2 Stroke Compression Ratio/Tuning Help

JoeKtm2
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JoeKtm2
Edited Date/Time 12/5/2017 8:13am
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?? Thanks

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Acidreamer
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Acidreamer 11/30/2017 11:43am
11/30/2017 11:43am
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.
JoeKtm2
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JoeKtm2 11/30/2017 11:51am
11/30/2017 11:51am
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.
Micahdogg
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Micahdogg 11/30/2017 1:14pm
11/30/2017 1:14pm
What kind of compression have you measured with a gauge?
JoeKtm2
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JoeKtm2 11/30/2017 1:26pm
11/30/2017 1:26pm Edited Date/Time 11/30/2017 1:28pm
175psi, which isnt high :/, i never trust gauges though.

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Paw Paw 271
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Paw Paw 271 11/30/2017 3:53pm
11/30/2017 3:53pm Edited Date/Time 11/30/2017 3:56pm
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
rollin64caddy 11/30/2017 4:40pm
11/30/2017 4:40pm
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...
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.
Sheriff245 11/30/2017 5:42pm
11/30/2017 5:42pm
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.
Paw Paw 271
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Paw Paw 271 11/30/2017 7:27pm
11/30/2017 7:27pm
There is really no such thing as an absolute in engine building. You photo and statements have lead me to my statement.

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ktm-5
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ktm-5 11/30/2017 8:59pm
11/30/2017 8:59pm
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
JoeKtm2
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JoeKtm2 12/1/2017 1:48am
12/1/2017 1:48am
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.
JoeKtm2
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JoeKtm2 12/1/2017 2:27am
12/1/2017 2:27am
Im running a 9 its fine, ive tried a 10 and was too cold.
c0ncEpT
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c0ncEpT 12/1/2017 5:47am
12/1/2017 5:47am
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.
RCF
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RCF 12/1/2017 6:59am
12/1/2017 6:59am
I've been having the same problem with a 250

JoeKtm2
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JoeKtm2 12/1/2017 7:46am
12/1/2017 7:46am
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
JoeKtm2
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JoeKtm2 12/1/2017 7:48am
12/1/2017 7:48am
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?
Paw Paw 271
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Paw Paw 271 12/1/2017 9:10am
12/1/2017 9:10am
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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JoeKtm2
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JoeKtm2 12/1/2017 9:18am
12/1/2017 9:18am
Paw paw what squish would you recomend for 98 octane? Ktm 125 sx 2018. Thanks
Micahdogg
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Micahdogg 12/1/2017 10:07am
12/1/2017 10:07am
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.
scooter5002
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scooter5002 12/1/2017 10:26am
12/1/2017 10:26am
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.
JoeKtm2
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JoeKtm2 12/1/2017 10:30am
12/1/2017 10:30am
Its burning on the bottem side of the o ring (side touching the cylinder), not just in 1 spot the whole oring.
Paw Paw 271
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Paw Paw 271 12/1/2017 1:01pm
12/1/2017 1:01pm
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
Paw Paw 271
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Paw Paw 271 12/1/2017 1:02pm
12/1/2017 1:02pm
JoeKtm2 wrote:
Paw paw what squish would you recomend for 98 octane? Ktm 125 sx 2018. Thanks
I would suggest about 1mm.

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ktm-5
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ktm-5 12/1/2017 1:06pm
12/1/2017 1:06pm
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...
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
Micahdogg
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Micahdogg 12/1/2017 1:26pm
12/1/2017 1:26pm
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...
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.
JoeKtm2
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JoeKtm2 12/1/2017 1:47pm
12/1/2017 1:47pm Edited Date/Time 12/1/2017 1:48pm
Ive only got 175psi lol
c0ncEpT
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c0ncEpT 12/2/2017 1:18pm
12/2/2017 1:18pm
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.
Paw Paw 271
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Paw Paw 271 12/2/2017 1:36pm
12/2/2017 1:36pm
c0ncEpT wrote:
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...
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.
THat will not work on a 2 stroke as they need the added squish clearance to cool the piston with the fuel charge. What you speak of will work to a point on 4 strokes only.

Paw Paw

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