2 Stroke Compression Ratio/Tuning Help

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11/30/2017 11:36 AM

Hi guys, new to the forum, need some advice please on compression ratio.
Bike ktm 125 sx 2018
I have milled the head 0.3mm, squish= 0.85-0.90mm, bike is fast
My problem is when i tear down the top end at 10hrs the inner o ring gasket is starting to melt which would suggest detonation... But i have 0 marks on the piston or head...
Uncorrected: 14.8:1 high i believe? Stock is 13.5:1. A friend recommends not to go over 14.2:1 on 98 octane.
Volume cc is 9cc
Corrected: 8:1 , why so low?
Running 98 octane fuel.
Jetting is ok.
Any suggestions?? ThanksPhoto

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11/30/2017 11:43 AM

This post is definitely for someone with more knowledge than I have.

My gut says you’re running a little lean on your jetting or you need to bump up your fuel octane.

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2009 Kawasaki KX450F
2009 Kawasaki KX250F
2002 Suzuki GSXR 600

11/30/2017 11:51 AM

Thanks acid, jetting is fine i cant go no richer (it sputters if i do) i belive also i need more octane. Just trying to get confirmation that my compression is the cause and is too high for 98 octane.

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11/30/2017 1:14 PM

What kind of compression have you measured with a gauge?

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11/30/2017 1:26 PM
Edited Date/Time: 11/30/2017 1:28 PM

175psi, which isnt high :/, i never trust gauges though.

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11/30/2017 3:53 PM
Edited Date/Time: 11/30/2017 3:56 PM

May I suggest that your squish is just a bit tight and causing excess heat at the edges of the piston and head and it is reducing the fuel mix at the edges and the fuel mixture aids in the cooling. More octane may help, but most likely not.

Paw Paw

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11/30/2017 4:40 PM

Paw Paw 271 wrote:

May I suggest that your squish is just a bit tight and causing excess heat at the edges of the piston and head and it is reducing the fuel mix at the edges and the fuel mixture aids in the cooling. More octane may help, but most likely not.

Paw Paw

Is there an ideal squish gap? I thought i read that it was .045" 1.143mm. Or is it all relative to fuel octane and oil ratio? ive done some researching and havent found a descriptive explination.

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11/30/2017 5:42 PM

I had 0.035´´ on my YZ144 and no detonation issue. Same with the stock engine. I used a mix of C12 and premium pump fuel.

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11/30/2017 7:27 PM

There is really no such thing as an absolute in engine building. You photo and statements have lead me to my statement.

Paw Paw

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11/30/2017 8:59 PM

So your running a squish of .033-.035in? I'd say it's too little of squish also, if I remember right I thought .040 was the minimum you want to run on the ktms

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12/1/2017 1:48 AM

Thanks guys, i belive you can go down to a minimum of 0.7mm squish on a 125 but i will try 1mm though. I think my main problem is that i have decked the head from 11.60cc to 11cc which raises compression from 14:1 to 14:8, am i just running too much compression? There for too much heat?

Corrected compression is only at 8:1 though..

By the way its freezing and snowing here in france sad lol

Thanks guys.

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12/1/2017 2:14 AM

What's a cooler plug going to do?

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12/1/2017 2:27 AM

Im running a 9 its fine, ive tried a 10 and was too cold.

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12/1/2017 5:47 AM

When it burns the o-ring its detonating. Put higher octane fuel in it. The squish is very tight as well. 1mm is a better area to be in with a KTM that already has a wide squish band.

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Race Bike: 2018 KTM 350SXF

Other Bikes: 1985 CR80R, 1990 CR250R, 1998 PW80, Specialized Fuse Comp 29.

Sold: 2016 YZ250F, 2012 CRF250R

12/1/2017 6:56 AM

I

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12/1/2017 6:59 AM

I've been having the same problem with a 250Photo

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12/1/2017 7:46 AM

1st thing check the head and cylinder matteing surfaces to make sure they are perfectly flat. Yours seems to be melted in 1 area to suggest its leaking there, mine was melting on the entire o ring. Good luck

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12/1/2017 7:48 AM

Today i have cut the bowl to drop the compression to 14:15-1, hopefully that sure cure the dettonation, im running 0.85mm of squish, isnt the idea to head as little squish as possible? I know people who run road race 125's at 0.65mm... Whats your thoughts?

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12/1/2017 9:10 AM

Don't confuse squish with compression ratios as they are each different. Changing the dome will not effect the squish.
You can have a very low compression and still have issues with the squish being incorrect. Squish issues are not corrected by increasing fuel octane.


Paw Paw

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12/1/2017 9:18 AM

Paw paw what squish would you recomend for 98 octane? Ktm 125 sx 2018. Thanks

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12/1/2017 10:07 AM

I don't know why 98 octane would not be sufficient. I would look for possible mechanical issues before deciding its a fuel related issue. Is it possible that your radiator is damaged (ballooned?) or water pump issue, or as someone else mentioned, the mating surfaces are suspect? I would think from what you described that 98 octane would be enough.

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12/1/2017 10:26 AM

Only as a reference: I had a 94 YZ 250 pumping out 245 psi compression, huge work done to it, running a 440 main, I believe. Was also an o-ring head, and I had zero issues with that motor. Check the mating surfaces.

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12/1/2017 10:30 AM

Its burning on the bottem side of the o ring (side touching the cylinder), not just in 1 spot the whole oring.

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12/1/2017 1:01 PM

JoeKtm2 wrote:

Its burning on the bottem side of the o ring (side touching the cylinder), not just in 1 spot the whole oring.

That indicates that the edge of the piston and the head are getting too hot. Adjust your squish, not your compression. The added squish clearance is needed to cool those areas.

Paw Paw

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12/1/2017 1:02 PM

JoeKtm2 wrote:

Paw paw what squish would you recomend for 98 octane? Ktm 125 sx 2018. Thanks

I would suggest about 1mm.

Paw Paw

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12/1/2017 1:06 PM

Paw Paw 271 wrote:

Don't confuse squish with compression ratios as they are each different. Changing the dome will not effect the squish.
You can have a very low compression and still have issues with the squish being incorrect. Squish issues are not corrected by increasing fuel octane.


Paw Paw

X2

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12/1/2017 1:13 PM

Thanks

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12/1/2017 1:26 PM

scooter5002 wrote:

Only as a reference: I had a 94 YZ 250 pumping out 245 psi compression, huge work done to it, running a 440 main, I believe. Was also an o-ring head, and I had zero issues with that motor. Check the mating surfaces.

I had a 01 RM250, o-ring head, making 240 psi with a milled cylinder and on 93 octane it knocked so bad that a chunk of the piston crown popped off and spit out the exhaust port. I tried 99 octane and it didn't break pistons, but it did ping a bit. So it said screw it and ran straight 110 Sunoco. I don't miss those days. Honestly, the bike wasn't even that fast. It was torquey and had gobs of midrange, but for tight woods stuff it was annoying.

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12/1/2017 1:47 PM
Edited Date/Time: 12/1/2017 1:48 PM

Ive only got 175psi lol

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12/2/2017 1:18 PM

When it burns the oring it's detonating. Put higher octane fuel in it. Running tight squish speeds the flame travel up. It makes power but you need the octane to control the burn.

The closer you run the piston to the head the more power it will make. You just need the proper fuel to control the burn.

In NHRA prostock they reduce piston pin height by .001 until the head stops marking the piston on the deck surface.

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Race Bike: 2018 KTM 350SXF

Other Bikes: 1985 CR80R, 1990 CR250R, 1998 PW80, Specialized Fuse Comp 29.

Sold: 2016 YZ250F, 2012 CRF250R