YZ250 Building a mean 78' yz250

[Nicholas Sibley]

Hello, I recently got a 1978 yz250 from a swap meet in San Diego. The bike was $500 and is almost complete. I plan on going through the entire bike and mechanically restoring it but I want to make it my every day rider because I've fallen in love with vintage bikes. I usually ride a 2000 yz250 but it's just not cool to me. My question is, what all can I do to this bike to pull more power out of it while remaining reliable, and what chassis upgrades/mods can I do to make it perform better where I ride? (a motocross style track built in some trails by my house) I know it's never gonna outperform my newer bike but I just want to see how far I can take it while still keeping it vintage. I'm only 17 so go easy on me ? I'm new to this game. I know that I can port the cylinder and do head work as well as a pipe but what about boring the cylinder? Are there aftermarket reeds? What are some suspension swaps I can do? Any input would be greatly appreciated. I'll upload some pics as soon as I get the bike up here to Washington. Thanks!

[Turboloader]

For plastics and other parts DC Plastics is your source for cosmetics. Ebay might have a D.G pipe you can use. I wouldnt alter anything on the motor, except fresh boyseen reeds, pipe and muffler combo, maybe some jetting< tricky, and sprockets and chain for gearing changes. Keep it stock! Maybe a fresh suspension fluid and seal change. Dont waste your money on frivolous engine portings and millings, leave it stock and enjoy it for what it is...

[SlowDinoDog]

Yeah...the stock engine is plenty fast...maybe, not as powerful as a modern YZ250, but neither is the chassis.

If it were mine, would split the cases for new crank bearings and crank seals. You'll be chasing jetting, otherwise.

Install a fresh top end with a hone or rebore...and gaskets. Probably a fresh clutch and cables.

Rebuild the forks with new seals...and add Racetech Valves and springs will be your best speed mod. Have the rear shock rebuilt. New tires and tubes...clean up the brake shoes.

New triple stem bearings, swingarm bearings, wheel bearings. Rebuild the Mikuni carb with new jets...probably new reeds.

 

Edited July 15, 2020 by SlowDinoDog

[Turboloader]

1 hour ago, SlowDinoDog said:

Yeah...the stock engine is plenty fast...maybe, not as powerful as a modern YZ250, but neither is the chassis.

If it were mine, would split the cases for new crank bearings and crank seals. You'll be chasing jetting, otherwise.

Install a fresh top end with a hone or rebore...and gaskets. Probably a fresh clutch and cables.

Rebuild the forks with new seals...and add Racetech Valves and springs will be your best speed mod. Have the rear shock rebuilt. New tires and tubes...clean up the brake shoes.

New triple stem bearings, swingarm bearings, wheel bearings. Rebuild the Mikuni carb with new jets...probably new reeds.

 

Man, SDD, your yz is sooo purty!!?

[CaptDan]

1 hour ago, SlowDinoDog said:

...New triple stem bearings...

I believe the '78 has the dreadful ball bearings in the stem.

There should be a tapered roller bearing kit available; a very worthwhile upgrade.

[SlowDinoDog]

13 minutes ago, M E T A L A C I D said:

Man, SDD, your yz is sooo purty!!?

 

The pics at the end of the video were as nice as it got. You know how it is....you ride them and they get scuffed.

I'm going to pull the engine for crank bearings and seals in a bit. I'll start a thread.

I'm still getting the cleaned up carbs back in my old Shadow 750 so I can sell it.

I'm not riding the YZ much...I'm thinking of selling it...but, its really cool just to look at. ?

Edited July 15, 2020 by SlowDinoDog

[CaptDan]

^ ^ ^  Upgraded to a '79 swingarm?

[Turboloader]

8 minutes ago, SlowDinoDog said:

 

The pics at the end of the video were as nice as it got. You know how it is....you ride them and they get scuffed.

I'm going to pull the engine for crank bearings and seals in a bit. I'll start a thread.

I'm still getting the cleaned up carbs back in my old Shadow 750 so I can sell it.

I'm not riding the YZ much...I'm thinking of selling it...but, its really cool just to look at. ?

She is a beauty! Looks brand new, Yeup Id love to see the rebuild. Always fun to check out!?

[SlowDinoDog]

6 minutes ago, CaptDan said:

^ ^ ^  Upgraded to a '79 swingarm?

 

I don't think so...it seems pretty stock all around except the plastics, bars, clutch perch,...

It came from Yuma. I met the guy in Chandler.

I fired it up and test rode it in a CrackerBarrel parking lot on a Sunday morning. ?

[Turboloader]

2 minutes ago, SlowDinoDog said:

 

I don't think so...it seems pretty stock all around except the plastics, bars, clutch perch,...

It came from Yuma. I met the guy in Chandler.

I fired it up and test rode it in a CrackerBarrel parking lot on a Sunday morning. ?

You rode in a cracker barrel parking lot?, LOL! Coool bro! Your state is layed back, over here, we would have cops all round, and people tripping out. Looks cool on your trailer. bran new!?

[CaptDan]

Sweet.

One of the updates on the 1979 YZs was the trick-looking aluminum swingarm. The 1976, '77, & '78 bikes had a black tubular swingarm. I mentioned that because putting the AL 'arm on the older bikes was a popular swap back in the day.

[SlowDinoDog]

3 minutes ago, M E T A L A C I D said:

You rode in a cracker barrel parking lot?, LOL! Coool bro! Your state is layed back, over here, we would have cops all round, and people tripping out. Looks cool on your trailer. bran new!?

 

yeah...it was smoking like a mosquito fogger and its LOUD with that steel 'silencer'.

The guy said it was in good condition when he found it and he 'rebuilt' it. Unfortunately, his skills were a bit limited. He used the wrong cylinder base gasket, it leaked and I had to put in a fresh top end. Then, she started drinking Rotella through the crank seal...so, that's where its at.

[Sullysully]

2 hours ago, SlowDinoDog said:

Yeah...the stock engine is plenty fast...maybe, not as powerful as a modern YZ250, but neither is the chassis.

If it were mine, would split the cases for new crank bearings and crank seals. You'll be chasing jetting, otherwise.

Install a fresh top end with a hone or rebore...and gaskets. Probably a fresh clutch and cables.

Rebuild the forks with new seals...and add Racetech Valves and springs will be your best speed mod. Have the rear shock rebuilt. New tires and tubes...clean up the brake shoes.

New triple stem bearings, swingarm bearings, wheel bearings. Rebuild the Mikuni carb with new jets...probably new reeds.

 

Stayin’ alive...

[SlowDinoDog]

2 minutes ago, CaptDan said:

Sweet.

One of the updates on the 1979 YZs was the trick-looking aluminum swingarm. The 1976, '77, & '78 bikes had a black tubular swingarm. I mentioned that because putting the AL 'arm on the older bikes was a popular swap back in the day.

 

I hijacked this guys thread big time...but, they are really cool bikes to talk about.

They got the aluminum swingarm in '78. I think the swingarm got longer in subsequent years...and the swingarm pivot moved closer to the drive sprocket in later models. The '78 has a pretty elaborate chain tensioner.

[SlowDinoDog]

Looks like the '78 250 and 400 had an aluminum swingarm but the 125/100 still used steel tubing.

[CaptDan]

According to the parts lists (and my suspect memory ? ) the 1978 swingarm was tube steel.

https://www.partzilla.com/catalog/yamaha/motorcycle/1978/yz250e/swing-arm-yz250c

But no real hijack done! I was going the recommend the AL swingarm to the O.P. (if it doesn't have it), so we're good. ?

https://www.partzilla.com/catalog/yamaha/motorcycle/1979/yz250f/rear-arm

[CaptDan]

Not the first time Yamaha parts lists (or my memory) have been wrong!  

 

[SlowDinoDog]

14 minutes ago, CaptDan said:

Not the first time Yamaha parts lists (or my memory) have been wrong!  

 

 

that's even more odd...the white (Euro version?) '78 YZ 125 had an aluminum swingarm in that brochure and the yellow '78 YZ 125 had a steel swing arm in the ad I posted. ??‍♂️?

[Turboloader]

6 hours ago, SlowDinoDog said:

Looks like the '78 250 and 400 had an aluminum swingarm but the 125/100 still used steel tubing.

The YZ's owned the 70's, period. Love those classic ads, good to read on my lunch break, sammich, bag o' chips, and greasy fingerprints on my cycle world.

5 hours ago, CaptDan said:

Not the first time Yamaha parts lists (or my memory) have been wrong!  

 

Euro Y-zingers..?

[Turboloader]

6 hours ago, SlowDinoDog said:

 

yeah...it was smoking like a mosquito fogger and its LOUD with that steel 'silencer'.

The guy said it was in good condition when he found it and he 'rebuilt' it. Unfortunately, his skills were a bit limited. He used the wrong cylinder base gasket, it leaked and I had to put in a fresh top end. Then, she started drinking Rotella through the crank seal...so, that's where its at.

You rescued a nice yz from the scrap yard and worked your magic....?