Detonation-identifying-causes?

stevo450
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stevo450
Just finished off building my CR125, sounds to me like its detonating ( rattling cans is the best description). I can't decide if it's just really crisp 2T sound.....lol.... It could be the older gas I chucked in to start it.... So I'll be starting there....
I'm curious as to what other signs there are to tell if I have detonation issues...
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Paw Paw 271
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Paw Paw 271 12/20/2016 7:12am
12/20/2016 7:12am
Spark plug reading is the first line of observation.
Piston condition on the edges, by observation.
Measured squish is also important.

If as stated you used old fuel, then it would by nature cause some noise and could be justify your concerns.
Dump the old fuel and put in a fresh mix using at least 93 oct and see if it sounds better.

Paw Paw
stevo450
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stevo450 12/20/2016 9:38am
12/20/2016 9:38am
Paw Paw 271 wrote:
Spark plug reading is the first line of observation. Piston condition on the edges, by observation. Measured squish is also important. If as stated you used...
Spark plug reading is the first line of observation.
Piston condition on the edges, by observation.
Measured squish is also important.

If as stated you used old fuel, then it would by nature cause some noise and could be justify your concerns.
Dump the old fuel and put in a fresh mix using at least 93 oct and see if it sounds better.

Paw Paw
Thanks Paw for the reply! I think the old gas isn't helping matters... I'll definitely get fresh gas in.

What would you look for with the plug colour??
What would should the squish be?
Paw Paw 271
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Paw Paw 271 12/20/2016 10:43am
12/20/2016 10:43am
You don't want light grey or white on the plug tip as it can be caused by a lean condition or compression being too high. If it is gray then you may want to go up one jet size on the pilot and jets until you are sure all is ok. You are looking for a Tan or light Brown color on the plug tip.
On the piston you can look for pitting on the piston edges next to the cylinder walls. If these pits are there you have pre-ignition or detonation that in most cases is caused by the squish being too tight or the use of low octane fuel.
You should be able to see this through the exhaust port with the piston at the bottom of the stroke and this cam be done by just removing the exhaust pipe.
If all above looks good then, just make sure the jetting is good and you are using good fresh fuel and you should be good to go.
To check the squish you need to remove the head and will use soft large solder. The exact squish gap for your engine should show up in the service manual. You can also look up the book "Two stroke tuning guide" for these figures and how to calculate them. If the squish is too tight you don't get the needed fresh fuel cooling to the edges of the piston and thus generate excess heat that will cause the ring to stick in the ring groves of the piston and or the piston edge to get so hot that it will deform on the ring and cause it to stick and then you have no compression. Conversly, if the squish is too big then you don't get a full burn of the fuel.

Paw Paw

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Ktm 5
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Ktm 5 12/20/2016 2:06pm
12/20/2016 2:06pm
Never use old gas

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stevo450
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stevo450 12/21/2016 3:32am
12/21/2016 3:32am
Paw Paw 271 wrote:
You don't want light grey or white on the plug tip as it can be caused by a lean condition or compression being too high. If...
You don't want light grey or white on the plug tip as it can be caused by a lean condition or compression being too high. If it is gray then you may want to go up one jet size on the pilot and jets until you are sure all is ok. You are looking for a Tan or light Brown color on the plug tip.
On the piston you can look for pitting on the piston edges next to the cylinder walls. If these pits are there you have pre-ignition or detonation that in most cases is caused by the squish being too tight or the use of low octane fuel.
You should be able to see this through the exhaust port with the piston at the bottom of the stroke and this cam be done by just removing the exhaust pipe.
If all above looks good then, just make sure the jetting is good and you are using good fresh fuel and you should be good to go.
To check the squish you need to remove the head and will use soft large solder. The exact squish gap for your engine should show up in the service manual. You can also look up the book "Two stroke tuning guide" for these figures and how to calculate them. If the squish is too tight you don't get the needed fresh fuel cooling to the edges of the piston and thus generate excess heat that will cause the ring to stick in the ring groves of the piston and or the piston edge to get so hot that it will deform on the ring and cause it to stick and then you have no compression. Conversly, if the squish is too big then you don't get a full burn of the fuel.

Paw Paw

Paw Paw
Thank you for the in depth response... Since this is a fresh engine from crank to piston to cylinder I'll have to check the squish to be sure it is ok. Is the 'dome volume' of much importance when ruling out detonation? I know that the relationship between the squish and dome volume is important ( correct compression ratio etc ) but would it also cause detonation issues? Of corse I have not changed the dome volume since rebuilding the engine but I had to get the cyclinder faced as I had a small coolant leak. I countered the material lose ( and consequently the inevitable reduction in squish clearance ) by using two base gaskets.

I'll get onto the squish ASAP and get fresh 98 octane.
Paw Paw 271
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Paw Paw 271 12/21/2016 6:44am
12/21/2016 6:44am
Don't go too high on the fuel octane or you will run into other issues. Also understand that using 2 base gaskets will help with the squish, but at the expense of the port timing.

Paw Paw
speed_racer
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speed_racer 12/21/2016 7:49am
12/21/2016 7:49am
What year CR125? It matters.
stevo450
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stevo450 12/21/2016 1:15pm
12/21/2016 1:15pm
Paw Paw 271 wrote:
Don't go too high on the fuel octane or you will run into other issues. Also understand that using 2 base gaskets will help with the...
Don't go too high on the fuel octane or you will run into other issues. Also understand that using 2 base gaskets will help with the squish, but at the expense of the port timing.

Paw Paw
I wasn't happy about using two base gaskets because of the port timing alteration in the future I will be replacing the cylinder all together and starting fresh.

98 is the standard here for most bikes, for some reason the fuel ratings and quality vary from that in the US... prett much anyone here will use 98 as the baseline.
It's a 2006 cr125

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