ignition timing.

[outdoorjoe]

Hi all,

something that ive been thinking about for a while now, so thought id throw the question out there see what comes back!

with timing a petrol engine car in my case a spitfire 1500, using the timing light method I:

1. mark the tdc marker on the crank cover with tippex
2. mark The notch on the crank pulley with tippex
3. find out what it should be (6 degrees BTDC)
3. set up the timing light
4. loosen the dizzy clamp
5. start the car and rotate the dizzy as required to make the white mark on the pulley appear at the correct setting under the strobe light.

Ok so my question is

1. why is it any car i ever does runs like a bag of nails at the manufactureres setting ie my car runs better at 4 degress btdc.

and

2. what methods do people use of ahieveing the correct timing ie et it close then drive the car and alter by a tiny bit at a time etc???

cheers guys
joe

[OneCarefulOwner]

Initial figures are likely to be based on using a different RON fuel to what's currently on sale, unless you're using an octane booster. This will affect running.

[Luxobarge]

As above - the fuels used by the manufacturer when they did the original research to decide on the "factory" settings are quite significantly different to what you'll be using now, not just in RON value but in overall chemical composition in many ways. For example, modern fuels are significantly more volatile, and inside the combustion chamber the flame front travels across the compressed mixture faster than it would do with old fuels, and that's why you often don't need as much ignition advance as with the older fuels.

Not to mention that the manufacturer would have done the research on a brand-new "perfect" engine, and (please don't take this the wrong way!) it's unlikely that your engine performs in quite the same way, if you know what I mean!

Your problem is quite common, I've come across loads of engines that don't run that well at "factory" settings.

Best method I've found is to get the engine running roughly right as you have done, then take it up a long hill in a high gear at medium revs and keep advancing the timing a couple of degrees at a time until it pinks. Then back it off until it just stops pinking under load, and that will be pretty close to the optimum setting. Ideally you should then look at it with your strobe light, and note down what the setting is for future reference.

Only better way than this would be on a rolling road IMHO.

Do make sure that the dizzy is in good nick first though, bob-weights operating smoothly and points gap correct. Oh, and I'd take the vacuum advance pipe off before doing the exercise too.

Hope that helps?

Cheers

[outdoorjoe]

thanks very much for your replies, didnt consider the fuel difference but did consider the engine 'might' not be as tight as she once was!

Ill keep playing with it and trying to sort it as best as i can! one more thing can anyone reccomend a good tuner/rolling road garage? im in south glos/north bristol so that area would be ideal!

cheers
joe

[Talking Hoarse]

As above - the fuels used by the manufacturer when they did the original research to decide on the "factory" settings are quite significantly different to what you'll be using now, not just in RON value but in overall chemical composition in many ways. For example, modern fuels are significantly more volatile, and inside the combustion chamber the flame front travels across the compressed mixture faster than it would do with old fuels, and that's why you often don't need as much ignition advance as with the older fuels.
Not to mention that the manufacturer would have done the research on a brand-new "perfect" engine, and (please don't take this the wrong way!) it's unlikely that your engine performs in quite the same way, if you know what I mean!
Your problem is quite common, I've come across loads of engines that don't run that well at "factory" settings.
Best method I've found is to get the engine running roughly right as you have done, then take it up a long hill in a high gear at medium revs and keep advancing the timing a couple of degrees at a time until it pinks. Then back it off until it just stops pinking under load, and that will be pretty close to the optimum setting. Ideally you should then look at it with your strobe light, and note down what the setting is for future reference.
Only better way than this would be on a rolling road IMHO.
Do make sure that the dizzy is in good nick first though, bob-weights operating smoothly and points gap correct. Oh, and I'd take the vacuum advance pipe off before doing the exercise too.
Hope that helps?
Cheers

Many thanks from me also for the petrol explanation Luxobarge. I was delving into this too a while back - see http://www.practicallyclassics.co.uk/vi ... f=8&t=3042
So - to take it a step further - what is the optimimum, best or most practical way to set the mixture where we are able to adjust?

[Wicksy]

I absolutely concur with Luxo on the ignition side, I've found over the years that's the only way to get an "old" engine to run sweet.
On the mixture - bought my self a Gunson exhaust analyser recently and it takes the guesswork out of setting the mixture and I now wouldn't be without one.
The other equipment must is a digital multimeter to measure the engine RPM and set the dwell if you have points ignition.
It's taken me a long time to acumulate the right kit and I now realise how straightforward the procedure actually is
A decent Strobe, multimeter and analyser are peanuts costwise, compared to a tank of petrol say, and the payback is, the engine runs right, you save fuel and are not killing the planet
Another nice to have but not essential is a compression tester - bought mine new on E-bay for about £20 - no point in setting the ignition and mixture if the engine is knackered internally
The price of all this kit incidentally is probably less than a rolling road test and they would have to go through all the basic setups, charging for it too, before attempting a test

[JPB]

I too would go along with every word of the above apart from the suggestion that a compression tester isn't essential.

They're not an expensive thing to buy, require very little space and no upkeep and even if you only use it once on your own car, people will find out that you have one and you'll be called upon to test other folks' engines with it.

[Luxobarge]

Yeah, I've got all the kit mentioned above and wouldn't be without it.

Having said that, I tried using the gas analyser on my Midget and it gave crazy readings. What I do is this (which won't be quite the same if you're not running SU carbs):

Get a nice idle speed. Use a Gunson carb-balancer (or other balancing tube thingamy of your choice) to get the idle identical on both carbs, then wedge the throttle open a bit so it's running a fast idle, to balance the carbs running off the idle stops. Now your carbs are in total alignment with each other apart from the mixture. Next: There's a little rod down under the side of the dashpot that lifts the carb slide. This will lift the slide but obviously not open the butterfly. Now just lift it and drop it straight away. If the revs drop, then it's a little lean, if the revs rise, it's a little rich (have I got that the right way round?) If the revs remain roughly the same then it's about right.

The main thing is to get this the same on both carbs, so they are aligned to each other.

Then I take it for various road trips (the fun part) to check the mixture at open throttle settings, and wind the mixture up or down a notch at a time and see if it goes better. Obviously, do this equally on both carbs so they stay in alignment. A bit of a rough and ready method, but without a rolling road I don't have any better way of checking mixture at open throttle settings, which after all is where it really matters.

Another option is to use a Gunson colour-tune, but again, you're only going to be able to get it right at fast tickover at best, it won't tell you anything about the mixture under load at open throttle, unless you can stick your head under the bonnet while barrelling up a hill at 60 mph!

The "lift the car slide" method will get you most of the way to be honest IMHO.

HTH

[Luxobarge]

Many thanks from me also for the petrol explanation Luxobarge. I was delving into this too a while back - see http://www.practicallyclassics.co.uk/vi ... f=8&t=3042

Yeah, I looked into this quite a while back. I was curious as to why we need ignition to be advanced at all. The reason is that the compressed fuel starts to burn as soon as the spark occurs, but there's a flame front that travels across the compressed mixture, burning as it goes, and this takes a finite amount of time to happen. Furthermore, there's a further miniscule delay before the burning mixture expands as it "explodes" so that it can do useful work in forcing the piston down the bore.

Now all these delays are tiny fractions-of-a-microsecond stuff, but on a highly revving engine they matter. Think of the engine revving slowly - the flame front has got more time to travel before the piston goes over TDC, so the ignition doesn't need to be advanced so much. Now think of the engine doing 6,000 revs or more - the time between the spark and the piston going over TDC is much less, so the spark has to occur earlier (i.e. advanced) in terms of rotational degrees (but not necessarily in elapsed time) in order for the "explosion" to happen at the right time.

Now all of this is dependant on the speed at which the flame front travels and the fuel expands for that particular fuel - if the composition of the fuel changes in any way, then these parameters are likely to change too, and the engine will need to be timed differently. And, as I say, modern fuels have changed from the old '60s and '70s 4-star quite significantly, not just in RON value but in actual chemical composition too - I don't have the details to hand but you get the picture. By the way, other "modifications" in the engine such as raised or lowered compression ratio will have an affect on the above too, and will often require slightly different ignition timing. Hence, unless your engine is in the same condition as the one the factory used to determine the "book" settings, then they will not be quite correct for your engine.

I would say that the extent to which the above is true will vary with different engine designs, and if the "factory" settings work well then that's great, use those and leave well alone, and in any event they are a good place to start. It's just that many of us have found that old engines don't run optimally on factory settings, and the above is the reason why.

[outdoorjoe]

quick question:

my neighbour has given me a timing light which has two wires (i was expecting 3) the two wires have crocodile stlye clips so look like battery connections.

anyone know how this works? it doesnt come with any instructions

cheers
joe