Detonation probs.

[evo550]

I recently experienced some pretty bad detonation after about 2 laps into a moto.
I had fitted a new set of rings before race day and after about two laps the tell tale sounds started from the midrange up.
The track is deep sand/loam so the bikes working pretty hard (KTM250 2t)
Never experienced the problem before, just after fitting new rings.
Only did it from about 3rd gear up from midrange up.
Jetting is standard except needle is on leanest clip and main is 2 sizes richer. r30 @ 50:1 98 unleaded caltex.
Any thoughts anyone ?

[caps 999]

way too lean

[head]

Try dropping needle clip (Raising needle) one clip at a time. New rings more compression, deep sand is more load, over cast day is more air. Should fix by just adjusting needle.

[evo550]

Does needle affect jetting at 3/4 to wfo ?

[Lozza]

Does needle affect jetting at 3/4 to wfo ?

Yes and no, it has an effect but not the major influence. A pic of the piston crown or do a plug chop will answer all the questions. New ring would not give that much more compression. Sounds silly but did you reuse a base gasket? A pinched O ring can set off deto to.

over cast day is more air.

There is never 'more' oxygen in the air(stable at 20.9%), what changes is relative air density, effectively leaning out jetting, but this has to change a fair bit to upset jetting that much.

[evo550]

Funny you mention base gasket Lozza, I put a new one on, but it was in a cometic kit and was tin as opposed to paper genuine ktm one.
At the time of instillation it looked thinner.
I don't feel there is an air leak as it runs fine otherwise, no over rev, high idles that kind of stuff.
It has only started after the ring change, nothing else has changed and 2 weeks ago was running fine.
I would have thought that a main jet that was 2 steps richer, would offset any lean needle position at 3/4 to wfo throttle settings.
Will get pics of plug and piston tonight.

[evo550]

Posted pics of plug

[Moto]

If you have installed a thinner base gasket you will have moved the cylinder+head down and the piston will go further up,if you get my drift.

 KTM in all their workshop manuals make a lot of reference to dimension "X" this is the distance between the top of the piston@TDC and the top of the cylinder.It is different for each model. This must be the correct measurement (increased compression ratio) and is adjusted by various thickness base gaskets, ( .2mm,.3mm,.5mm and 1mm are available)Is the old gasket made up of more than one gasket?  I would take the head off and check this measurement if you think you have put a dodgy base gasket on.

[oz555ktm]

  What year is it ..

  you Must all way rest the X-Factor on a KTM  when you do a over-haul .

  Need to Know the year   Engine No or Date Wheel under front  sproket..

     

[oldyzman]

From what i have heard, experience and what i have read - of which i am open to correction.....

If the head has a squish band then you should consider the measuremeant between it and the piston at TDC. A 250 should have 0.8mm to 1.0mm of squish. if you get this right by adjusting the thickness of the base gasket then the comp shoud probably be automaticaly correct thats if the pison is correct for the bike. You can get this measurement by putting  a line plastascene in the comb chamber or if you are keen then get a piece of soft solder approx 1.5mm thick-feed it into the comb chamber through the spark plug kick the bike over by hand - pull out the solder and measure the flat spot of the solder with a vernier caliper,  i do this measurement left and right and fore and aft on the piston, fore and aft are not too accurate cause of piston rock. you can even soften the solder a bit more by putting it in a cup of boiling water first.
Brett

[evo550]

  What year is it ..

  you Must all way rest the X-Factor on a KTM  when you do a over-haul .

  Need to Know the year   Engine No or Date Wheel under front  sproket..

     

......'09.

There is no head gasket, just "O" rings. So a diff thickness base gasket would alter compression. But short of puillng the new one off and measuring them both it's hard to tell.
Does the plug give away any clues, looks o.k to me.
How does higher compression bring on detonation? Heat?

[Lozza]

Dropping the cylinder will give a higher corrected comp ratio due to a lower  exhaust port, it also closes up the squish gap which alters Maximum Squish Velocity(MSV) which can cause the onset of deto. But mx 250's can handle high MSV numbers anyway. The plug looks OK but look with a magnifying glass for any ceramic boil. Come on lift that head evo

[evo550]

Come on lift that head evo

Noooooo, don't make me do it
Tommorrow, stay tuned.

[Ji Gantor]

Another easy test to see it there is a base, head gasket or intake manifold air leak is to start the bike, while running spray start you bastard around test areas. If the engine revs up you have an air leak.

Ji

[evo550]

Just for Lozza's viewing pleasure,