OzVMX Forum
Clubroom => Tech Talk => Topic started by: evo550 on May 30, 2010, 09:27:51 pm
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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 ?
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way too lean
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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.
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Does needle affect jetting at 3/4 to wfo ?
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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.
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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.
(http://i118.photobucket.com/albums/o82/evo550/Image0034.jpg)
(http://i118.photobucket.com/albums/o82/evo550/Image0020.jpg)
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Posted pics of plug
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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.
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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..
?????????????????????
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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
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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?
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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 ;D
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Come on lift that head evo ;D
Noooooo, don't make me do it :P
Tommorrow, stay tuned.
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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
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Just for Lozza's viewing pleasure,
(http://i118.photobucket.com/albums/o82/evo550/Image0045.jpg)
(http://i118.photobucket.com/albums/o82/evo550/Image0056.jpg)
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There appears to be no hints of detonation there.
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What model + size KTM is it? We can then tell you what dimension " X " you should have.Once you have measured this is and know it is correct it can be eliminated from being your problem.
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'09 250 2t.
There appears to be no hints of detonation there.
Yeah,
I was suprised at how clean it was, considering it's up to 50 odd hrs running Castrol R30, which has a bit of a reputation for excessive carbon build up.
I think I will raise the needle a clip or two and replace the base gasket with a genuine item as a first step.
Thanks for your help guys, will keep you posted, racing next w/end.
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'09 250 2t.
There appears to be no hints of detonation there.
Yeah,
I was suprised at how clean it was, considering it's up to 50 odd hrs running Castrol R30, which has a bit of a reputation for excessive carbon build up.
I think I will raise the needle a clip or two and replace the base gasket with a genuine item as a first step.
Thanks for your help guys, will keep you posted, racing next w/end.
I think the 'reputation' is a load of shite
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That Looks Like a wisco Piston????
Mahle (KTM ) Piston are flater on the top of the piston .
Wisco seam to be allways Higher .
I have found that some time the Crown could be .4 of a mm Higher ..
So you need to all way check the X factor .
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Piston is Vertex, OEM.
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If all you have done is changed the rings and replaced the base casket and it was running fine before. You have raised the compression.
You need to measure the thickness of the old casket and replace with one the same.
You could try BP 100 fuel and see if that stops it.
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..............but where is the deto? Plug looks fine (even the burnback looks good)and there is no evidence on the head and piston??? There is something at around 8 o'clock on the head but it does not look like detonation.
p.s. You have bad 'pie cut' which means the head/cylinder mating is not straight.
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What are the signs of Deto?
What's a pie cut ?
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What are the signs of Deto?
What's a pie cut ?
On the head at about 8 o'clock there are some marks without looking at it closely .I can't make it out from the pic. 'Pie Cut'.........I dunno either I just said that to make myself seem intelligent/scientific ::) ;D But study the burn pattern closely ;)
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Bit of a difference in thickness, the genuine one being the thicker of the two.
Although KTM list 4 different sizes, the thinnest being .25mm :o
Is it possible to run higher compression and jet to suit.
(http://i118.photobucket.com/albums/o82/evo550/Image0058.jpg)
(http://i118.photobucket.com/albums/o82/evo550/Image0059.jpg)
There is something very Ji about this thread ;D
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The thinner cometic gasket has raised your compression enough to ping on your current fuel. It will probably run well on better gas, but if you don't want to switch fuels just reinstall a 0.8mm base gasket.
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My calculation says you have `1cc less due to the gasket, I still can't see that causing problems. But I'll go back to the piston again and I still don't see any evidence of deto on the crown or the head. Here's a pic of some good deto, you can see the difference in the colour of the crown and the pitting under the plug.
(http://i155.photobucket.com/albums/s297/Lozza85_2007/post-24069-127559839242.jpg)
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Hey Lozza,
It might be a giant coincidence [with the ring change] but what about some kind of ignition problem?
Could some high rpm misfire be mistaken for pinging [or pinking for the US members]? A dodgy plug cap or high tension wire?
Thanks
VMX42
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Ignition timing looks perfect according to the plug.
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could it be a crap load of fuel?
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Hey Mick Ive noticed a pinging noise at Nepean when the outfits come 'round the speedway bend, I always assumed that it was the passengers head-butting the witches hats! cheers wally.
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it`s the metal plates in our heads
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OK.
It appears KTM offer 4 different thickness base gaskets.
When the cylinders are cast then gasket surfaces turned down there are slight variations in height between head and base surface of each cylinder, before the cylinder is bolted to the bike it's measured and matched to a base gasket. This is done to ensure each bike that comes out the factory is running the same (correct)compression ratio.
This process is also done for pistons.
Their advice was to stick with the original gasket thickness.
Seems that's what I'll do.
Their advice was also that you maybe able to jet to prevent deto with thinner base gasket, but would probaly blubber elsewhere (YZ490 comes to mind)
Lozza,
How long does it take for deto to cause the type of damage you demo'd ?
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Sometimes 1 WOT run sometimes a whole race but it doesn't take long, the delay is between when the deto occurs and you notice some drop off in performance.
Jetting to prevent deto sounds like pretty dumb advice to me, retarding the timing a bit or using a lighter power valve spring would be better.
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Jetting to prevent deto sounds like pretty dumb advice to me, retarding the timing a bit or using a lighter power valve spring would be better.
Yeah, KTM didn't recommend it either, I asked if it was possible, they said maybe, but........
It's far less hassle to just spend $4 and put the right size gasket in.
Race day this Sunday, will see how it goes.
Thanks for everyone's help.
P.S. already running the "Angry" spring.
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already running the "Angry" spring.
Always good to know!