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Author Topic: Detonation.  (Read 11865 times)

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Offline evo550

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Detonation.
« on: May 30, 2010, 04:17:13 AM »
 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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Offline 125mx.com

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Re: Detonation.
« Reply #1 on: May 30, 2010, 05:58:41 AM »
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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If it aint broke, fix it till it is

Offline RedFoxCR

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Re: Detonation.
« Reply #2 on: May 30, 2010, 06:13:47 AM »
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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Neytiri:Now you choose your CR 500R. This you must
feel - inside.If he also chooses you,move quick, like I showed.You will have one chance, Jake.
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Offline SachsGS

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Re: Detonation.
« Reply #3 on: May 30, 2010, 02:40:32 PM »
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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Offline Chokey

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Re: Detonation.
« Reply #4 on: May 30, 2010, 05:30:59 PM »
Quote from: SachsGS on May 30, 2010, 02:40:32 PM
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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Offline graham472

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Re: Detonation.
« Reply #5 on: May 30, 2010, 06:32:38 PM »
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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Offline evo550

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Re: Detonation.
« Reply #6 on: May 31, 2010, 01:02:31 AM »
Couple of pics of plug, see what you think.

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Offline 2T Institute

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Re: Detonation.
« Reply #7 on: May 31, 2010, 02:03:25 AM »
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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Offline RedFoxCR

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Re: Detonation.
« Reply #8 on: May 31, 2010, 04:58:53 AM »
The plug seems OK, the insulator is brown, no metal fragments...
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Neytiri:Now you choose your CR 500R. This you must
feel - inside.If he also chooses you,move quick, like I showed.You will have one chance, Jake.
Jake:How will I know if he chooses me?
Neytiri:He will try to kill you.
Jake: Outstanding.

Offline evo550

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Re: Detonation.
« Reply #9 on: May 31, 2010, 05:18:35 AM »
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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Offline RedFoxCR

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Re: Detonation.
« Reply #10 on: May 31, 2010, 05:26:41 AM »
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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Neytiri:Now you choose your CR 500R. This you must
feel - inside.If he also chooses you,move quick, like I showed.You will have one chance, Jake.
Jake:How will I know if he chooses me?
Neytiri:He will try to kill you.
Jake: Outstanding.

Offline Chokey

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Re: Detonation.
« Reply #11 on: May 31, 2010, 09:15:27 AM »
Quote from: RedFoxCR on May 31, 2010, 05:26:41 AM
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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Offline RedFoxCR

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Re: Detonation.
« Reply #12 on: May 31, 2010, 12:47:05 PM »
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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Neytiri:Now you choose your CR 500R. This you must
feel - inside.If he also chooses you,move quick, like I showed.You will have one chance, Jake.
Jake:How will I know if he chooses me?
Neytiri:He will try to kill you.
Jake: Outstanding.

Offline Chokey

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Re: Detonation.
« Reply #13 on: May 31, 2010, 01:04:09 PM »
Quote from: RedFoxCR on May 31, 2010, 12:47:05 PM
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?

Quote
Base gaskets don't really affect compression - i counted! The only affecting thing is  the head gasket.
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Offline ford832

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Re: Detonation.
« Reply #14 on: May 31, 2010, 04:56:37 PM »
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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