Ignition timing on 2 stroke

[JTurn36178]

I, or should I say we, have two YZ 250's.
(Both have stock expansion chambers, & carbs)
Mine a '96 with a 3" flywheel (with goofy power valve chamber.) Mine has good mid to top pull. But has always had a hesitation off the bottom. I have advanced the timing slowly over a loooong periond of time. As best as I can measure about 3/16" ahead of the factory mark. No ill effects as yet. (By the way It's a real dog at the stock setting.) Every move I have made has improved the performance.

My son's a '94 with a 4" flywheel. His runs great with a little change in jetting once in a while. His timing is barely moved in the advanced direction past the factory mark.

During the rain this week I have done a little research. I have installed a dial indicator in both motors to check the difference from top dead center to the mark on the flywheels.

My son's flywheel mark is .056" before TDC.

My engine flywheel mark is .008" before TDC.(after I have advanced it.) It runs so well at that I sure wasn't going to go back to stock.

I realize they are two different cylinders with possibly different port heights etc. that would cause the differences.

Assuming that the mark on the flywheel is some sore of reference point related to the cdi trigger, on both bikes.

I would like to know if any of you have expierences with these and could explain why his engine tolerates the large difference in advance. (or so it seems a difference.)

I discussed this topic in the general repairs forum once and was blown off as some type of idiot. I'm real close but still like to have fun.

Thanks for any ideas.
Jim

 

[ellandoh]

this subject is beyond me , but i do have one piece of the puzzle which someone who is experimenting with timing and jetting :cool: this is words from an older 2stroke guru i got some advice from at the dunes.
this guy can tell where my timing is at by looking at my plug which he ties all in to jetting.
he was using some 50 cent words about the type of fuel i was using

apparently the point on the electrode where the oil goes from black to cleaned off is a "timing mark" and will move up or down the electrode from timing adjustment?

in case you werent aware of this......you may be but i thought id toss that in for ya to :think:

 

[JTurn36178]

Huh??? Never heard that one before. I'll try to keep and eye out for that..

By the way, while I had the two flywheels off I marked them both with a sharpie to see the difference between the marks. The flat plate that is on the 3" flywheel extends out to 4".

I did notice that the pick up on the two different bikes were located at two different locations. One approximatly 9:30 and the other approx. 11:00 as you look at the clock face. This still does not change the difference between the marks and tdc.

I thought maybe, one of the brains: et Eric or Rich might have heard some explaination over the years..

Well thanks for the input.
Jim

 

[usgpru27]

Ellandoh is correct. You can check your timing by simply looking at the plugs wire over the electrode. It will have a distinctive line where it goes from light to dark. On our roadracing bikes we run this about 80% light and 20% dark. On the CR125 it seems that about 30%dark is a good start.

 

[JTurn36178]

The wire!!!! I will check the difference between the two yz's. I probably ought to start out with new plugs.

I read in Eric Gorr's new book that the more advanced the timing the harder mid range hit and flat top,,
retard, and you get rev to the moon. I also read that I need to raise the ports but that will have to wait till cold weather

I just wonder why the timing marks are so far apart from tdc on these two engines that are only two years apart. They probably have different compressions etc.

Thanks again, all ideas are appreciated
Jim

 

[motometal]

if you do the calculations, .050" on the outside of the flywheel is not really much (I hope I am understanding your situation correctly). Math: (.050/(diam of flywhwl at mark x pi)) x 360 = degrees difference in timing. You will probably come out at around two degrees, which I think would be barely noticeable. When playing with timing, it's a good idea to use at least part race fuel. I'm not a big fan of minor, gradual changes...you can't ever feel what the bike is really doing. Move it a ways, try it, move it back the other way, etc.

If it's too far advanced you should be able to hear it pinging or see it on the plug. Running it a short time this way shouldn't hurt it, unless you are WAY advanced.

If you haven't lost your overrev yet, chances are it isn't too far advanced.

 

[Sherby24501]

Timing effect on a gasoline engine are usually such that at lower volumetric efficencys (lower load) the engine requires more advance to give the best torque. Unfortunately at low speeds detonation (pinking) is caused by end gases starting to burn before the flame front from the spark plug propagates to them (usually only below 6000 revs/min). The noise is caused by the flame fronts coliding (like thunder). Over advance (past minimum advance best torque) puts a higher load on the piston (may caus more mechanical noise) and cools the exhaust gas (more complete combustion before exhaust event) this changes the effect of the expansion chamber because the speed of sound in the exhaust gas is linked to temperature. Excessive over advance also puts an extra load on the plug (heat has to go somewere) and may cause pre-ignition - this is what typically causes holes in pistons, but the leaning of the mixture too far also increases the burn rate (and hence reduces the advance required) and increases the exhaust gas temperature. The effects you descibe could be due to an overly rich mixture increasing the ignition requirements and the pipe tuning better due to the change in exhaust gas temperature.

It is worth noting that the stroke of the engine effects the real timing when using a Dial gauge, but your sons is actually more advanced than yours ! maybe yuors was retarded as originally set (which may explain the very poor performance).

There is a lot of mis-information on the web and in general I would suggest that you try to establish the ignition timing so as to be safe (overadvance is bad for bottom ends). and concentrate on getting the fueling right. Also try using a colder plug if messing around with the ignition as these on 4 strokes (especially high output) are often the weakes link.

Hope this helps

 

[motometal]

"There is a lot of mis-information on the web..."

Sherby...if you were referring to my post or one of the other responses, please detail why, in your opinion, this is mis-information, rather than just generalizing.

 

[Rcannon]

This thread is excellent. I appreciate the info.

 

[Sherby24501]

Sorry no offence to anybody in this thread intended - I find the standard of support offered by all the forum members to be excellent and helpful. Motometals comment on using a knock resistant fuel is a very good additional pre-caution, you can see knock damage on the plug if the mixture is not overly rich, the porcelain around the electrode can look nibbled or eroded - however as I commented before it is advisable to run a cooler plug when experimenting with ignition as pre-ignition can occur even with knock resistant fuels (especially with overadvanced ignition).