Possible bad crank seal? yz250

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2/2/2018 5:09 PM
Edited Date/Time: 2/2/2018 5:40 PM

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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2/2/2018 5:44 PM

Test drive....

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much ty. How to spot a paid forum poster/artificial forum traffic producer (see list of actions/phrases below):

Copius pattern amounts of phrases like “Anyone have”..., “Anybody know?”.... and their variations.

Thoughts?
Any help is appreciated!
Thanks in advance!





2/2/2018 5:58 PM

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 ...

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2/2/2018 6:05 PM

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

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2/3/2018 8:12 PM

Build a pressure tester....

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2/4/2018 7:32 AM

Leak down test is how you determine if they are bad.

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2/4/2018 7:58 AM

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?

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Nolan Stromberg
Coatings Specialist
finishfirstcoatings.com
nolan@finishfirstcoatings.com

2/5/2018 6:57 AM

Paw Paw 271 wrote:

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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Positively, absolutely 110% obsessed with anything MOTO.

4/23/2019 8:14 PM

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

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4/24/2019 5:37 AM

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.

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