1978 YZ250?

alphado
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alphado
Edited Date/Time 8/28/2012 10:12pm
I trade my pitbike last night for a 1978 Yamaha YZ250. It is all there and ready to ride. Could use new plastic to make it super nice. Tell me about the bike.
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Craze
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Craze 7/30/2009 9:50am
7/30/2009 9:50am Edited Date/Time 4/17/2016 1:13am
It's a 6 speed and it's FAAAAST!!!!, you can raise the footpegs 5mm up, and 5mm back by installing "79" footpeg mounts, adding a DG pipe or "79" pipe gives it a little more on Bottom & top end. I ran K & N filter's (Two sided airbox) on mine...Great bike.
lumpy790
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lumpy790 7/30/2009 11:34am
7/30/2009 11:34am
Did the 250's have cases that wore out where the main bearings went in like the 125 did?
project racer 7/30/2009 2:22pm
7/30/2009 2:22pm Edited Date/Time 4/17/2016 1:13am
Those were very peaky bikes. Good mid to top. The '79 had a much better range of power.  I raced the '78 125E, '78 250 &400E, '79 250 &400F and '80 250G models. Also had a Champion framed dirt tracker that used the '78/'79 YZ250 motors. In '95 I won the 250B class at the Southeast regional dirt track championship at Daytona short track on it. I was the only aircooled bike in the final. The water coolers had nothing for me.Falicon crank, stock 80 over cylinder, 39mm Keihin D slide carb,'87 YZ80 external flywheel ignition,Jemco pipe. Super fast. I had heard of the main bearing issues on those models, but never had the problem myself. I lost the tranny in the G 250 in three month's. Good bike, but weak gearbox.
The two sided air box was last used in '76. The '77,'78'and'79 were single sided airboxes that were accessed from the RH side panel.

The Shop

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project racer 7/30/2009 2:31pm
7/30/2009 2:31pm Edited Date/Time 4/17/2016 1:13am


The blue and orange bike is the '78/'79 engined 250 YZ Champion. The white bike is a '74 MX360 Champion. The guy on the yellow bike in front of me is on a '75 MX400B powered Shell framed Yamaha. This was the 251-600cc class.
project racer 7/30/2009 2:32pm
7/30/2009 2:32pm Edited Date/Time 4/17/2016 1:13am
This is the start of the 250B final in '95 during bike week /SE championships. Lots of "wet barrel" guys were wondering what I had "done to the motor". It was a home built "special". Got hard up for money and sold it in '96.
ThePipe
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ThePipe 7/30/2009 7:14pm
7/30/2009 7:14pm Edited Date/Time 4/17/2016 1:14am
ThePipe
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ThePipe 7/30/2009 7:15pm
7/30/2009 7:15pm Edited Date/Time 4/17/2016 1:14am
vetarider
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vetarider 7/31/2009 10:19am
7/31/2009 10:19am
I confirm the peaky motor. Weak down low but comes on hard mid to top. Definitely want to keep it on the pipe. Seemed like a lot of work to ride it hard but then again I did get lazy riding four strokes for so long.

Someone on this board also recommended a reed spacer to help the low end. Moose has one that goes back to the '86 model that seems like it should fit the '78 because they use the same intake manifold gasket so the hole pattern in the cylinder must be the same. I can't verify if it works or not, just passing along a recommendation.

I can email you the parts pages for the '78 and I have an Excel file with all of the part numbers used on the bike. For some reason, Yamaha took down the parts pages from their corporate website for the '78 YZ250.

Also, there are several Yamaha dealer websites that have a cross reference feature so you can see what other models used the same part numbers. They also show superseded numbers.

Sadly, I just sold mine this past weekend here in NC... but I don't think you were the buyer.
project racer 7/31/2009 7:33pm
7/31/2009 7:33pm Edited Date/Time 7/31/2009 7:38pm
vetarider wrote:
I confirm the peaky motor. Weak down low but comes on hard mid to top. Definitely want to keep it on the pipe. Seemed like a...
I confirm the peaky motor. Weak down low but comes on hard mid to top. Definitely want to keep it on the pipe. Seemed like a lot of work to ride it hard but then again I did get lazy riding four strokes for so long.

Someone on this board also recommended a reed spacer to help the low end. Moose has one that goes back to the '86 model that seems like it should fit the '78 because they use the same intake manifold gasket so the hole pattern in the cylinder must be the same. I can't verify if it works or not, just passing along a recommendation.

I can email you the parts pages for the '78 and I have an Excel file with all of the part numbers used on the bike. For some reason, Yamaha took down the parts pages from their corporate website for the '78 YZ250.

Also, there are several Yamaha dealer websites that have a cross reference feature so you can see what other models used the same part numbers. They also show superseded numbers.

Sadly, I just sold mine this past weekend here in NC... but I don't think you were the buyer.
I tried a reed spacer, but it was hardly noticeable. More improvement by just adding a set of Boyesen reeds. The best way to ride the '78 YZ250 is to treat it like a 125.
MXVet261
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MXVet261 8/1/2009 4:15pm
8/1/2009 4:15pm
Junk. Sell it and get an '81 RM125.
vetarider
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vetarider 8/3/2009 10:11am
8/3/2009 10:11am
Are the forks off of a '79 or '80 model? They look longer than stock '78 forks.

You can find replica front/rear fenders from a couple of sources. They're supposed to be working on side panels but the bad economy may have delayed the project because they were supposed to be out already.

Clarke now makes a replica tank but you can always find a good used one or re-condition the current one if it's not too far gone.

GIwasB4
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GIwasB4 8/4/2009 8:27am
8/4/2009 8:27am
That bike is sweet!!
You out of the Lube and TV Land racing?
alphado
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alphado 8/5/2009 3:22am
8/5/2009 3:22am
I still race my kid's KLX110 at the Lube, pretty much done with TV Land.
burker1307
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burker1307 8/10/2009 3:49pm
8/10/2009 3:49pm
I had a 1978 YZ 125E. Junk. Case halfs had the inserts that cracked two times in one year. Only way around it was new cases. I think the 250 had the same. I rebuilt a 1977 YZ 400D and they had the inserts too. I drift punched them to help keep them from spinning.

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project racer 8/10/2009 11:12pm
8/10/2009 11:12pm
burker1307 wrote:
I had a 1978 YZ 125E. Junk. Case halfs had the inserts that cracked two times in one year. Only way around it was new cases...
I had a 1978 YZ 125E. Junk. Case halfs had the inserts that cracked two times in one year. Only way around it was new cases. I think the 250 had the same. I rebuilt a 1977 YZ 400D and they had the inserts too. I drift punched them to help keep them from spinning.
I've owned YZ's from '76 to '83 , even dug '78-'79 YZ 250 motors out of junk yards for my dirt tracker ,and have never encountered the insert problem. If you take care when replacing the main bearings, by heating the cases and freezing the bearings, they practically fall in.

I can see where the inserts may come loose when beating old bearings out, or hammering new ones in. If you again heat the cases properly, even old bearings will just about fall out of the cases with hardly any force.

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