This small engine fires 4 degrees ATDC. I was under the impression it occurs BTDC ? Any advice is much appreciated 
Model Engine Spark Timing
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because its low rpm?
Spark timing is all about ensuring the flame front (ignition of fuel, and thus the controlled burn of combustable fuel air to provide pressure on the top of the piston, and thus the power stroke) provides the most duration to the piston surface during the power stroke.
With a fast rpm motor, you need to advance the timing, so that the spark ignited flame front can in effect catch the piston at the right time, high rpm, and too late ignition would mean the flame front would not catch the piston surface as it was already on the down stroke, and thus provide little to no power.
the other end to that is timing too far advanced for the piston speed, and the flame front reaches the piston top before the piston is actually at top dead center, ie still moving up, so now you have the flame front, ie combustion pressure trying to push the piston back down before it is ready, and pinging occurs, it sounds like someone is hammering the tops of the pistons, its the noise of the flame front hitting the piston and the piston still trying to push up against it. Not good for the engine.
So some motors have static timing, and they are rpm limited by lack of spark advancement.
Others have basic weight and spring, and others have both weights and springs, and vacuum advance for further advancement of timing at high vacuum, low load situations.
All modern computer controlled engines have pre mapped ignition curves that are dependant on rpm, manifold vaccum, throttle position, gear selected, fuel type in use, road speed etc etc
In your little motor, its such a low rpm motor, any advance of spark timing before tdc would result in the flame front hitting the piston surface before the piston is on its power stroke, ie btdc, and it wouldnt run, just kick back the other way.
Make sence ?
Pre ignition is what happens when your fuel air mix ignites before the spark plug ignites the mix, resulting in both 2 or more flame fronts hitting each other, and most usually, the resultant flame front/s hitting the piston before its reached tdc.
Thus fuel has different octane ratings, ie its resistance to ignite prior to the spark plug firing, for motors that might cause pre ignition. (higher the octane rating, the harder it is to ignite the mixture, or less likely it is to be ignited by a secondary ignition source).
Causes can be, high compression, poorly designed combustion chambers, with hot spots that can cause the fuel air to ignite prior to the spark, or build up in the combustion chamber, piston top of carbon that can ignite/ glow, and cause the fuel air mix to pre ignite prior to the spark plug firing.
Thus endeth the lesson.
With a fast rpm motor, you need to advance the timing, so that the spark ignited flame front can in effect catch the piston at the right time, high rpm, and too late ignition would mean the flame front would not catch the piston surface as it was already on the down stroke, and thus provide little to no power.
the other end to that is timing too far advanced for the piston speed, and the flame front reaches the piston top before the piston is actually at top dead center, ie still moving up, so now you have the flame front, ie combustion pressure trying to push the piston back down before it is ready, and pinging occurs, it sounds like someone is hammering the tops of the pistons, its the noise of the flame front hitting the piston and the piston still trying to push up against it. Not good for the engine.
So some motors have static timing, and they are rpm limited by lack of spark advancement.
Others have basic weight and spring, and others have both weights and springs, and vacuum advance for further advancement of timing at high vacuum, low load situations.
All modern computer controlled engines have pre mapped ignition curves that are dependant on rpm, manifold vaccum, throttle position, gear selected, fuel type in use, road speed etc etc
In your little motor, its such a low rpm motor, any advance of spark timing before tdc would result in the flame front hitting the piston surface before the piston is on its power stroke, ie btdc, and it wouldnt run, just kick back the other way.
Make sence ?
Pre ignition is what happens when your fuel air mix ignites before the spark plug ignites the mix, resulting in both 2 or more flame fronts hitting each other, and most usually, the resultant flame front/s hitting the piston before its reached tdc.
Thus fuel has different octane ratings, ie its resistance to ignite prior to the spark plug firing, for motors that might cause pre ignition. (higher the octane rating, the harder it is to ignite the mixture, or less likely it is to be ignited by a secondary ignition source).
Causes can be, high compression, poorly designed combustion chambers, with hot spots that can cause the fuel air to ignite prior to the spark, or build up in the combustion chamber, piston top of carbon that can ignite/ glow, and cause the fuel air mix to pre ignite prior to the spark plug firing.
Thus endeth the lesson.
Thanks Grant, really, thank you. I will have to re read this a few times to take on board all that you have shared here, but I get the crux of your message, thank you!Spark timing is all about ensuring the flame front (ignition of fuel, and thus the controlled burn of combustable fuel air to provide pressure on the top of the piston, and thus the power stroke) provides the most duration to the piston surface during the power stroke.
With a fast rpm motor, you need to advance the timing, so that the spark ignited flame front can in effect catch the piston at the right time, high rpm, and too late ignition would mean the flame front would not catch the piston surface as it was already on the down stroke, and thus provide little to no power.
the other end to that is timing too far advanced for the piston speed, and the flame front reaches the piston top before the piston is actually at top dead center, ie still moving up, so now you have the flame front, ie combustion pressure trying to push the piston back down before it is ready, and pinging occurs, it sounds like someone is hammering the tops of the pistons, its the noise of the flame front hitting the piston and the piston still trying to push up against it. Not good for the engine.
So some motors have static timing, and they are rpm limited by lack of spark advancement.
Others have basic weight and spring, and others have both weights and springs, and vacuum advance for further advancement of timing at high vacuum, low load situations.
All modern computer controlled engines have pre mapped ignition curves that are dependant on rpm, manifold vaccum, throttle position, gear selected, fuel type in use, road speed etc etc
In your little motor, its such a low rpm motor, any advance of spark timing before tdc would result in the flame front hitting the piston surface before the piston is on its power stroke, ie btdc, and it wouldnt run, just kick back the other way.
Make sence ?
Pre ignition is what happens when your fuel air mix ignites before the spark plug ignites the mix, resulting in both 2 or more flame fronts hitting each other, and most usually, the resultant flame front/s hitting the piston before its reached tdc.
Thus fuel has different octane ratings, ie its resistance to ignite prior to the spark plug firing, for motors that might cause pre ignition. (higher the octane rating, the harder it is to ignite the mixture, or less likely it is to be ignited by a secondary ignition source).
Causes can be, high compression, poorly designed combustion chambers, with hot spots that can cause the fuel air to ignite prior to the spark, or build up in the combustion chamber, piston top of carbon that can ignite/ glow, and cause the fuel air mix to pre ignite prior to the spark plug firing.
Thus endeth the lesson.
That was very good Trains, next lesson maybe you could tell him about the evils of lean burn engines and detonation. Too many don't understand the difference between pre-ignition (pinging) and detonation.
some light bedside reading here.
Further reading
and another good one.
http://www.contactmagazine.com/Issue54/EngineBasics.html
Excellent article ! Best one I've read for some time.
