kdx 250 problem running 2 hours after rebuild

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rybear
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kdx 250 problem running 2 hours after rebuild

Post by rybear »

i purchased a 1999 KDX 250 for $300 about 2 months ago. it needed a new top end. i took the motor out to get a better look and removed the jug. The piston skirt came apart and the jibletts were all jammed down underneath the crank. i cleaned all that crap out with oil and compressed air and thoroughly cleaned the bearings, as well as i could without splitting the cases. ordered the gasket kit, piston, plug. I had my cylinder bored and assembled it all. Started 3rd kick. ran great. did a very good top end break in on it. warm it up.....let it cool....warm it up....let it cool. It took some serious self control. so i gt it all fixed up. took it to the track (three palms -Houston TX) do 1/2 a lap and it boggs down and dies. Forgot to turn the fuel on like a dumb dumb. I get back going, do 2 good laps then it starts to act so stupid. boggs horrible! wont rev! wont idle! and was smoking so bad people were waving me down! it literally smoked out the track. my first thought was its the reeds or the plug. because my brother owned a 96' cr250 and it was acting like that and it was a split reed. So checked my reeds and the do not seam bad. cleaned the carb even though it looked brand new on the inside. took my pipe off and looked in the cylinder and it still has the hone marks nothing black. Plug is brand new. compression test shows 120psi. put it all back together and it runs the same. its very hard to start and when you get it to, it will barely run. started it again and sprayed carb cleaner all over everything to see if i had a leak on the boot or something.... no dice. took the carb to air cleaner boot off and started it. was trying to get it to rev and it bogged down and died, right before it threw a flame out the back of the carb. took it back apart to closer look at the reeds because reeds are a one way valve. nothing should come back like that! and the only thing that i could see (besides some slight scuffing where it touched the housing) was that one side did not 100% lay flat with the housing, it may have been enough to slide a piece of paper through it. so my question is........could that be it?
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Julien D
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Post by Julien D »

120 psi is pretty low. I'm guessing crank seals are leaking, and badly from the sound of it. No big surprise if there were pieces of piston in the crankcase.
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Post by Colorado Mike »

Yep. The cloud of smoke was most likely transmission oil getting sucked in through the clutch side crank seal. Split the cases and change the crank bearings and seals. I would think your rod bearing is toast too. Always thoroughly clean your intake and exhaust tracts when dealing with a blown motor. The reed block especially can hold pieces of motor that will blow the new one.
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Post by Slick_Nick »

Running it without the fuel on like that starved the engine of oil. YOu probably cooked all of your bearings, if not the piston too.
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Post by Julien D »

>|<>QBB<
Slick_Nick wrote:Running it without the fuel on like that starved the engine of oil. YOu probably cooked all of your bearings, if not the piston too.
Wow, that's a stretch. Especially when you take into account the recently shattered piston. I've run my bike out of gas more than once and haven't cooked anything yet.

:roll:
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took clutch bits off

Post by rybear »

ok i took apart that whole side, and my oil seal does not look bad at all. i was under the impression that if they were bad you could really tell by looking at it. is that true?
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Post by Slick_Nick »

>|<>QBB<
juliend wrote:>|<>QBB<
Slick_Nick wrote:Running it without the fuel on like that starved the engine of oil. YOu probably cooked all of your bearings, if not the piston too.
Wow, that's a stretch. Especially when you take into account the recently shattered piston. I've run my bike out of gas more than once and haven't cooked anything yet.

:roll:
Run it hard on a track with the fuel turned off, and tell me what happens.

Let me know if you need help rebuilding your engine... :roll:
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Post by Julien D »

LMAO. It happens. People run their bikes out of fuel all the time. Aside from the rare occurrence of the bike leaning out and pinning, it's not going to cause a problem such as the OP is experiencing with his 250.

No, I won't need any help from you with my next rebuild. I prefer to replace the parts I take out, not clean the rust off and call them "good as new".

:butthead:
ok i took apart that whole side, and my oil seal does not look bad at all. i was under the impression that if they were bad you could really tell by looking at it. is that true?
No, you really can't tell by looking. Especially not from the outside...
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Post by radonc73 »

Not to be nit picky here but they didin't make a 99 KDX 250. I am pretty sure that the early 90s KDX motor was from an 88 KX. I know that it is hard to own the baddest KDX EVER but you have to be prepared to split the 2 worlds. If you look on the powerplant it should say KX 250 on it.

If you don't have one already get a KDX manual with the addendum for the 250. It helps alot to clarify the differences between that years 200/220 and the 250 as the 250 had the USD forks and all. I bought a disk off of Ebay for like $20 and now can just print stuff out instead of getting the manual greasy.
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Post by Julien D »

Good point. I think they sold a 95 model in EU.
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Post by rybear »

yea its a 93 'i get the year mixed up between my bikes
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Post by Slick_Nick »

>|<>QBB<
juliend wrote:LMAO. It happens. People run their bikes out of fuel all the time. Aside from the rare occurrence of the bike leaning out and pinning, it's not going to cause a problem such as the OP is experiencing with his 250.

No, I won't need any help from you with my next rebuild. I prefer to replace the parts I take out, not clean the rust off and call them "good as new".
We have been over this a number of times, and the bearings, etc were good as new once cleaned up, I don't know why you insist on bringing it up time and time again as though it makes a difference. The bike now has almost 50 hours on it since the rebuild, and not a single thing has gone wrong. I inspected all bearings and regreased at 25 hours, and they all looked great still, once again, proving that all of the parts that I refurbished and reused are functioning normally, and I avoided unnessicary replacement.

Are you implying that running a bike out of fuel and oil can not under any circumstance cause a siezure? Time to refresh yourself on the principles of an internal combustion engine... :roll:
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Post by Indawoods »

It didn't seize...

Regardless of what you call good Nick... you have no means of checking for metal loss or know what tolerances are for each bearing that you "recondition". The moisture that the bearings were exposed too and the contaninants all contribute to metal loss on the races and the bearings themselves. You may have bought a little time but there is no validity to your claim that they are good as new.

You have no way of gauging when they will fail. 2 months, 6 months? I like the piece of mind that my bearings are new and up to full manufacturers spec when I put them in.
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Post by Julien D »

Nick, I'm just giving you a hard time. And I enjoy bringing that up whenever you say something ignorant.


Inda is correct. It did NOT seize. If it ran out of fuel, pinned, and seized, then yes, it can and most likely will cause a problem. However, 99 out of 100 times there will be no damage from running out of fuel, whether the tank runs dry or the petcock is off makes no difference. If it simply sputters out and dies, there should be no ill effect whatsoever. Seems foolish to me to make an assumption like that when the OP has just stated he had a shattered piston and did not split the cases to clean it out.
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Post by rybear »

it was a twisted keyway on my flywheel. it was 180 off. if it was the crank seals than i would only run at high rpm and not idle. mine wouldnt do ether. its a brand new rebuild so the piston and rings are not yet broken in, so 120psi is not abnormal. maby for a kx250 that is wicked high comp, but for my little ole kdx its good (the kx head gasket is literally half as thin as the kdx 250 one) . and the psi would not be affected by my seals if it were to be leaking. on my motor there is no gasket between the cases its just really really tight. so it is unlikely that it would be sucking in from there unless they were leaking awfully. thank you for your help though.
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Post by Julien D »

120 is indeed pretty low. And your explanation of failed crank bearings is off as well. At any rate, I'm glad you figured it out. Good luck with that bottom end holding up for you!
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Post by rybear »

what is the psi suppose to be?
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Post by Julien D »

Not sure, you got a manual handy? I can try to look it up, but I would guess 120 is on the low side of good. I would expect higher after a new top end.
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Post by rybear »

i do plain on soon installing a 88 kx 250 head gasket to raise comp.
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Post by twenny »

Alright rybear ,

Ive just rebuilt my 89 200 SR , new bearings , seals , piston, big end etc , before the first heat cycle my compression test showed 145 psi , after 2 heat cycles and a 15 min ride its 160 psi . yours is defo to low bud .

and crank seals will affect compression , if there bad when the piston comes down crankcase pressure can be lost thru the seals instead of been transferd into the combustion chamber . there for losing compression .

Are you holding the throttle fully open ? When you did your test ,you should be matey .

do you under stand the basics of a 2 stroke engine fella , sorry if that sounds harsh not meant too , it just if you fully understand somthing youll find it easier to diagnose problems

my advice is read the service manuel cover to cover , and research how a 2 stroke engine actually works bud . Even buy a scrap engine and strip it taking into account were all the bits go and there relationship with each other .

hope this helps

Twenny
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