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I seized my crank this weekend... let's guess how!

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A nice surprise for a Sunday afternoon... and a pleasant 5 mile walk out of the desert through some gnarly uphills and sand washes.

Bike: 05 YZ250

Jetting: 172/50

Alt/Temp: ~2500-3000/50*-65*

Back story:

Bike ran great last weekend. It's beginning to get warmer here so I noticed my jetting was a little off, but not dramatically. I decided before I go out again I was going to rejet my bike. However, I did run out of my normal two cycle (Yamalube) so I borrowed some of my friend's two cycle (Kawasaki brand). Didn't seem to be an issue all day last Sunday, though.

Mid week:

Washed my bike up clean. Garden hose and scrub brushes; no pressure washer.

Sunday morning:

Get up early, swap my stock 05 pipe for my SST pipe. Loosen the carb brackets, turn my carb and swap brass - went from 172/50 to 170/48. Not a major jetting change. Also cleaned my air filter and reoiled it.

(A thought I had just now is that maybe I didn't squeeze all of the oil from the filter before installation. I thought I did, but maybe not...)

Get to the staging area, unload and gas up my bike. I'm absolutely sure it was mixed properly, however, again, it was with the Kawasaki brand two cycle. Also check my fluids (oil and coolant). Usually when I kick it over it will start the first few kicks no matter what. This time it took about 15 kicks, but I thought maybe that was from having drained the carb this morning.

The bike finally starts up and immediately revs to the moon. I instantly kill and it and know right away what the deal is - my throttle cable must be stuck. It's happened before on me, and I knew I needed to get a new cable, but it's one of those things that when you get it "fixed" you forget about it and forget to replace it. So I take my seat and tank off, fiddle with the cable, and try to start it again. Revs to the moon. Repeat process, this time taking the top of the carb off, taking the needle and slide out, re-checking the jets, and putting everything back together. Success. It starts, revs fine, and I let her warm up and off we go.

About 20 minutes into the ride we get to a fence and stop to open it. The bike was running just fine to this point. When I stop, however, we notice that gas is running straight through the carb, through three different vent/overflow hoses. I shut off the gas, tell my friend to go ride, and I'm going to slowly perform surgery on the carb trailside.

I unpack my tools, lay down a bandanna to set my parts on and keep dirt from getting in them. Important to note there is no wind or rain. So this time I completely take my carb off, look at the floats (they're fine, the needle moves fine), take out the jets and make sure they're clean, and take out the needle. This time, however, I notice the problem: the cable wasn't letting the slide close all of the way.

Q: Would this cause the gas to pour through the carb like it did?

I decide to turn in all of my throttle cable adjusters until I notice the slide close fully. I then turn the adjuster on the bars until the slide begins to lift, then back until it fully seats and leave it there. I put everything back together and decide that we need to get back to the truck. I also put in a new spark plug.

The bike starts after a few kicks, and we let it idle. No hanging idle, no revving. I adjust the air screw until it idles smooth, and we sit and wait to see if gas drains through it. Nothing. I turn it out. Still no gas draining from the overflow. Seemingly good news!

So we start heading back to the truck and I ride for about 10 minutes before stopping to check up on everything. Bike seems to run just fine along the trail. However, when we stop again it's draining gas. I decide to start it up and just make it back to the truck. When I start it the bike has a weird idle, so I over up the throttle to what the engine does. It has a really weird, surging rev, especially when held wide open. After holding it wide open for about 10 seconds, it finally smokes up and seizes up. I knew what happened instantly. The kick starter is locked.

After getting it back to the truck and back home, I tear into it slowly. The bike still has plenty of coolant. I take the carb off, then the reeds. Reeds are fine. I take the head off - everything looks fine. Maybe a slight blemish slightly off center, but nothing worrisome. From the top the piston looks normal. I can't see any damage anywhere. Important to note that the piston is "seized" about 3/4 or more up the cylinder.

Also, the head gaskets look fine. Next step: attempt to take the cylinder off. I remove the nuts and start to carefully slide the cylinder up, and the piston slides down normally. I go slow and try to observe any flakes or metal chips - nothing. However, I did notice that one of my cylinder peg bolts was really white and corroded. This would left side, back bolt. Base gasket also looks okay.

I get the cylinder up as far as I can raise it and notice that although the piston is moving down the cylinder, the rod isn't moving. I immediately realize that I've seized the crank. I try to "work" it a little bit; the crank will move backward a little bit, but not enough to drop in its stroke enough to move the piston down enough to pull the cylinder. It will not move forward past a certain point. The kick starter is also locked.

So that's where I'm at. I think I'm going to have to pull my engine completely off to get the jug off, but I'm going to have to do that anyway to fix the thing. I'm not so worried about fixing it - though I'm undecided if I want to do it myself (I don't have any of the tools required) or just send it to Eric Gorr. Best case scenario the crank is okay and the piston and sleeve are okay; likely I'll just replace everything and get it right.

But I'm more interested in what may have caused this seizure. Revving would indicate an air leak, but that seemed to have been mitigated when I tinkered with the cable. Did dirt get inside of the engine and cause it? Change in two cycle oil? High revving? Any thoughts are appreciated.

Pics will follow tonight. Also, I haven't drained the motor oil yet. I have a mag drain plug so something should show up when I do that.

Thanks guys, for helping solve this mystery and helping me learn more about these sorts of things.

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you failed to mention how many hours you possibly have on the bike plus switching between the yamaha or kawasaki brand oils probably made little to no difference since they are JASO spec oils.

Also the white corriosion on the cylinder Stud is normal, cranks don't last forever. Since YZ's have plastic crank stuffers they can melt and cause the bottom end bearing to lack lubrication which then can cause the crank siezure.

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man...that sucks

I'm wondering if the 170 main might be a bit lean for riding the sand? The Yami jetting chart (knowing they are going to be a bit conservative) calls for a 175 with our current temps and altitude....? ....but on the same note a slightly lean main I wouldn't think would cause the other symptoms you described?

Just a thought?

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170 🙂 Id say lean seize.. BTW, I have a used crank I can sell ya from my 05 👍

In addition to being lean on your main (IMHO) it sounds like you had a crank seal or base gasket air leak?? 🤣

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you failed to mention how many hours you possibly have on the bike plus switching between the yamaha or kawasaki brand oils probably made little to no difference since they are JASO spec oils.

The bike is an 05 and hasn't had anything done to it but a new top end last year (I bought it last year); age is definitely a factor.

man...that sucks

I'm wondering if the 170 main might be a bit lean for riding the sand? The Yami jetting chart (knowing they are going to be a bit conservative) calls for a 175 with our current temps and altitude....? ....but on the same note a slightly lean main I wouldn't think would cause the other symptoms you described?

Just a thought?

You might be right, actually. It did seize when I was revving it out (in neutral, however, and not under load). I do jet my bike slightly leaner, though (I mix at 32:1, also) because that's how the plug test worked out last year. Maybe I misdiagnosed my plug test last summer and maybe I should stay closer to the chart despite the test?

Dammit. If that was the culprit that's definitely a head thumper. 🙂

170 :applause: Id say lean seize.. BTW, I have a used crank I can sell ya from my 05 👍

In addition to being lean on your main (IMHO) it sounds like you had a crank seal or base gasket air leak?? 🤣

I think I'm just going to go with a kit - not sure if Wiseco or HotRods is the better option - the forum has both good and bad reviews of both.

Depending on what everything looks like I could do a bottom end for under 200 and a top end for 120 going with Wiseco/HotRods and an OEM top end kit. 320 + tools to split the cases (or pay someone to split them) = under 400.

OEM bottom looks to be much more expensive.

-------

I'm thinking more and more my problem was a) pinched throttle cable holding the slide open and :jawdrop: lean seize caused by lean main jet, improper warm up due to throttle cable rev and wringing it out for a few seconds.

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It has a really weird, surging rev, especially when held wide open. After holding it wide open for about 10 seconds, it finally smokes up and seizes up. I knew what happened instantly. The kick starter is locked.

You might be right, actually. It did seize when I was revving it out (in neutral, however, and not under load).

What?!👍

It was surging-(lean symptom) and you decided to rev it out WOT for 10 seconds without a load?! You may have played a larger role to your bike seizing than you think. I feel that you revving the shyte out of it in neutral under lean conditions ultimately seized it.

Its common knowledge that a motor (even perfectly jetted) should never be revved WOT without a load for any period of time, the most I give mine is a 1 sec blip or two to clear the combustion chamber after a 2min warm up (and that’s only a ¼ - ½ throttle openings).

With regards to the carb leaking , have you replaced the little fuel inlet needle valve inside the carb? My old YZ also leaked abit after stopping and even the though the valve looked 100% , replacing it proved it was the culprit to my carb overflowing problems.

This might help you if you haven’t seen it already:

https://thumpertalk.com/forum/showthread.php?t=412662

Good luck!

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What?!🤣

It was surging-(lean symptom) and you decided to rev it out WOT for 10 seconds without a load?! You may have played a larger role to your bike seizing than you think. I feel that you revving the shyte out of it in neutral under lean conditions ultimately seized it.

Its common knowledge that a motor (even perfectly jetted) should never be revved WOT without a load for any period of time, the most I give mine is a 1 sec blip or two to clear the combustion chamber after a 2min warm up (and that’s only a ¼ - ½ throttle openings).

With regards to the carb leaking , have you replaced the little fuel inlet needle valve inside the carb? My old YZ also leaked abit after stopping and even the though the valve looked 100% , replacing it proved it was the culprit to my carb overflowing problems.

This might help you if you haven’t seen it already:

https://thumpertalk.com/forum/showthread.php?t=412662

Good luck!

When I cleaned out the carb to begin with and got it started it leaked a bunch from the power valve overflow, and the old spark plug was wet and oily. After riding it for about 10 minutes and it displaying no odd tendencies while riding, we stopped to check on it. That is when we noticed the gas basically going straight through.

I did get it started again and that's when I revved it. 10 seconds is probably exaggeration on my part, but I did hold it open for a few seconds only right after I noticed the surging.

Right or wrong this isn't something that I haven't seen before (revving). If it is indeed the case that you should never hold your throttle open while not under load - even for a few seconds - I'll chalk it up to learning a lesson. I'm not an expert by any means but I feel as if I'm not a novice either, so I guess you learn something everyday.

But you're probably right - my idiocy at a few points caused the seize. I think I jetted too lean as well as revved it too much. What's annoying about this is I'm the only person I ride with that even cares about all of this crap; the rest of them don't do a single thing to their bikes - save an oil change once a month and a filter clean about that often - and they rarely have problems. Maybe it's luck? 👍

Also, I've pretty much studied that thread through and through. Which is why I arrived at 170 for my MJ even though the chart suggests about 172-175. Maybe I'm misreading my plug tests.

I'll look at the float valve, but I'm not sure there's a problem with that - never was a problem before. I think the gas overflow was due to the slide not fully seating because of the jacked throttle cable. Am I far off on this?

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I would have thought that a lean condition especially with the SST could be an issue but it is the crank that has seized not the piston to cylinder. I would have thought the 170 main and the oil mix you are using would be sufficient for crank lubrication. Mind you holding it wide open for 10 seconds in neutral is a pretty good way to highlight any failings. Looks like with a good rebuild you are good to go and the barrels still good. It was probably just it's time and the rest never helped.

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I would have thought that a lean condition especially with the SST could be an issue but it is the crank that has seized not the piston to cylinder. I would have thought the 170 main and the oil mix you are using would be sufficient for crank lubrication. Mind you holding it wide open for 10 seconds in neutral is a pretty good way to highlight any failings. Looks like with a good rebuild you are good to go and the barrels still good. It was probably just it's time and the rest never helped.

This is what I'm thinking.

As far as I can tell the piston and rings are okay. I haven't noticed any material and they move in the piston - it's the crank that's seized.

Thanks everyone!

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What's annoying about this is I'm the only person I ride with that even cares about all of this crap; the rest of them don't do a single thing to their bikes - save an oil change once a month and a filter clean about that often - and they rarely have problems. Maybe it's luck? 👍

.......I feel your pain

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I got my motor off the frame tonight and took a look see...

Wimpiest seized crank ever! The stupid thing still moves most of its stroke. Just not smoothly. I suppose I should just be glad that there's no visible carnage (yet, anyway).

The rest remains to be seen. I need to order a crank kit, top end kit (even though it all looks good I might as well replace that) and a case splitter/crank press before I can find anything else out. I think I'm going with the Wiseco kit. The reviews here have been mixed between Wiseco and HotRods; most prefer OEM but I think the complete kit just makes more sense: cheaper and assembled.

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What's annoying about this is I'm the only person I ride with that even cares about all of this crap; the rest of them don't do a single thing to their bikes - save an oil change once a month and a filter clean about that often - and they rarely have problems. Maybe it's luck? 👍

Also, I've pretty much studied that thread through and through. Which is why I arrived at 170 for my MJ even though the chart suggests about 172-175. Maybe I'm misreading my plug tests.

I see this SOOOO often at my job (electronics---unrelated to mechanics work). Usually messing with stuff does more harm than good unless it is absolutly necessary. There are so many ways to introduce errors and screw things up when people constantly mess with things. Contacts get dirty from adjustments, adjusting one thing messes up another thing....etc. Sometimes it is best to leave good enough alone and run to fail. Not saying that that applies at all times but from my experiences it seems to apply more often than not.

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"I seized my crank this weekend -- let's guess how."

Other than to note that my computer blushed, no comment... 👍

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jwherb......join the club.I locked up my 1-2-5 this weekend at REM race at Glen Helen.S##t happens i suppose.

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most cranks lock up from debris, balance issues that kill bearings, lack of lube and old age......them are my guesses.

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When I cleaned out the carb to begin with and got it started it leaked a bunch from the power valve overflow, and the old spark plug was wet and oily. After riding it for about 10 minutes and it displaying no odd tendencies while riding, we stopped to check on it. That is when we noticed the gas basically going straight through.

I did get it started again and that's when I revved it. 10 seconds is probably exaggeration on my part, but I did hold it open for a few seconds only right after I noticed the surging.

Right or wrong this isn't something that I haven't seen before (revving). If it is indeed the case that you should never hold your throttle open while not under load - even for a few seconds - I'll chalk it up to learning a lesson. I'm not an expert by any means but I feel as if I'm not a novice either, so I guess you learn something everyday.

But you're probably right - my idiocy at a few points caused the seize. I think I jetted too lean as well as revved it too much. What's annoying about this is I'm the only person I ride with that even cares about all of this crap; the rest of them don't do a single thing to their bikes - save an oil change once a month and a filter clean about that often - and they rarely have problems. Maybe it's luck? 👍

Also, I've pretty much studied that thread through and through. Which is why I arrived at 170 for my MJ even though the chart suggests about 172-175. Maybe I'm misreading my plug tests.

I'll look at the float valve, but I'm not sure there's a problem with that - never was a problem before. I think the gas overflow was due to the slide not fully seating because of the jacked throttle cable. Am I far off on this?

your throttle cable has nothing to do with your bike puking fuel.float too high, bad float needle so it,s not sealing or you have crap in the hose(hose is breakingup internally)and pieces are getting jambed in the needle seat.

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I agree with CJ, the needle valve sounds suspect. On many of my YZ's I have had the float stick and replacing the needle valve has always fixed the problem. Look carefully for galling on the aluminium below the rubber tip of the valve. This is where the contact area where valve slides inside the brass needle seat and wear occurs over time.

What exactly seized? Was it the lower rod bearing or the crank bearings? Another thing I saw with my Yz's is that the crank seal on the right side (clutch side) of the engine has been sucked in towards the crank bearing blocking off the oil passage. I have never had this happen with OEM seals though. However it has happened to me twice with aftermarket seals. Check to see if maybe this was the problem.

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I'm still stubbornly convinced it's not the needle valve wearing: it worked fine the weekend before, worked fine intermittently before I seized. Perhaps something got down in there and obstructed it, however. I'm going through my carb and thoroughly cleaning it during this rebuild.

I'm not sure what seized yet as I haven't torn into it just yet. I know the rod freezes up but can be loosened and brought through it's stroke. But at certain points something "catches" and it freezes up. This is just from having the engine on the bench with the cylinder off and me trying to move it through it's stroke.

I'll definitely check all the seals - I'll likely end up replacing everything since I'm into the motor anyway.

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You're on your own for figuring out your carb problem, but I soaked a crank in Marvel Mystery oil a few months ago, it wouldn't move one bit, two weeks later (changed the oil once in between to clean out the crap) and a little persuasion daily (move forward/back a few times as far as it would go) and you couldn't tell there was anything wrong with it ever.

I never did get to test that engine since I didn't have a top end. I gave that bottom end away for a pile of junk two weeks ago, still don't know what I'm doing with the pile o' junk.

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Have you ever considered it was just your cranks time to go?

I mean they DO break eventually.

To add more:

Usually a lean issue rears its head with the top end first...not the crank.

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