timming advance

Steamboat Aka J. R. Stevens /

I'm not really into 2-strokes but I would like to know if they have a means to advance the ignition timming either vacuum, centrifical using springs or???? jim

Re: timming advance

Don Pflueger /

all you can do is rotate the stator plates. there is no advance like in a car. the only thing that comes close is a cdi ignition which advances itself electronically as rpm increases.

Re: timming advance

Steamboat Aka J. R. Stevens /

Thanks Donp,I kind of thought that. Every thing else being the same, wouldn't a CDI ped out perform a points ped? jim

Re: timming advance

Yep,usually. But the best way to check timing is with an induction type timing light and hook it to any ,usually 12 volt battery. Paint a line at the proper timing setting,say 23 degrees BTDC or so, and start it up on the stand. The light doesn't lie. don-ohio (:^)

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Don Pflueger /

ignition system wise yes. probably part of the reason a tomos a35 is faster than an a3n.

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BOO, don, I'd have to disagree... but I've riden only one A-35. I love my points-type A-3! lol.

Re: timming advance

Don Pflueger /

disagree? why? cdi ignitions electronically advance the timing at higher rpms much in the same way as weights do in an automotive high performance ignition. this makes the engine fire more in time with the piston movement therefore making it run a little faster and more precise.

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Blah with all that technologly junk! Why am I at this computer?! I'm stuck, lost, in the good ole days, when the A-3 ruled Tomos' realm... I like the old stuff, I can work on it without all those special tools and stuff.

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Don Pflueger /

aahhh but the nice thing about cdi is it either works, or it dont. nothing to adjust, set, ortime other than making sure you have the flywheel key in place.

Re: timming advance

Hi, I found a site, Dan's motorcycle repair and engine repair. He has an index and a section on ignition timing of motorcycle engines. He does not recommend advancing the timing on small two strokes. Some good reading there.

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ageed.. the ideal 2-stroke timing advance/retardation curve is almost the exact opposite of 4-stroke engines.

If a programmable ignition is available for a 2-stroke, there's lots of advance at low RPM and it falls off as the engine speeds up ..

Reasons include improved volumetric efficiency at higher RPMs... A larger quantity of cleaner, faster burning mixture requires less advance.

Re: timming advance

Don Pflueger /

if you ask me, i would think you would want to advance the timing as rpm increases. the faster the piston is moving, the sooner you want the spark. otherwise it would fire closer to tdc which would not allow for maximum use of the combusted fuel. you want to time the explosion so that it occurs just as the piston reaches the point just past tdc. so in order to achieve that at a faster rpm, you need the spark sooner. retarding the timing would cause spark at a later time which would mean the piston may be on the downstroke when the explosion occurs. it seems like it would push it down faster, but wouldnt you want that push from the maximum point in which the piston has just started its down stroke? which should be 1 degree atdc.

Re: timming advance

it's an odd concept and counter-intuitive but it seems that two strokes are different than 4-strokes.. Actually things are not so different. Modified 4-strokes with high lift cams and other things experience it too, to some degree.

The objective of ignition timing is to produce a spark that ignites the fuel mix at the "right" time. But what is the right time? Right is whatever causes peak cylinder pressure at some stage in the piston's travel downward past TDC. But that "right" spark time can vary according to more factors than just crankshaft position.

For instance, a lean mix burns faster than a rich mix. A faster burning mixture requires less advance than a slow burn. Peak cylinder pressure would occur too soon if ajustments are not made to accomodate the faster burn.

Another thing is compression ratio. High compression ratios require less advance for similar reasons.. But can compression ratio change while an engine is running? Yes it can. If an engine is more efficient at some RPM than another, the effective compression ratio gets higher. More mixture is being compressed into the same combustion chamber space and the actual, true compression ratio is raised. A retarded spark would be called for.

Evidently, cylinder filling in 2-strokes is very inneficient at low rpms, no doubt due to the lack of valves.. At higher RPMs the inertia of the fuel mix and other factors improve cylinder filling. In contrast, volumetric efficiency in unmodified 4-strokes is more efficient at low RPMs than at high.

Add to this 2-stroke a performance exhaust and the engine suddenly gets very efficient volumetrically in a narrow, high RPM ranvge. .. at the same time the mix is clearer of residual exhaust gasses when that expansion chamber kicks in.. So timing should be retarded from what it is below that power band... because the fuel mix is much quicker burning.

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btw, don .. you're probably saying to yourself "But the engine is spinning faster at high RPMS so there is less time for even a fast burning mixture to burn."

Actually this is true to a degree and it is part of the equation. But the cleaner, more compressed mixture burns so much faster that timing still needs to be retarded.

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Don Pflueger /

you must have been reading my mind. that is what i was thinking about and not thinking about a leaner mix or compression ratios. so i understand totally what you are saying.

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if i didnt come across this in a couple books it never would have occured to me.. and it took me a long while to understand what the they were saying.

So many tuners in another forum are fixated on jetting. Larger jets are the name of the game. Why? Because when they add an expansion chamber, plug chops suddenly show a lot of heat up in the power band..

Nobody wants to retard their timing .. I guess the word 'retard' or 'retarded' is uncomfortable. So, drown the heat caused by too much spark-advance with more fuel and run way too rich. At least nothing got 'retarded' ;)

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Steamboat Aka J. R. Stevens /

http://www.pwctoday.com/viewtopic.php?p=470168 Check these posts, they have lots of questions.

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yup, but, I still likes the old junk. Too much $$ for new stuff like that! "You can get ______ (fill in blank) for a dime a dozen! If you aren't gettin 13, you got ripped off!" My grandpa says that.

Re: timming advance

From what I've read,it depends upon the engine rpms, and a lot of times when you retard timing at a high rpm, you can gaIN POWER.

Tab researched this and documented it on a DYNO and it is in the Mopedriders' forum. Search there to see comparative HP and rpm with different timing settings. don-ohio

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