Detonation? - Like a lot....2009 yz125

bierbower105
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Uhrichsville, OH US
11/6/2019 9:57am Edited Date/Time 11/7/2019 11:17am
So I bought a used 2009 YZ125 off a guy here in Ohio - said the engine was just rebuilt (I know the guy that rebuilt the motor - good wrench and he has built me engines before - I trust his work - he confirmed recent rebuild)

Picked up the bike - started it in the shop - everything sounded good - jetting crisp - adjusted the air screw just a bit to get a flat spot out right off idle - other than that carb is 100% stock jetting

Vforce reed cage - bills pipe

Drained renegade race fuel previous owner had in it - unknown pre mix ratio - I put in pump 93 with 927 32:1

First day I rode it - 3rd session out - bogs and no compression - won't start.

Plug was brand new - it had some oil on it when I pulled it at the track - doesn't look to have been running extremely lean?

Only thing I can think that would have caused this is an air leak - I'm not totally thrilled with the seal between the carb and the reed cage - I ordered a new carb boot to see if it might seal better on the reed cage.

I just ordered a new head, cylinder etc - I don't want to put all this money into the bike and have it blow up first ride again.

Any help will be appreciated.


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mag23
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Lincoln, CA US
11/6/2019 10:04am
Man... I'm not sure if that was detonation. Is there any play in lower rod bearing (up-down), or, crank bearings?
Air filter clean? Detonating has a pretty distinct sound while you're riding, normally.
bierbower105
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Uhrichsville, OH US
11/6/2019 10:06am Edited Date/Time 11/6/2019 10:11am
mag23 wrote:
Man... I'm not sure if that was detonation. Is there any play in lower rod bearing (up-down), or, crank bearings? Air filter clean? Detonating has a...
Man... I'm not sure if that was detonation. Is there any play in lower rod bearing (up-down), or, crank bearings?
Air filter clean? Detonating has a pretty distinct sound while you're riding, normally.
I didn't feel any play in the bottom - he said the bottom was redone too.....those gouges in the head wig me the more I look at them.

Air Filter and Air Box brand new - I didn't see any dirt or anything in the boot or carb when I pulled everything apart.
bierbower105
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Uhrichsville, OH US
11/6/2019 10:27am






Airboot when I pulled it off. Super clean bike. - Unless the previous owner sucked dirt and then cleaned it really really good and sold it to me. I know the guy from seeing him around the track - but still.
Matt Fisher
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Visalia, CA US
Fantasy
1084th
11/6/2019 11:26am
Those marks are not from detonation. Something hard, very likely metallic, bounced around in the combustion chamber.
2

The Shop

soggy
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UT US
Fantasy
3783rd
11/6/2019 11:30am
was the head cut for race fuel?
bierbower105
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Uhrichsville, OH US
11/6/2019 11:32am
What could have been bouncing around in there? - Spark plug tip maybe? - any chance it's dropped down into the crank.....?


And the head does not appear to be cut.
kyle-vick
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Manton, MI US
11/6/2019 12:05pm
Maybe the alignment pin on the piston for the ring came out?
Sixeightone
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Central, CT US
11/6/2019 12:27pm
Looks like one of two things, a cylinder out of round, or like a DC size piston in an AB cylinder. Or a spark plug tip broke off and bounced around and that's why the guy sold it.
bierbower105
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Uhrichsville, OH US
11/7/2019 4:59am Edited Date/Time 11/7/2019 5:19am
Pin that holds the rings was still there - I didn't see anything there were a few little chunks of metal in the top of the piston- I'll check the power valve when I pull it out tonight. Other than that I didn't see anything - Rebuild and cross my fingers - thanks for the input fellas.

New cylinder, head, and piston should all be here tonight.

Used bikes seem to turn into projects even if you don't want them too.
nicko-31
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NY US
11/7/2019 5:35am
I only run Yamaha Lube R at 30:1 with richer main and next

richer needle from stock with 100LL AV-GAS in my 125 YZ .
Although my motor is not stock Yamah's like good high quality fuel.
Markopolo400
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St. Paul-ish, MN US
11/7/2019 7:07am
Almost looks like the bottom end gernaded at one point and previous owner left the same piston in there and didnt cut the head.
sesker15
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Upper Marlboro, MD US
11/7/2019 11:15am
Is this bike suppose to run race gas? Head cut for race fuel?
bierbower105
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Uhrichsville, OH US
11/7/2019 11:17am
sesker15 wrote:
Is this bike suppose to run race gas? Head cut for race fuel?
I don't think the head was cut - the previous owner had race fuel in it. I drained it before I ever rode it - but I'm sure he rode with race fuel in it.
11/25/2019 11:54am
I had an RM that the previous owner put a standard plug in instead of a race plug with a welded electrode. Electrode broke off and it looked just like that. Before you put it back together pull the whole engine out and flush the bottom end a few times with diesel fuel in case any metal made its way down there.

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