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Big bore for 1982 xt250 ?

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I just purchased a 1982 xt250. I like the bike , being light ECT. I'm 60yrs. Young been riding since 10 yrs old  started racing at 12 on a xr 75. I need more power & possibly some suspension help. I enjoy working on my bike.  Make it my own.

Thanks for any help.   Bill 

Edited by Xt4fun

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I want to thank you for post tt250& xt250hot rod project. I think I'll go with yfm 350 big bear. I have a question would the 350 barrel work or do I need to reserve. Also what about the yfm400 instead?? More ccs[emoji16]

Thanks xt4fun

 

 

 

 

 

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Typo error - I meant resleeve not reserve . Ill attach photo's soon. Thanks again for any help or advice you can give
me

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On 7/11/2019 at 2:26 PM, Xt4fun said:

I want to thank you for post tt250& xt250hot rod project. I think I'll go with yfm 350 big bear. I have a question would the 350 barrel work or do I need to reserve. Also what about the yfm400 instead?? More ccsemoji16.png

Thanks xt4fun

I built a couple 357cc engines out of SR250 and XT250s when I was at Yamaha. The Big Bear cylinder will work because the stud spacing is the same but you need to bore the cases. The XT is simple to do but the SR was hard because it required the right side crank from the BW350 to retain an electric starter. If you go that large however you really should use the Warrior cylinder head that has larger valves. The SR I built had the 250 head and was torguey but didn't make a lot more power than the stock SR. One of my co-workers at Yamaha built a 305 using a XV700 piston I believe and that was a bore in to the stock cylinder. He loved that bike and used it to good effect in Enduros back in the 80s.    

Guy

'

 

 

 

 

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I want to thank you very much Guy for the quick response quick response, & great info. Ya I think 350cc would be great .I agree that many cc's would definitely need larger valves . As they say "If it flows it goes" I agree... I worked late today.family night I'll get back to you tomorrow. What do you suggest for a cam ?


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The cam depends on what you want to do with the bike. For a mild one, but stronger than the XT250 profile, try to find a TT250 cam. Above that you can use a YFM350 Warrior cam or go aftermarket for one of the hotter ones for that same engine.

Guy

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Thanks again for the info . Just wondering how you came up with 357cc ? Does the yzf 400 barrel & piston fit ? I'm trying to build a duel purpose bike that's fun to ride on & off the road . Even an occasional run on the freeway . I do enjoy doing an occasional wheelie.ill go with a wiesco piston 10.5 - 11.0 compression. Bigger valves,I w/aftermarket cam make my own exhaust any recommendations for a carb.



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Sorry, I was remembering the SR250 I built using a BW350 crank and 4th over 84mm piston that made it a 357. With the 84mm piston and your XT250 crank you'll be at 321cc, still enough for a healthy boost in torque. 

Guy

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Thanks again guy lr . What do you think about advantage parts they have a kit87-1996 comes with piston,barrel,head & gaskets etc. Very reasonable price. I think all I need except carb & cam. Would the yzf350 carb rejetted perform well or should I consideter something larger? It's good to use some ones experience to help others not make the same mistaks others have made. [emoji4]

.

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I'm not familiar with their parts but it sounds like a complete package. Is that the Warrior big valve head? As far a carburetion goes I never really got to experiment with it on the other engine I built after I did the SR357 and for that I used a 34MM Mikuni CV which would not be the hot setup for your bike. You might try the stock XT carb to begin with just to see how it runs. 

Guy

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I'm not familiar with their parts but it sounds like a complete package. Is that the Warrior big valve head? As far a carburetion goes I never really got to experiment with it on the other engine I built after I did the SR357 and for that I used a 34MM Mikuni CV which would not be the hot setup for your bike. You might try the stock XT carb to begin with just to see how it runs. 
Guy
What years did the big valve head's come out for my application? Great info
. This motor is going to rock & roll. VERY COOL.

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The big valve head started with the 1987 YFM350XT Warrior. Look for used heads with the model prefix 1UY cast into the underside of the fins. The intake and exhaust valves also have the 1UY prefix instead of the 3Y1 parts used on the XT250 and the utility model 350 ATVS. 

Guy

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Hello my friend , you know if the rear shock is rebuildable or maybe a shock off another bike?the front forks maybe thicker oil, spacersfor a lil.more preload ? I heard someone installed a reservoir on the stock rear shock. Also one question guy on putting the 350 piston & barrel on the 250 i understand
I'll need to increase the rod pin dia. By 1 mm. Correct? I truly appreciate your assistance in this project.


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Guy LR, sorry to reopen this discussion, but I was wondering if you have done this conversion on a tt250 or an xt250.  I see the crank snouts look different on both the yfm350 crank and the bw350 crank, compared to the xt/tt250 crank.  The main bearings are the same.  Is there some way it will still go together, or does it require different crankcase covers, or machining of the 350 crank ends to make fit?  I am really interested in doing this build, and any help is greatly appreciated!  Thank you

 

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7 hours ago, Cody Knutson said:

Guy LR, sorry to reopen this discussion, but I was wondering if you have done this conversion on a tt250 or an xt250.  I see the crank snouts look different on both the yfm350 crank and the bw350 crank, compared to the xt/tt250 crank.  The main bearings are the same.  Is there some way it will still go together, or does it require different crankcase covers, or machining of the 350 crank ends to make fit?  I am really interested in doing this build, and any help is greatly appreciated!  Thank you

 

The BW crank is what you need to make it work. Hard to find now. Back when I was building them in 1990-92 they were still available to me at Yamaha as were the other parts like the clutch covers and CDI mag. If I were doing it now I'd try to find a junk BW motor to rebuild and just remove the electric starter and starter gears. 

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