Two Stroke ignition timing - Two Questions.

[T250K]

G'day guys,
               Most of my time fooling with engines was with 4 strokes, so I was a bit surprised to check the timing on my PE 250 recently. 

  Eleven degrees BTDC at idle, advancing to 19 deg BTDC at around 5,000 rpm, and then rapidly dropping back to 11 deg as engine speed increased beyond 5,000.    Even with the totally different breathing characteristics and pipe supercharge etc this feels like very late timing at high engine speeds.

Question (1)    Anyone have any comments please ?

Question (2)    The above got me thinking that the timing on some older engines with fixed timing could be pretty crook.    I have an Ossa Phantom GP2 which I hope to rebuild shortly.    The Motoplat rotor was damaged when purchased and will need to be replaced.     Can possibly find a good used one on ebay but if not, a new rotor is almost as expensive as a PVL or similar aftermarket system with inbuilt timing advance.
                     I don't need to have the engine standard to be eligible for competition at this stage, and if I go to aftermarket ignition I can work around any change in flywheel weight.

Thoughts and advice appreciated.              Thanks,      T250

[lukeb1961]

Sounds correct.

PE250B  23.0 BTDC @ 6000 rpm
PE250N  18.0 BTDC @ 6000 rpm
PE250T  13.5 BTDC @ 6000 rpm
PE250X  13.5 BTDC @ 6000 rpm
PE250Z  13.5 BTDC @ 6000 rpm

Have a read of this.
http://www.geocities.com/a57ngel/moto/CDI.html
remade the CDI on his KDX and the link at the very bottom goes to the page where he alters the timing and explains the changes he did versus an RM250 curve.

I currently have a CDI nearly down to the circuit board and in a week or three should have an alternate built.
Do expand on what you are doing and what you have found!!

Luke

[T250K]

Thanks Luke,
                 My PE is a Z model so it looks like the timing is only a couple of degrees late.   I was just surprised to find so little advance at high engine speed.      I know 2 strokes need to run late to rev, and that detonation is bad news but an equivalent 4 stroke from the eighties would have run 35 to 40 degrees BTDC at 6,000 rpm.

In my case the PE ignition needs to be tidied up and I think I'll go for a replacement on the Phantom, so was thinking about maybe improving engine performance with modified advance curves while I'm at it.     

It's encouraging that in the reference you gave, the best ( modified ) curve for their KDX was with 19 degrees of advance at 6,000 rpm and then retarding fairly rapidly as the revs increased.    OK, there will be design differences which might allow more advance on the KDX but that's still a 50% increase in advance compared to the PE at the same speed and they're both sold as enduros. 

Could be a lot of room in here for tweaking the ignition curve for a particular use, or for improved performance to complement porting, head or pipe changes so wiil keep working on it.

 



   

   

[Lozza]

But this is  'proper' engine innit, it doesn't need all that advance especialy on pipe, off pipe advance seems a little low. Playing with programmable ignitions has revealed a certain advance that seems to be universal amoungst both OEM and custom curves.

[Lozza]

Only briefly, what's up? Better options these days,

[T250K]

For sure Lozza,
                    ' Proper ' engine is right, much cleaner combustion chamber shape and better flame speed in a 2 stroke.     Seems there's still scope in there for fiddling tho, for example once you had settled on pipe and porting mods for a particular engine would you use a different advance curve to move the torque around in the speed range to suit different circuits ?

And how much to be gained in a thirty or forty year old engine by switching from fixed timing to modern electronics with ( to some degree  ) programmable advance ?