Seized Piston Advice

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jc7622
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Seized Piston Advice

Post by jc7622 »

My kid seized/stuck his piston on Sunday. Or at least that is what I think happened - the kickstarter wouldn't budge. When we got it home I washed it, removed the spark plug and tried to push the kickstarter with my hand. It appeared to free itself and then would go up and down normally.

I took it apart to make sure everything was ok. The cylinder looked fine to me and the piston looked good also. I plan to take both in to the shop tomorrow morning for a second opinion and to order new parts.

I do have a question though. It has been a long time since I have done this and have forgotten a lot. On the crank it is very dark where it connecting rod is - it is almost black and over about 1.5" eith way it goes to purple, blue, gold and then to the silver color of the crank the rest of the way around. It has obviously experienced some heat. My question is, how much of this discoloration is normal and how much is cause for alarm?

Another question I have has to so with the play in the bearings on the lower part of the connecting rod. How much should there be side to side and up/down? We definately have some play there.
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Post by Indawoods »

None of that sounds good at all.... :?
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Post by fuzzy »

If the piston and cyl came apart easily, and look OK then you didn't stick it. Sounds like you cooked that rod bearing, and if you did then you'll need to have a builder rebuild the bottom end. 2-strokes (most all) have multiple-piece crankshafts, and have to be pressed together in a precision manner....Then aligned in the case during assembly....Not a general garage job unless you build little engines all the time.

To answer your questions...none of that discoloration is normal, and there shouldn't be much play in the crank bearing. :cry:

[edit] HA, vince beat me to it.
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Post by GS »

Zero up and down play(that you can feel) is what you are looking for. However a little bit of side to side play is normal.

Rainbow-colored rod is a problem that needs investigating, unfortunately................. :?
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Post by jc7622 »

Well, that's probably it then. Bummer. We have lots of up/down play (1mm+) and lots of side-to-side leaning play. It's really pretty sloppy. I'm probably lucky nothing worse happened and we caught it when we did and how we did. I guess I will drop off the motor in the morning.

I made the daredevil (who is 11) take the whole thing apart himself to teach him a lesson. I sat and instructed him on which bolt to take off next. I've got to somehow convince that kid that the meat of the powerband is not at 20,000 RPM.
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Post by GS »

Just to be fair, it's pretty hard to kill a KDX engine with rpm, unless it's extensively modded. They just don't go there.

Good on ya for helping/instructing him in the engine removal.

Now you need to figure out what caused the problem! Sometimes NOBODY (that bugger) at all can be blamed.
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Post by jc7622 »

Oops. I left out the bike. It was a KX65.
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Post by canyncarvr »

A KX65 tachs a tad higher than the KDX....

The 1mm+ up and down is toast.

You should try to look in on a crank rebuild sometime. Or..have you read about the process? Yes..it is not a job for your average shade-tree mech (imo). Heck...the thought of splitting cases gives me a headache!

I watched it done once. The process included banging on the thing with a HAMMER 'fer crine out loud. No..not a steel 'hammer', a dead blow. Still..as fine a 'tuned' unit as the crank needs to be, the way of going about putting the thing together is strange......

Does the kid tweak the bike in 3rd when he should've been in 4th already? Seems most kids ride like that. Extended high revs at little to no load isn't a good thing.

Oh...
I took it apart to make sure everything was ok.
I made the daredevil (who is 11) take the whole thing apart himself to teach him a lesson.
I really Really REALLY like the second one better!!! Nnnnooooo...I'm not thinking of it in a 'punishment' sort of way. BS to that. If he's going to ride, he needs to know how to wrench on his stuff. It's a matter of teaching responsibility, showing that effects indeed have causes. It's a matter of raising your kid up right. Haven taken the time to instruct and guide him on the take-apart is a great thing!!! Indeed, kudos to you on that!!

Good luck!

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Post by fuzzy »

I wanna see pics :mrgreen:

What were you running for fuel mixture out of curiosity? Sometimes bearings just fail. Also, there are good, and MO betta bearings for that crank. You can bet your butt that no works engine has ever been built with OEM rod OR bearings. Might want to inquire and splurge on a good aftermarket rod and bearings. Hopefully your crank is salvagable...Sounds like chances are good here.
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Post by jc7622 »

Yeah I took the motor in this morning and the rod was toast. The shop is going to replace it. I've replaced a whole crank before, but aren't up to this task. He estimated the labor to be about $240 to replace the rod and put it back together.

I used the word "punishment" tongue in cheek. I was never mad at him or anything like that. He is actually pretty good at wrenching. About half way through the tear down his riding buddies started rolling up on their bicycles and he was the star of the show. He thought he was hot stuff. None of them had ever seen a piston before and thought the whole thing was pretty cool.

He needs to learn a little responsibility. He is the kind of kid that would never wash his bike, boots, gear, or do any maintenance. It's time he learns some proper habits. Besides, getting four bikes ready and taking care of gear for four people is wearing me out.

We run Amsoil at 40:1. My guess is that the high revs had something to do with it, but we bought it used and don't know what kind of maintenance it had before we bought it. I'll see if I can get them to pose for a pic.
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Post by fuzzy »

Smaller, high-revving engines can benefit from more oil. I's switch to 32:1...If not more if he's on the pipe all the time.

Good point. Poor prior maintance could be the culprit. IMO, a general rule of thumb for a quick-piston-eating MXer is 3-4 pistons tops before goingn for a full rebuild.
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Post by canyncarvr »

Not to beat a dead horse here...but I'm with fuzzy on this one. An MX65 @ 40:1?

Not me, man!

Maxima says:

Service Ratios: Normal use ratios are 32:1 to 50:1. Racing requires greater oil content. Ratios no higher than 40:1 should be used. Small displacement, high-RPM racing engines should use rations no higher than 32:1.


From: http://www.maximausa.com/technical/oilratio.html

I DID read the amsoil part. Still...not me, man!!


Please...no 'I run my 125 to 18KRPM at 100:1 with DynoWhoppin oil, and it works FINE!' input.

Thanks.

BTW...my opinion AND $2.00 will get you a cuppa across the street!!

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Post by jc7622 »

I was initially running it (and mine) at 32:1 but switched to 40:1 a couple months ago. I talk to people who have run it at 40:1 for years with no trouble in any of their bikes and then talk to people who think 32:1 is too high for even a 250.

There was quite a bit of residual oil in the crank area, more than I have seen on any of my past bikes. Maybe the shop can help to diagnose it once they have it apart.

Are there negatives to running at 32:1 in a bigger bike? It seemed to run fine at that ratio and it did make me feel better doing it.
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Post by canyncarvr »

Certainly there is a correct answer to your question...but I doubt you will find it. Way too many opinions based on not a lot..that's what you WILL find.

I fall back on that old McCullogh white paper (that I cannot now find on the web. I read it years back.) from decades ago. Their testing said that when jetted for correct air/fuel ratio they got increasing power output from a 2-stroke engine to the point of plug failure (having SO much oil..and therefore DO much volume of A/F ratio adjusted fuel they couldn't light it off) the more oil they ran.

I've tried richer premixes in my KDX...jetted to suit..and 32:1 did NOT suit me. I've tried 50:1 too..and went back to 40:1.

My Banshee had such an obvious seat-of-the-pants improvement at 32:1 over the 40:1 I was running that 32:1 is the obvious choice.

I say that to refute the common knowledge that smaller engines (the 200) will 'like' richer premix..and bigger engines (the 350 Banshee) don't need as much. Wrong or right (admittedly it may not BE right..just my experience) the two engines I'm running at the moment are the opposite of the 'common' knowledge.


Another anecdote of absolutely no value to the current discussion...

I started riding 2-strokes a long time back. My first was a Yamaha 350cc street twin, an R5. It was an oil injected engine with an oil res of maybe close to 1.5 qt. Of course, I was told that I HAD to run a quality 2-stroke oil. For whatever reason, I put Quaker State 30W motor oil in the injector tank. Cheap, I guess. The bike was supposed to run an '8' plug, I ran a 77 (hotter). I was told that a 77 would put a hole in the piston(s). OEM jetting was something like a 120MJ in the round slide Mik carbs. I didn't KNOW it (didn't check that sort of thing back then!) but the bike had been rejetted..to a 90MJ. I guess that was to accommodate the 4100'el.

Oh...the manual said you were to take the heads off every 1000km to scrape the carbon off.

I ran at sealevel a good bit. The 90 mains gave me a clatter and rattle (detonation) that I addressed by giving the thing more gas!

I got over 1000 miles to a quart of 30W oil. I NEVER took the heads off until my first rebuild. I may have changed those 77HC plugs once in awhile, but not for years at a time. Most of the time I rode it, I had a pack on the back...I was GOIN' places!!

That first rebuild? 47,000 miles.

The point of the diatribe: I've beat 'common' knowledge over the head concerning 2-strokes a good bit. I'm likely to disagree with what is 'fact' about 'em...'cuz I know better!

Yep...it's alllllll according to me!! :wink:

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Post by jc7622 »

I remember Banshees back in my Quad days (mid to late 80's). I really loved the way those things sounded. I remember the ones Dean Sundahl used to race in the desert - awesome. I've also always wanted a twin two-stroke street bike.
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Post by canyncarvr »

I pondered getting an RZ when they came out.

Didn't.

Stupid Stupid Stupid!!!


It wasn't too long ago I was still riding a 2-t-twin..my '77 RD400 (put about 65,000 miles on that before I gave it away). I went riding with a guy on a Ninja about twice my size. He couldn't understand howcome he couldn't shake me off his arse!

A couple years after that, 'The Fastest Motorcycle on the Planet'..a ZX1100 rider said it really sucked to not be able to keep up with me in the twisties. He was kind'a irritated about it....... :wink:



Yeah...I love that B-shee sound!! I've been listening to Yamaha twins for going on four decades.

YeOW. I must be OLD!

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Post by fuzzy »

RD's / RZ's rule!
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