2020 kx85 blows up

Bendover24
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Bendover24
My nightmares have become reality...The day has come for me to join the community of pu**y 9-year-olds that have blown up their kx85. (I'm 14 tho) I bought a 2020 kx85 a little less than a year ago by now. (and no, not daddy's money h*e. I bought it with my money for ur info) It's been an awesome bike and I had big plans for it. Just today I was riding at a comfortable pace on a long street road and I started to feel the bike bog and engine break so I start to let off the throttle. It all happened really quickly. Beforehand, it was running great. I was in 6th gear on the throttle a bit. I wasn't revved to the moon or anything either. I pulled the clutch in when I noticed it and then it shut the f*ck off and locked the rear wheel. I hopped off and tried kicking it over and that's when I had that oh sh*t moment. It lost quite a bit of compression but it's not far from before. It's not locked up or anything. I had to push it 2 miles and up a hill, until a guy offered to help, and at that point, I was like f*ck it. If I get kidnapped or if he steals my bike then...idfk. Luckily he was nice and I think the fact that we both spoke Spanish help a bit. (not important) I know this probably isn't the right place to be sharing this but oh well. If anybody happens to know anything or of what could have happened I would love to hear about it. I am very certain it blew up. Given it is a very vague term but regardless I don't care at this point. Also if anybody knows of good places to rebuild my motor in the bay area ish, I would very much appreciate it. I can't wait for all the 18-29-year-olds giving me sh*t in the comments or whatever. Any info would be greatly appreciated and thank you.
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mx317
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mx317 12/9/2020 1:29pm
12/9/2020 1:29pm
on pavement?
Skerby
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Skerby 12/9/2020 2:22pm
12/9/2020 2:22pm
Rebuild it yourself.

Something that will break these bikes fast: coasting off the gas at high speeds on pavement.
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beamer
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beamer 12/9/2020 2:23pm
12/9/2020 2:23pm
Melted the top end, my guess, running in 6th on pavement
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Tracktor
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Tracktor 12/9/2020 2:34pm
12/9/2020 2:34pm
Pop the spark plug out & take a pic to post. Most times loss of compression = detonation/lean out which could be bad fuel or air leak/carb issue..............

The Shop

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Bendover24
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Bendover24 12/9/2020 5:37pm
12/9/2020 5:37pm
beamer wrote:
Melted the top end, my guess, running in 6th on pavement
What's the problem with riding in 6th?? I'm always seeing people getting cr*p for riding a certain way. Apparently, I don't know how to ride. How am I supposed to do it then? Im genuinely confused.
AJ565
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AJ565 12/9/2020 8:27pm
12/9/2020 8:27pm
beamer wrote:
Melted the top end, my guess, running in 6th on pavement
Bendover24 wrote:
What's the problem with riding in 6th?? I'm always seeing people getting cr*p for riding a certain way. Apparently, I don't know how to ride. How...
What's the problem with riding in 6th?? I'm always seeing people getting cr*p for riding a certain way. Apparently, I don't know how to ride. How am I supposed to do it then? Im genuinely confused.
Dirt bike. DDDIIIIRRRRRRTTTTTTT bike. You’re not jetted for pavement, that’s a whole different ball game. If you saw the jetting guys with shifter karts ran to keep those engines alive at sustained rpm you’d wonder how it even runs.
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mx317
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mx317 12/10/2020 5:02am
12/10/2020 5:02am
They tend to expire on pavement.
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timobijnen84 12/10/2020 5:41am
12/10/2020 5:41am
I probably think that the jetting was off or some bad fuel and you were running lean. The piston got to hot and blew up.
Rebuild it yourself you will learn how a engine works and will get to know your bike for the next time it breaks.
FGR01
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FGR01 12/10/2020 9:57am
12/10/2020 9:57am
Any possibility you forgot to put oil in the gas or didn't put enough in? I just rebuilt a KX100 for a kid (basically same engine as yours) because he forgot to put oil in his gas.
Bendover24
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Bendover24 12/10/2020 10:51am
12/10/2020 10:51am
FGR01 wrote:
Any possibility you forgot to put oil in the gas or didn't put enough in? I just rebuilt a KX100 for a kid (basically same engine...
Any possibility you forgot to put oil in the gas or didn't put enough in? I just rebuilt a KX100 for a kid (basically same engine as yours) because he forgot to put oil in his gas.
My gas was mixed 32:1. I definitely had my gas mixed.
Bendover24
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Bendover24 12/10/2020 10:53am
12/10/2020 10:53am
beamer wrote:
Melted the top end, my guess, running in 6th on pavement
Bendover24 wrote:
What's the problem with riding in 6th?? I'm always seeing people getting cr*p for riding a certain way. Apparently, I don't know how to ride. How...
What's the problem with riding in 6th?? I'm always seeing people getting cr*p for riding a certain way. Apparently, I don't know how to ride. How am I supposed to do it then? Im genuinely confused.
AJ565 wrote:
Dirt bike. DDDIIIIRRRRRRTTTTTTT bike. You’re not jetted for pavement, that’s a whole different ball game. If you saw the jetting guys with shifter karts ran to...
Dirt bike. DDDIIIIRRRRRRTTTTTTT bike. You’re not jetted for pavement, that’s a whole different ball game. If you saw the jetting guys with shifter karts ran to keep those engines alive at sustained rpm you’d wonder how it even runs.
10-4 but dang guy..
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dkurtd
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dkurtd 12/10/2020 10:59am
12/10/2020 10:59am
It's not so much the pavement but how they are ridden on the pavement. Most new 2 stroke engines aren't jetted for extended heat soaks at high RPM (greater then 7k) before they go into mild/moderate detonation, especially with todays 91 octane fuel. This either ends with a seizure or a hole in the piston. This is why the go-cart guys run a Mychron to keep an eye on EGT's.
Falcon
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Falcon 12/10/2020 1:58pm
12/10/2020 1:58pm
Perfect time to learn a top end rebuild! Get out your owner's manual or consult YouTube University and learn.

It's a lot of work, but not too difficult to perform. Take your time and follow the steps. Chime in here again if you run into trouble or have questions.

Donkey Kong
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Donkey Kong 12/10/2020 2:33pm
12/10/2020 2:33pm
How many hours were on the bike?
LKHill
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LKHill 12/10/2020 3:14pm
12/10/2020 3:14pm
Clean your air cleaner more often. No way you slam wore out a 2020 with proper maintenance.
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FFDan
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FFDan 12/10/2020 4:48pm
12/10/2020 4:48pm
I agree with most on here that say to tear it down and rebuild it. 2 stroke rebuild is about as simple as it gets. Youtube is a great reference along with your owners manual. The tools needed will be a couple screw drivers, a pliers, and a mm socket set. Im guessing most on here will even give you some friendly good advise while you are in the process, if you run into problem or are having a difficult time with something. You can order everything you need online to put it back together. Good luck.
Bendover24
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Bendover24 12/10/2020 6:06pm
12/10/2020 6:06pm Edited Date/Time 12/10/2020 6:15pm
LKHill wrote:
Clean your air cleaner more often. No way you slam wore out a 2020 with proper maintenance.
She was still fresh. The filters come with little oil so I re-oiled it when I got it. I kept up good on the maintenance. I have yet to check the coolant or spark plug after it happened. I should have time tomorrow to look into it and I will try and post pics when I can.
timobijnen84 12/11/2020 12:54am
12/11/2020 12:54am
LKHill wrote:
Clean your air cleaner more often. No way you slam wore out a 2020 with proper maintenance.
Bendover24 wrote:
She was still fresh. The filters come with little oil so I re-oiled it when I got it. I kept up good on the maintenance. I...
She was still fresh. The filters come with little oil so I re-oiled it when I got it. I kept up good on the maintenance. I have yet to check the coolant or spark plug after it happened. I should have time tomorrow to look into it and I will try and post pics when I can.
When did you oil your air filter the last time? On pavement air filters dont get dirty but still have to be re-oiled.
Post some pics of the progress of rebuilding the engine. You will get some tips and advice from members.

Pit Row

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LKHill
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LKHill 12/11/2020 9:10am
12/11/2020 9:10am Edited Date/Time 12/11/2020 9:12am
2 strokes just don't blow up. You did something wrong. You could' not blow one up that is running right other than to cold seize it. Post pictures of the carnage if you want to know what it was.
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Bendover24
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Bendover24 12/11/2020 5:07pm
12/11/2020 5:07pm
LKHill wrote:
2 strokes just don't blow up. You did something wrong. You could' not blow one up that is running right other than to cold seize it...
2 strokes just don't blow up. You did something wrong. You could' not blow one up that is running right other than to cold seize it. Post pictures of the carnage if you want to know what it was.
It was running great. I got it set up by east bay motorsports not too long ago. She was mint. I could only think of it getting a bad clog of fuel, or I didn't warm it up enough, or something along the lines of that. I'll try getting pics as soon as possible.
Leeham
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Leeham 12/12/2020 5:08am
12/12/2020 5:08am
Cruising for a long time on a small bore 2 stroke usually ends up bad, unless properly setup for it. Its likely going to be a costly fix. I dont understand the whole riding on public streets deal. Dangerous, illegal, and you can kill bikes.
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Bendover24
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Bendover24 12/14/2020 8:13am
12/14/2020 8:13am
Leeham wrote:
Cruising for a long time on a small bore 2 stroke usually ends up bad, unless properly setup for it. Its likely going to be a...
Cruising for a long time on a small bore 2 stroke usually ends up bad, unless properly setup for it. Its likely going to be a costly fix. I dont understand the whole riding on public streets deal. Dangerous, illegal, and you can kill bikes.
I wasn't riding on public streets, it was a long private road. What do you mean by costly? full rebuild?
jamiechapman418 12/14/2020 10:36am
12/14/2020 10:36am
Get your head and cylinder off and post pics of piston, head, and cylinder inside. That will give some clues as to what extent of damage you are dealing with.
LKHill
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LKHill 12/14/2020 2:03pm
12/14/2020 2:03pm
If your lucky the cylinder will be usable. If you don't take the head off you may be able to save the gasket.
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FahQ
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FahQ 12/14/2020 5:57pm
12/14/2020 5:57pm
Plug heat range, jetting, and fuel are three things that need to be changed to rip down the street in 6th gear.
My little brother blew up his 80 many moons ago doing this.
He bought an RS125 and went road racing, turned out good for him.
Bendover24
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Bendover24 1/3/2021 1:51pm
1/3/2021 1:51pm
FahQ wrote:
Plug heat range, jetting, and fuel are three things that need to be changed to rip down the street in 6th gear. My little brother blew...
Plug heat range, jetting, and fuel are three things that need to be changed to rip down the street in 6th gear.
My little brother blew up his 80 many moons ago doing this.
He bought an RS125 and went road racing, turned out good for him.
I wasn't road racing or doing dumb shit on the street. I was cruising in 6th to keep down on the noise on the way to my riding spot. I wasn't ripping it down the long backroad either.
Bendover24
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Bendover24 1/3/2021 1:55pm
1/3/2021 1:55pm
jamiechapman418 wrote:
Get your head and cylinder off and post pics of piston, head, and cylinder inside. That will give some clues as to what extent of damage...
Get your head and cylinder off and post pics of piston, head, and cylinder inside. That will give some clues as to what extent of damage you are dealing with.
I already took some pics but I don't know how to post them on where can I get some help? I took off the head but didn't take off the cylinder. It looks to me like the rings expanded faster than the cylinder or sum and caught on the exhaust valves but I'm no expert.
Bendover24
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Bendover24 1/3/2021 1:57pm
1/3/2021 1:57pm
I won't be able to do the rebuild or get more pics until I get a better torque wrench that isn't so damn big and that goes lower than 20 ft-lbs.
jamiechapman418 1/4/2021 9:49am
1/4/2021 9:49am
Unfortunately, without the cylinder off and some pictures I am afraid there isn't too much more anyone can give for advice.
LKHill
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LKHill 1/4/2021 5:14pm
1/4/2021 5:14pm
Bendover24 wrote:
I won't be able to do the rebuild or get more pics until I get a better torque wrench that isn't so damn big and that...
I won't be able to do the rebuild or get more pics until I get a better torque wrench that isn't so damn big and that goes lower than 20 ft-lbs.
I didn't even know what a torque wrench was and I was splitting cases before I was a teenager. Never stopped me from riding or had any failures.

A buddy of mine had a '76 RM 125 that he didn't cross-torque the cylinder nuts and it blew a base gasket,lol.
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