 throttle

and i say we- but its Ken that deserves the credit as he has done most of the donkey work over the years on that

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November 22nd, 2010, 10:15 am
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Firing on two.
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Post Re: Pinking on 2nd Choke
I think I'm now happy that I understand this to a satisfactory level. But I'm still mulling it over from different angles in my head. So things that can cause detonation are, over advanced ignition timing, lean mixture, high cylinder head temperatures, low octane fuel, high compression ratio, high charge temperatures.

Now assuming that when an engine is set up exactly as the manufacturer intended it wont pink, then a change/increase in any of the things above, to a level outside what the manufacturer intended, could trigger the engine to start pinking. The most commonly suggested cure seems to be to retard the ignition, and I assume that this is because it is the thing most likely to have changed over a period of time.

However there must be times when the timing gets retarded in order to compensate for something else, for example an excessively skimmed cylinder head(not 2cv), poor(low octane) fuel, or abnormally high ambient temperatures, and I assume that in such circumstances the engine is not working at it's best.

As I understand it the precise timing of peak cylinder pressure is important to provide maximum torque, so having to set the ignition timing such that peak cylinder pressures are later and lower will lead to lower engine power. Are there times when the timing must be retarded from the setting that gives best power in order to avoid detonation? And in such situations would it be preferable, or even possible, to avoid detonation some other way, for example lowering the cylinder head temperature? I guess raising the octane rating of the fuel is a win/win solution, but I assume that enrichening the mixture past that that gives best power or lowering the compression ratio would only be used as last resorts.

I only just managed to follow that train of thought myself, so I understand if it makes no sense to anybody else. I think what I'm looking for is more information on how to best balance all the variables in a non-standard engine where the factory settings no longer apply. For example, assuming the objective is maximum power, and to avoid detonation you could only lower the compression ratio or retard the ignition timing, which would be preferable? That may not even be a valid question, and I won't be at all surprised if there's no black and white answer.

A slightly more comprehensible question. Some time ago I had to change the piston in my 2-stroke 125, after it developed a finger sized hole in the center of the piston crown, much like the one in viking's pic. Now I've always put the fault down to running lean, the symptoms being that it was very hard to start, and didn't do it again after I changed the crank seals. My belief had been that the lean mixture had burnt hot enough to simply melt it's way through the crown. However is it far more likely that detonation played a major role in the pistons demise? And if so, why is the center of destruction directly under the spark plug? It seems far more logical when it's at the extremities of the combustion camber, as per Sean's pic.

Apologies for what is probably a load of guff.

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November 23rd, 2010, 2:14 am
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Post Re: Pinking on 2nd Choke
Joolz,
re. your question about lowering compression ratio versus retarding ignition timing in order to avoid pinking, the former is the better option for 'old school' engines.

Modern engines don't usually have this problem, as they're mapped to run as close as possible to the limit and even if the line is crossed, the knock sensor will immediately retard the timing.

Going back to old school engine tuning, pushing the CR too high could be conter-productive.
On some race tuned motorcycle engines, reducing the CR by quite a small amount, say from 9.8 to 9.7:1, would mean that pinking did not occur.
If left at 9.8:1, the timing had to be retarded so much that overall performance suffered.

Re. holed pistons, that's because the centre of the piston is always the hottest part of a piston.
Exhaust gas is around 650C, the crown of the piston can be at 300C under hard use, at which temperature its strength is already reduced considerably, then if 'runaway' pinking/pre-ignition is allowed to continue for long enough, the centre of the piston melts...

The driver of the car in Viking's photo must have been deaf as a post, looking at the amount of hammer the piston had to endure before expiring... :lol:

ken

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November 23rd, 2010, 2:50 am
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Firing on two.
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Post Re: Pinking on 2nd Choke
Thank-you ken, I'm pleased that you considered that ramble worth responding to, with something as useful and informative as ever.

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November 23rd, 2010, 3:05 am
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Firing on two.

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Post Re: Pinking on 2nd Choke
your ramble made perfect sense-thinking out loud is good!

Sean

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November 23rd, 2010, 3:12 am
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Post Re: Pinking on 2nd Choke
ken wrote:


The driver of the car in Viking's photo must have been deaf as a post, looking at the amount of hammer the piston had to endure before expiring... :lol:

ken


Maybe deaf too, certainly very ignorant. No need to mention the driver(s) here as they do read and visit this site. Cut a long story short. Raid 2004 a 2cv arrives in my garage in Perth with instructions of getting the 2cv registered which I did. Wanted to service the entire car including an engine service as the 2cv was is such unprepared state and not really fit for raiding. Guess the reply. I informed the raiders of low quality octane only available in the outback but I was dismissed. The 2cv left Perth without any service what so ever. 3 days later in the middle of Australia the phone call came in ‘’something wrong with the engine’’ can you please organize another one? I did and the car completed the raid and I got the fu…ed engine back. I was horrified when stripping the engine, big hole in the middle of the piston and the list was endless. Main issues was pinging, timing no good, oil cooler blocked with shit, low oil level and filthy oil too and so on. Raid car sold after the raid and restored. This just shows that being deaf or outright ignorant not willing to understand and respecting the 2cv; you’ll pay the price later. What pissed me even more off? The next raid 2008, same driver(s) arrive with another load of shit, drove through rivers loosing fans. The last river took out another fan and guess what. Well the raid is over and we are heading back to Perth to fly home. We won’t fit another fan. F…ng hell by the time they got to Perth with hardly any fan blades left, the engine was cooked. Again the car sold to an innocent city person in Melbourne.
Raid 2012 and it may happen again unless they get the message of looking after the engine with proper service etc.

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November 23rd, 2010, 10:09 am
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