Possible bad crank seal? yz250

Samuel
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6/30/2014
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Independence, OR US
2/2/2018 5:09pm
Just picked up a yz 250. I rode it on the test drive and it was smoking a lot, so I figured it was a jetting issue, no big deal. I got home, changed the transmission oil, did a top end, new plug, new filter, 40:1 and fired up the bike and it ran exactly the same as when I test rode it. Reeds look good. Starts okay and runs off the bottom okay, but then at 1/4th throttle it chokes up, sputters, and smokes like mad. I leaned it out a few times, clip position, main, and pilot with almost no discernible difference. I also adjusted the float level and checked the float with the bowl off. The float needle is working as it should. So I'm thinking maybe crank seals. I've never had an issue with crank seals, so I'm not sure what it typically looks like. I just pulled everything to get to the seal on the clutch side, and I can't see anything wrong with it. If it were a bad seal, would I normally be able to tell by looking at it? Is there anything else I could check on why my bike is rich?
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kzizok
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AS US
Fantasy
1859th
2/2/2018 5:44pm
Test drive....
sandman768
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5955
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3/21/2014
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Saratoga Springs, NY US
2/2/2018 5:58pm
I would say rt side crank seal, sucking trans oil...sometimes you can smell Trans oil burning out silencer...left side crank seal usually results in lean condition... I would. Replace both crank seals, easy to do external seals...you did not mention year of bike but if your removing clutch side cover I would plan on rebuilding water pump & inspect clutch assy while your there....u tube YZ crank seal replacement, it's pretty easy ...
1
Paw Paw 271
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4/3/2013
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Benton, LA US
2/2/2018 6:05pm
You can't tell if a seal is bad as the inside sealing surface is not viewable. Remember that cranks seals have 2 sealing surfaces. One is to prevent dirt and oil from entering the lower end of the crank and the other is to prevent the blowing out of the compressed intake charge. So, both sealing edges must be working for the seal to work correctly.
I would suggest looking at the bottom side of the old piston to see if there is a dark deposit in it. If there is said deposit, then the clutch side seal is leaking, assuming as you stated that the carb is working correctly and your center crank halfs are sealed correctly and not leaking.

Paw Paw
2
Monk
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CA
2/3/2018 8:12pm
Build a pressure tester....

The Shop

Hondas4Life3
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7/30/2013
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Louisburg, KS US
2/4/2018 7:32am
Leak down test is how you determine if they are bad.
2/4/2018 7:58am
See if there is a noticable loss of gear oil. Sounds like a crank seal. Is there a large amount of oil residue on the silencer? Does it smell like burning gear oil?
2/5/2018 6:57am
You can't tell if a seal is bad as the inside sealing surface is not viewable. Remember that cranks seals have 2 sealing surfaces. One is...
You can't tell if a seal is bad as the inside sealing surface is not viewable. Remember that cranks seals have 2 sealing surfaces. One is to prevent dirt and oil from entering the lower end of the crank and the other is to prevent the blowing out of the compressed intake charge. So, both sealing edges must be working for the seal to work correctly.
I would suggest looking at the bottom side of the old piston to see if there is a dark deposit in it. If there is said deposit, then the clutch side seal is leaking, assuming as you stated that the carb is working correctly and your center crank halfs are sealed correctly and not leaking.

Paw Paw
Always enjoy reading your tech posts, Paw Paw... You may think you're responding to just one person's question, but I assure you many others like myself are eagerly to picking up all the knowledge you're dropping Smile
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psorace
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1
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4/23/2019
Location
Nashville, TN US
4/23/2019 8:14pm
I bought a new 2018 yz250x from a dealer in July 2018. The day I brought it home the countershaft sprocket seal was leaking transmission oil ! I replaced it. She ran very rich. I dropped the main jet down to a 172 and the pilot to a 45. I tried every thicker and longer taper needle. I installed a JD Jetting kit and went 3 steps leaner than recommended on the needle and was still fouling plugs ! JD techs couldn’t believe it. One hour and it was fouled. I figured that the crank seal must be fine because it was a brand new bike !!! I ordered a custom Lectron carb and installed it. It took most of the day to foul a plug . Went one quarter turn leaner on the rod, then another turn leaner and it was definitely too lean. I finally figured it was a bad right crank seal. I replaced it and she is spot on perfect plugs every time. I have over 60 hours of jetting changes wrenching on this bike and it was a bad right crank seal the whole time. DNK if Yamaha had a bad run on seals or a Japanese crack head installed the seals, because 2 seals were bad the day I bought it NEW. Now she is as sweet as a prom queen. Dealer couldn’t care less. They made me pay to replace the countershaft seal saying the carb overflow tube melted to the seal because it was improperly routed. Sure they charged Yamaha for the warranty work and double dipped into my wallet as well. Lesson learned. I installed the crank seal myself. F Them
1
lumpy790
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9140
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9/18/2007
Location
York, SC US
4/24/2019 5:37am
A cold 2st rose will always smoke till it is warmed up and blown out to get any left over oil out of the bottom end.

Not the side that it would get the oil from but Pull the ignition cover off and see if there is any up or down play. Worn main bearings could pull upward on seal letting some oils slip by.

Could be a leak from the yamabond between the cases too.
1/13/2021 1:17am
psorace wrote:
I bought a new 2018 yz250x from a dealer in July 2018. The day I brought it home the countershaft sprocket seal was leaking transmission oil...
I bought a new 2018 yz250x from a dealer in July 2018. The day I brought it home the countershaft sprocket seal was leaking transmission oil ! I replaced it. She ran very rich. I dropped the main jet down to a 172 and the pilot to a 45. I tried every thicker and longer taper needle. I installed a JD Jetting kit and went 3 steps leaner than recommended on the needle and was still fouling plugs ! JD techs couldn’t believe it. One hour and it was fouled. I figured that the crank seal must be fine because it was a brand new bike !!! I ordered a custom Lectron carb and installed it. It took most of the day to foul a plug . Went one quarter turn leaner on the rod, then another turn leaner and it was definitely too lean. I finally figured it was a bad right crank seal. I replaced it and she is spot on perfect plugs every time. I have over 60 hours of jetting changes wrenching on this bike and it was a bad right crank seal the whole time. DNK if Yamaha had a bad run on seals or a Japanese crack head installed the seals, because 2 seals were bad the day I bought it NEW. Now she is as sweet as a prom queen. Dealer couldn’t care less. They made me pay to replace the countershaft seal saying the carb overflow tube melted to the seal because it was improperly routed. Sure they charged Yamaha for the warranty work and double dipped into my wallet as well. Lesson learned. I installed the crank seal myself. F Them
I have the same issue sorry for late reply my kx 125 is rich as after a topend an replate been messing with jetting but no good not fouling because I'm always on the pipe but when I ho slow it does anyways I thought your post interesting as I had a countershaft leak so I thought about it an if its pressurizing the tranny then it pops the countershaft an leaks there I'm not getting any tranny loss because theres 100 MLS extra in there because I think its leaking gas in to the oil aswell you said two bad seals naa it was just one but then leaked the countershaft I think this is a symptom of a bad right crank seal yet no one has said it is just makes sense to me an my rocket scientist brain cheers
1

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