Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: evo550 on May 30, 2010, 04:17:13 AM
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KTM 250SX
Last race day I experienced bad detonation from mid range up after about 2 laps of racing. Was a nice autumn day not too hot.
Track is deep sand/loam, bike does work hard, but has never happened before.
Fitted it's first set of new rings before race.
Jetting is standard, except needle is set on leanest clip and main is 2 steps richer.
I don't think I have a air leak as runs great up until a few laps into moto, then sounds like ball bearings in a tin can. Pull over, revs out in netural no prob , take off again, then after 1/2 lap under load it comes back.
98 octane fuel r30 @ 50:1
Any thoughts ?
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Your getting hot fella, try lifting the needle a notch or two, and check ign timing has not slipped, we also had this on two of our bikes, one was to lean and the other had the inner core in the tailpipe crack and partially block the tailpipe causeing heat to build in the cylinder.
Just thinking about what you have said abount new rings would suggest you have more compression and/or got the squish wrong ?
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Hi there. I have one strange sound on lo rpm. Like some loud drumming sound inside the front exhaust pipe. That stuuff disappears when i hit the gas. and the bike runs a bit loud in general. 1999 cr 250r
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Some of the KTM's have plastic water pump impellors and they will spin on the water pump shaft. It doesn't happen that often but I've learned the hard way to check the water pump when trying to figure out why the jetting has suddenly "changed".
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Some of the KTM's have plastic water pump impellors and they will spin on the water pump shaft. It doesn't happen that often but I've learned the hard way to check the water pump when trying to figure out why the jetting has suddenly "changed".
Along those same lines, using coolant/antifreeze that is high in silicates can literaly eat the impellor vanes.
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Had a similar problem (almost identical conditions) at the first race meeting I went to with my brand new 144. Richened the jetting progressively after each race and the problem occurred later in each race. Thought it needed a colder plug (didn't have one) and by this stage it was that rich it was blubbering. I was devastated, my shiny knew KTM was a piece of crap :(
Anyway, after kicking the dog and thinking "KTM couldn't have f@#ked up that badly", put everything back to the standard settings and went and got some more fuel. Bugger me, a few minor adjustments and it ran perfectly and has done ever since.
All I can put it down too was a dud batch of fuel (BP Ultimate). Maybe get some fresh fuel and give it another shot. If not, I'd raise the needle.
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Couple of pics of plug, see what you think.
(http://i118.photobucket.com/albums/o82/evo550/Image0034.jpg)
(http://i118.photobucket.com/albums/o82/evo550/Image0020.jpg)
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You look to still be a tad rich by the tar on the plug, the ignition looks OK, time to lift the head.
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The plug seems OK, the insulator is brown, no metal fragments...
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O.K.
Seems as though jetting is o.k.
Started it up, coolant circulating fine.
Dodgee fuel (gas) option or
Thinner base gasket, excess compression ??? is this a possibility?
What signs do detonation leave on piston top ?
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Eh, forget that! Base gaskets don't really affect compression - i counted! The only affecting thing is the head gasket. And the head itself. :)
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Eh, forget that! Base gaskets don't really affect compression - i counted! The only affecting thing is the head gasket. And the head itself. :)
A thinner base gasket has the exact same effect on compression that a thinner head gasket has. Either way the head is moved slightly closer to the piston, raising compression and reducing squish clearance.
The base gasket also affects port timing.
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Not really!!! They're so thin, let's count:
A usual gasket is 0,45mm thin. My lower gasket is made of magazine cover and it's 0,27 mm.
the hight difference is 0,18 mm only. Multiply it into Pi and a sqr of half bore and you get 0.6 cc as a difference. Huge, isn't it...
That Doesnt really change anything. Maybe the compression will be 11,3 instead of 11.
When someone cuts the cylinder head, they cut a 1 mm of metall! That can make difference!
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Not really!!! They're so thin, let's count:
A usual gasket is 0,45mm thin. My lower gasket is made of magazine cover and it's 0,27 mm.
the hight difference is 0,18 mm only. Multiply it into Pi and a sqr of half bore and you get 0.6 cc as a difference. Huge, isn't it...
That Doesnt really change anything. Maybe the compression will be 11,3 instead of 11.
When someone cuts the cylinder head, they cut a 1 mm of metall! That can make difference!
While I certainly agree that cutting 1mm off the head will make a much greater difference, we weren't talking about cutting the head, now were we?
Is this not what you said?
Base gaskets don't really affect compression - i counted! The only affecting thing is the head gasket.
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On a well properly tuned engine the difference between detonation and not can be a very fine line.If it just happened after a rebuild I'd be tearing it down and cking for gasket thicknesses and air/coolant leaks.If not,I'd drain it and make sure you've got fresh gas,clean carb/passages and no air leaks.Failing that,I'd ck for a head gasket and likely be going in.
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Is this not what you said?
Yep, i'm about to say that again and again. Thin gasket never leads to detonation.
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It depends how closely jetted/timed you are as well as what fuel you use.If you picture your piston at tdc,a thinner base gasket lowers your cylinder in relation to that which puts your piston closer to the top of your head which then means you have less comb chamber area therefore........
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Is this not what you said?
Yep, i'm about to say that again and again. Thin gasket never leads to detonation.
Seriously? You really believe that? Because I have personal experience with a thinner head gasket on my KX putting the engine over the edge and causing it to ping on 93 octane. I had to run 100 octane with the thin head gasket to not have detonation. Replace the head gasket with the standard thickness gasket and the pinging goes away. Imagine that...
Many modern two-strokes are right on the verge of detonating right off the showrrom floor, and it takes very littel to push them over the edge. Anything that raises compression increases the possibility of detonation.
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I'm sayin about base gasket only! The head gasket is way much thicker.
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For your viewing pleasure,
(http://i118.photobucket.com/albums/o82/evo550/Image0056.jpg)
(http://i118.photobucket.com/albums/o82/evo550/Image0045.jpg)
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Eh, forget that! Base gaskets don't really affect compression - i counted! The only affecting thing is the head gasket. And the head itself. :)
Are you serious ?
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I think you should listen to the above people RedFoxCR.
Base gaskets change both port timing AND compression.
It's a bit hard to put different size head gaskets on when there is not one.
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Considerable diffeence in gasket thickness, the thicker one it originally.
Although KTM offer one at .25mm thickness which is thinner again.
It would seem higher compression can be run, but jetting chnbes are required.
(http://i118.photobucket.com/albums/o82/evo550/Image0059.jpg)
(http://i118.photobucket.com/albums/o82/evo550/Image0058.jpg)
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Considerable diffeence in gasket thickness, the thicker one it originally.
Although KTM offer one at .25mm thickness which is thinner again.
It would seem higher compression can be run, but jetting chnbes are required.
(http://i118.photobucket.com/albums/o82/evo550/Image0059.jpg)
(http://i118.photobucket.com/albums/o82/evo550/Image0058.jpg)
KTM offer a wide choice of gaskets right down to 0.06mm allowing us to jack the cylinder around to suit applications, TM Racing also offer a good choice of sizes, must be a Euro thing !
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KTM offer a wide choice of gaskets right down to 0.06mm allowing us to jack the cylinder around to suit applications, TM Racing also offer a good choice of sizes, must be a Euro thing !
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0.06mm are you sure ?
My KTM parts diagram only goes down to 0.2mm.
It seems the reason they offer a few different gasket thickness is to match that particular cylinder the bike has been fitted with. When the cylinders are finished and ready to be fitted they vary slightly in size due to the casting and finishing process. The different gasket ensures that each bike comes out the same performance and tuning criteria.::) ::)
The same thing goes on with their pistons.
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It seems the reason they offer a few different gasket thickness is to match that particular cylinder the bike has been fitted with. When the cylinders are finished and ready to be fitted they vary slightly in size due to the casting and finishing process. The different gasket ensures that each bike comes out the same performance and tuning criteria.::) ::)
Exactly so.While you can play around with different thicknesses depending on what you're after,that is the reason for the discrepancy.