Ignition advance angles

[507VBH]

Hi,

I am a bit confused about ignition timing on Lambretta engines. With a 4 stroke engines the timing advances with speed but it seems from reading the forums the timing decreases with speed. Why is this? Does the timing retard from 19 degrees at idle as speed rises or does it tetard from some larger idling angle?

Cheers!

[missing lynx]

if you've got a std ignition set up it stays constant

[CHRIS in MARGATE]

If the timing components are in perfect condition and absolutely standard then the firing point should remain reasonably constant at its preset point. You might see it wobble by about 1 degree on a strobe when revved.
Our little old Lambrettas start better and perform better with the ignition timing set as per Innocenti.
However and a big however, they won't last long nowadays with 50+ years of tuning, fettling and fitting non standard parts BUT one of the biggest problems can be attributed to the fuel we can now buy which has far less pure petrol content and can lead to overheating and pre ignition and certain failure at the top end.
Many enthusiasts advocate setting the ignition anything up to 8 degrees retarded from the factory setting but this is far from ideal and as such a compromise can be achieved by fitting an aftermarket ignition retarder so that the engine starts better and runs better at low speed but retards to a safer firing point at higher revs.

A far cry from a machine designed to get us to work and back and do the shopping.

[Eden]

Standard timing should stay static if all parts are good but yes if using an advance retard system on any two stroke engine the ignition retards as rpm rises but only after a certain RPM which is dependent on the specific motor and set up, which as you rightly say is the opposite to what four stroke engines do.
In simple terms the reason two strokes need to retard as revs raise is due to the speed the flame front from combustion spreads across the combustion chamber, retarding (or delaying) the point of ignition prevents the combined forces of the expanding flame front together with the increasing compression as the piston reaches TDC reaching a point where the max pressure is reached before the piston has passed TDC. If it did it would soon hole the piston. Its desirable to have the max pressure occurring slightly after TDC and at the same point throughout the rev range, retarding makes this possible.
At higher rpm it takes less time for the piston to reach tdc than it does at lower rpm so the spark needs to happen earlier at low rpm and later at high rpm.
This is why motors running static timing which is set for safe running at high rpm feel laggy and under powered at low rpm and often use more fuel at lower rpm that they do when reving higher. Because the max pressure is occurring too late after TDC.
Retarding at higher rpm also allows the engine to rev on higher which in turn requires slightly more retard and so on.

These are the reasons I only use fully programmable advance retard systems so the ignition can be set to work with a specific engine or/and use rather than static which is the least desirable or an adv/retard system that has set generic curves.
In the two stroke world one shoe doesn't fit all!

[507VBH]

Ok - looks like the situation with 2 strokes is a lot more complex than with 4 strokes! Do the available ignition retarders use the same pick up on the stator as ordinary CDI units or is a special stator assembly required?

Cheers

[CHRIS in MARGATE]

I personally use an Agusto 6000. You can get a 7000 and an 8000. The 6000 denotes rpm and the point of maximum retard. This works for ME and has done so for about 9000 miles. My firing point is 21.5 BTDC at 1500 rpm; 17 at 4000; 15 @ 6000 and at 8000 it is about 14 - 14.5.
These units are retro fit and interrupt the feed to the CDI but you can't just plug it in and go. You need to make sure TDC point is accurate because you are going to advance ignition to a potentially harmful firing point. Must be set up with a strobe. For me I am normally cruising at 5500 so I am happy that my timing at this point is approx 16 degrees BTDC.

This as I said works for ME, and I have an engine that is fuelled correctly and runs fast and sweet.
No 2 engines are the same and you could invest in a programmable advance/retard unit and fine tune to your requirements.

[507VBH]

This all seems a bit odd to me! None of these retard units take account of engine load i.e. inlet manifold pressure. Is that right?

[Warkton Tornado No.1]

Two strokes are different to four strokes in their requirement for ignition timing.

Both 2T & 4T need to fire before the piston reaches TDC.

However, whereas the 4T needs further advance as the revs rise, the 2T needs less advance, in fact should be retarded automatically, in an ideal World.

This will only occur only with the addition of a module to the standardish electronic ignition or by using a specific electronic ignition with the advance/retard built in.

Many years ago, the likes of Motoplat discovered that a very rapid advance, then gradual retard was the better set-up for two strokes & if that principle has been bettered, I am unaware.

The reason that the ignition can, & should, be retarded is that as the volume & speed of input charge increase into the engine, the stratified fuel charge changes. Instead of large molecules of fuel comprising the ‘mist’ there will be many smaller molecules of fuel in the ‘mist’

This means that those molecules will have a much greater surface area & consequently burn better & quicker.

That is the reason for the preference in automatically retarding of a two stroke ignition.

[Eden]

I've used the programmable Kheper for the last 2+ years (no idea on miles but its a lot) and currently i'm beta testing the new version 3 Kheper which has many more options regarding setup and many more set and programmable curves which can be stored in the unit as apposed to the one the last version held.

The kheper is also retro fit as it just replaces the CDI, this is another bonus due to the much higher voltage output it gives compared to any standard type CDI.

Ive had the V3 Kheper on for about a month in which time ive covered over 1700 miles both in the UK and on track in France. Its been faultless to date.

Another feature it has which is useful if you find you have filled with bad fuel and are running hot or pinging or if you find you are at altitude and running rich or whatever other reason you may find your self in that would benefit from a tweak, is that you can alter the curve while the engine is running to adjust for the conditions you find yourself in using an app on your mobile phone, the curve resets to what it was originally programed as once the engine is turned off. Using the app you can also see how much advance or retard is being applied at any given RPM live while riding.

They are or will soon be available from here http://kheper-scooter.com/ and imo they are cheap considering what you get at only 140 Euros, A good standard CDI and an Augusto unit costs a little less but is by far an inferior option.

[Warkton Tornado No.1]

This all seems a bit odd to me! None of these retard units take account of engine load i.e. inlet manifold pressure. Is that right?

I think concerns of that nature to be restricted to forced induction.

Although it is possible to supercharge a two stroke engine (Ships diesel two stroke engines are supercharged & consequently are (or were!) the most efficient commercially available engine) there are very few manufacturers do so, although Lambretta did with their 200 kph 125 cc World Record breaker.

[507VBH]

Two strokes always were black magic!

[missing lynx]

what engine setup have you got? if it's more or less standard the just stick with standard ignition and don't worry about advance and retard units

[507VBH]

I have a GT186 with 25mm Dellorto and a clubman exhaust.

[Eden]

Just run it at 17 or 18 degrees fixed. With no expansion you don't need to worry much about any retard.

[burnside]

Both my GT186 and GT200 are on SIL electronic ignition with timing set to 17 degrees fixed....a few thousand miles under the belt and no issues

[507VBH]

Does using 98 octane petrol help?

[rossclark]

Increasing the octane level will certainly assist in preventing detonation though while most of the premium types may contain less or no ethanol they also include other additives such as detergents etc.

[CHRIS in MARGATE]

But NOT the Tesco High Octane because it is reported to have far too many other additives.

[burnside]

I always use high octane, BP and Shell usually but also Sainsburys at times

[Warkton Tornado No.1]

I know personally through experience that a good electronic ignition optimised through its advance/retard works very well & increases the flexibility of the engine with no detriment.

However, my advice to anybody would be to consider that option if buying an electronic ignition for the first time or replacing a failed system.

A well set-up ignition is not just for ‘tuned’ engines. They lessen the compromise we must otherwise take with our shopping machines in this current age.

Just for balance, though, the other thing I will reiterate to any readers is that the quickest Lambretta on the track is fielded by the Saxelbys & amazingly has fixed Motoplat ignition. They also removed the anti-dive @ the last Cadwell……