77 YZ250 Project

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dt400eman
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Post by dt400eman » Mon Dec 12, 2016 10:57 pm

Different Mikuni carbs for similar displacement on any 2 models, and rpm range, pipe specs, etc may draw differently from the jets due to their internal set up, so if the carbs are from similar years/models maybe you can just use the new carb as is with what jetting came stock.[dunno]
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Post by dt400eman » Mon Dec 12, 2016 11:00 pm

edit- came stock that is on the donor bike, not on what your old carb has.
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Post by dt400eman » Tue Dec 13, 2016 2:13 pm

From post 10-[quote]I would have to think that sticking with the 38's jetting specs may be too lean if it came off a 400.QUOTE]
I believe I didn't think this through- since the 400 has a larger displacement the jets could be overly large for the 250.
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UTBMXCruiser
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Post by UTBMXCruiser » Thu Dec 15, 2016 8:02 pm

I was referring to sticking the same jetting in a 38 I have on hand that came in the stock 38 on the 78 YZ250. I have these specs from Yamaha Wrench Reports. Already have a slide with the correct cutaway.
86 YZ250S, 84 RZ350L KR, 83 IT250K, 82 IT465J, 81 YZ465H X 2, 81 ZIT465H, 81 YZ250H, 79 DT250F, 79 DT100F, 78 DT400E X 2, 78 YZ400E, 78 DT175E, 77 YZ400D X 2, 77 YZ250D, 76 Bultaco 250 Pursang Mk 9, 75 DT250B, 74 Suzuki TM250L, 72 DT2 MX X 2
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Post by dt400eman » Tue Dec 20, 2016 4:21 pm

This guy is super knowledgeable about performance upgrades to the YZ engine. I have watched all his videos. He hasn't posted any for along time because real tuners don't have the time. Enjoy this assement of late 70's YZ. Check'm out- his other videos.dance

https://www.youtube.com/watch?v=9doC2H6OnEs
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Post by UTBMXCruiser » Sat Jan 07, 2017 3:06 pm

I received the new cylinder and head and it is as advertised brand new never been used. I have since bought 2 sets of standard pistons and rings. Both are the double ring piston for the 78 2K7.

So now I have another money pit project to press on with. I will soon be digging out that bottom end to start tearing into. An ignition and pipe are still on the acquisition list.

The target for all this is a legal AHRMA "Historic" class 250. I made inquiries with the sanction body about this class. Limited to 9 inches of suspension travel. I will have to use a 77 frame and swingarm. They would not allow me to use a 78 aluminum swingarm, even though the 78 arm is the same exact dimension as the 77 arm except the material it was made out of. Because the stock 78 arm came on a 78, it is not legal. I would have to invest big bucks to dig up a DG or ProFab aluminum aftermarket swingarm to run aluminum. They are out there but the prices are nuts! There is nothing in the rules that would stop me from adding the 78 configuration chain guide rollers to the 77 frame to aide in keeping the chain on the bike, which was a big issue on the 77. The 77 frame was made of mild steel and the 78 chrome moly. Other than the chain rollers, they are the same dimension. As with the swingarm, I have to run a 77 frame.
86 YZ250S, 84 RZ350L KR, 83 IT250K, 82 IT465J, 81 YZ465H X 2, 81 ZIT465H, 81 YZ250H, 79 DT250F, 79 DT100F, 78 DT400E X 2, 78 YZ400E, 78 DT175E, 77 YZ400D X 2, 77 YZ250D, 76 Bultaco 250 Pursang Mk 9, 75 DT250B, 74 Suzuki TM250L, 72 DT2 MX X 2
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Post by dt400eman » Sat Jan 07, 2017 4:35 pm

I was surprised at the goodies to be found on e-bay by just blind luck. By entering in search terms just for instance "motorcycle swing arm" or if you get 40,000 hits then narrow to "swing arm dirt bikes". Everyone pretty much knows the difference of what a street bike swing arm looks like compared to a dirt bike. Sometimes a general reference can net you a diamond in the rough, as they do not know exactly what they have and so do not have a label to give the part. I paid 110 bucks for my 79 DT250 front fender on e-bay. There was a chap looking for a front fender for his 78 400 on the forum at one time. He did not want the copies from overseas. After looking at the search terms "dirt bike fenders" I came across one listed for a Honda. It was there for along time. He got it for an unbelievable $5.50 + small shipping fee. Good luck on finding the parts and if you need any help.[o-o]
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Post by dt400eman » Sat Jan 07, 2017 5:01 pm

Also with e-bay when I am window shopping for parts for my bike I type in "78 DT400 used". The word "used" gets rid of all the new chains and sprockets and the riff raff, and I only get all the oem goodies. Also looking up "parts lots (fill in blank) can net results. Also on craigslist there are hordes of bikes and parts lots and generally I have found shooting an e-mail can sometimes net unexpected results.
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Post by dt400eman » Sat Jan 07, 2017 5:05 pm

Is this the one you are looking to get, albeit like you said very expensive.
http://www.ebay.com/itm/YAMAHA-YZ250-YZ ... 2471704918
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Post by dt400eman » Sat Jan 07, 2017 5:25 pm

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