siezed engine? Not sure what happened.

Magellan

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#1
I recently rebuilt the top end on my son's KDX 80. I had the cylinder re-plated and put in a new piston and rings. I got the piston from the company that did the cylinder repair (welded up a pit, bored to match piston and plated). I never checked the clearances other than the ring clearance.

The engine has about 10 hours on it and has been running fine...till yesterday.

My son was riding on a forest road to get to the trail head. He is just learning to use a clutch, so was probably running a pretty steady speed in first gear for a while. We were in a bit of a rush, so we didn't warm the engine up for very long.

About a mile up the road, the bike lost power. Seemed to me like it seized. The engine didn't lock up, but there was no power. It would start, but didn't run well.


I tore down the top end today. The cylinder and piston look fine. Some light scoring, only one that barely catches a nail. No signs of metal transfer and everything measures within spec. A compression test prior to the tear down showed 120 psi, which is within the service limit.

I did notice some signs of overheating on the flywheel near the connecting rod. Could the bottom end have seized? I was running a 42:1 mixture. The manual calls for 32:1, so technically this is lean. Could the bottom end overheating cause a loss of power without actually freezing up the engine?

The connecting rod has a bit of radial play. To the extent I am able to measure it without disassembling the bottom end, it is probably out of spec. I don't think it is any worse than when I did the top end though. There was a lot of piston slap when I bought the bike, so I just did the top end.

At this point, I'm thinking of just cleaning up the piston a bit and running it with a richer mixture.

Here's a link some pics of the piston, cylinder, and flywheel:
KDX Top End

Any input on what you think happened and what I should do are appreciated.

Thanks.
 
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#2
The heat mark around the big end pin is pretty normal. During the initial build process the crank weight is heated and the big end pin is cooled to allow for a proper press fit.
 

Magellan

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#3
that's good to know. At leat it probably rules out a bottom end issue.

I think I'm going to chalk this up to overheating due to too lean a mixture and not enough warm up time (cold siezure). There doesn't appear to be anything to "fix" at this point as there's minimal damage to the cylinder and piston..at least no metal transfer that I can see.

I think I'll just buff the piston a bit, reassemble, and see how it runs.
 

dirt bike dave

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#4
I agree on insufficient lube and not enough warm up.

FWIW, I've heard that when a cold motor seizes due to insufficient warm up, there is often evidence of damage on the on the intake side.

A contributing factor could also be engine braking while riding on the road, without pulling the clutch in. High rpm = lots of friction, and throttle closed = not much lube or fuel to cool things down.

I grenaded a CR250 after a sustained top gear run on a road, then coasting in gear...the crank bearings and big end bearing failed and a hole got punched in the cases. FWIW, the motor was plenty warm and I was mixing 32:1.

I now pull in the clutch and avoid high rpms with the throttle closed. It was an expensive lesson.
 

Magellan

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#5
So what do you guys think about just reassembling?
I may have a pro measure things to be sure, but I measured multiple times and pretty much got the same answer each time.
I really can't think of anything else to do assuming my measurements are accurate.
 

WoodsRider

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#6
Air-cooled 2-strokes seem to be more sensitive to 4-corner seizures when not properly warmed up. One thing to check for is that the piston rings do not stick in the ring grooves.

32:1 is the leanest fuel to oil ratio you should run. Always warm the bike up before riding making sure the cooling fins are warm to the touch with a bare hand.
 

Magellan

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#7
I just remebered one more thing that may be pertinent. when I disassembled the top end, the piston and cylinder had a pretty good coating of oil on them. It was pretty thick and a dark grey color.
I suppose it is possible that it is the tranny oil, though I don't seem to be missing any that is noticeable.
I also recall noticeing a lot of smoke coming from the pipe when I looked back to check on my son. I thought maybe it was just too rich on the mixture, but now I'm starting to think it was burning the tranny oil.

Could a right side crank seal leak cause a loss of compression/power?

I'll put it back together and do a leak down test, but as always, appreciate any input from the forum.
 
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dirt bike dave

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#8
My guess fwiw is that the dark grey oil coating on the piston and cylinder is a mix of powder material sloughed from your piston and unburned premix.

Probably as compression was being lost due to irregular heating / lube failure, your son was holding the throttle open to try to keep the bike running. So it was being flooded, and there was already excess heat on the exhaust side, so therefore lots of smoke but no power.
 

Magellan

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#9
That would make sense. the oil was a bit on the sludge side, and there is nothing on the crank. It was just on the piston and cylinder.
It sounds like I can probalby get away with reasembling and a Leakdown test to check things out. We probably only have about 8 weekends to ride before the weather turns and we start thinking about snowboarding. I'll have time to rebuild the crank over the winter to address the bearing play.

Thanks. I'll report back on the LD test in a few days.
 

WoodsRider

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#10
Magellan said:
We probably only have about 8 weekends to ride before the weather turns and we start thinking about snowboarding.
8 Weekends? Woodland MX, North of Vancouver, runs a Vintage Series all winter. That bike would be perfect for the Schoolboy Class.
 

Magellan

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#11
Good to know. We are primarily woods riders (I run some of the nma enduros), but some track time wood be fun.
Btw, I also ride an ec300.
 

RM_guy

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#12
Word to the wise...don't get sucked in by Woodsrider...otherwise know and WoodsWeenie :p
 

WoodsRider

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#13
RM_guy said:
Word to the wise...don't get sucked in by Woodsrider...otherwise know and WoodsWeenie :p
Still need hill climbing lessons?

Magellan - I rode enduros for a few years and still do a bit of trail riding with my boys. We started racing vintage mx a couple years ago after moving back to Seattle. It's a good way to stay in shape over winter.
 

Magellan

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#14
WR, I looked into Woodland, and think we'll give it a shot this next season. I need to call and verify what class my kid can ride in. I put a 97 front end on his 87 KDX, but I think disc brakes are allowed. In any case, we don't care about points, just getting some saddle time.

I did a leak down test today. At 6psi, I'm less than 1PSI/minute. It took about 1m 20s to lose 1PSI. I don't think this is enough to have been the cause of my problem.

I took a bit of wet 1200 grit to the piston to make sure it was smooth prior to reassembly. I give it a compression test after reassembling and am well north of 150PSI (this with fresh oil on the piston from assembly).

mag
 
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WoodsRider

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#15
Mag - There pretty open when it comes to the kids. We race the Pac NW series, pnwvmx.org, and they let kids race Schoolboy on modern bikes and my youngest races the XR75 class on a CRF50. We raced at Tieton (Yakima) yesterday and there were only 4 kids racing, two boys in Schoolboy on 85cc modern bikes and my boys in the XR75 class.
 
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Magellan

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#16
Then I'll look forward to seeing you there!
 

WoodsRider

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#17
Definitely! They start in October, but they run vintage on Saturday so we won't be going until football and fall baseball are done.
 

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#18
I once seized my KDX200SR riding it on the highway with it "on the pipe" and the throttle less open to keep from accelerating. The pipe was a Pro Circuit and my jetting habit was to have it almost too lean.
So I would suggest to you to raise the needle one notch if you can get away with it, just as a preventive measure. It's hard for kids to do everything right, especially when they are still amateurs.
And always warm up an engine till you can feel the heat.
 
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#19
Back when I was a kid with a mini enduro, I four corner seized that tough little engine 2 or 3 times. Same deal, just loss of power then engine died. Of course, I had no idea what it was doing or what caused it when I was 12. Every time, I just let the bike cool all the way down and started it up again and rode away. That engine should have exploded a dozen times the way I (didn't) take care of it.
 

Magellan

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#20
Well, after getting it all back together, with good results on the compression and leak down tests, it started right up and still sounded like sh_t.

I think it's the crank bearing....
 

motometal

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#21
Maybe this is a done deal...but I was going to put my two cents in...regarding oil ratio...while it might generally be a good idea to run 32:1...I highly doubt the ratio was your problem. Lots of folks out there successfully running 50:1 etc and while it isn't my choice...it works for them. By the way les oil in the mix actually behaves like a richer mixture...more fuel and lower viscosity