I picked up a Yamaha DT250 today

Rick Bergsma /

Got this sweet 1977 Yamaha DT250 today. It has 8000 kms on the odo .

It wouldn't start , when I checked for spark , the plug was sparking everywhere but the tip . I installed a new plug and poured in some premix and it fired right up but was kinda gutless . I checked the compression and sadly it was only 100 psi .

So ..... I tore it down and discovered it was still the std 70 mm piston . I'm going to order the next size up from Summit and rebuild the top end . The bottom end seems tight , I'll just leave it alone for now .

20201126_095918-001.jpg

Re: I picked up a Yamaha DT250 today

Pushrod Fifty /

Nice bike, if the bore isnt scored, maybe you just need to clean the rings, ports and pipe to get back some compression.

Re: I picked up a Yamaha DT250 today

Looking good.

Re: I picked up a Yamaha DT250 today

Hey Rick, congrats on the new addition!

I wanted to give you some information in regards to the compression check that you did. The factory compression ratio on these DTs is very low. I discovered this with a DT400 that I owned some years ago.

Your DT250 came from factory with 6.7:1 compression. https://www.yamaha-enduros.com/index.php/250cc-specs/45-77-80-dt250-d-f-specifications

6.7 x ~14.7 atmospheric pressure should give a reading of approximately 98.5 psi.

In other words, your compression is healthy and spot on! These DTs are somewhat gutless(some of the old timers swear “DT” stands for “De-Tuned”), but still a lot of fun. An expansion chamber/porting/head work might satisfy your want for a little more grunt. The Yamaha-Enduros forum is a tremendous resource for these bikes. https://www.yamaha-enduros.com/index.php/forums

Take care and stay safe!

Re: I picked up a Yamaha DT250 today

DT's have super low compression. I know it's different but my DT50 has such low compression I start it by putting it in first and gently walking next to it, no clutch required. They're all like that too. Definitely not a bump start, way more like a moped push start except even easier.

Re: I picked up a Yamaha DT250 today

Overpriced Parts /

> All Gas No Brakes Wrote:

> -------------------------------------------------------

> Hey Rick, congrats on the new addition!

>

> I wanted to give you some information in regards to the compression

> check that you did. The factory compression ratio on these DTs is very

> low. I discovered this with a DT400 that I owned some years ago.

>

> Your DT250 came from factory with 6.7:1 compression.

> https://www.yamaha-enduros.com/index.php/250cc-specs/45-77-80-dt250-d-f-specifications

>

> 6.7 x ~14.7 atmospheric pressure should give a reading of approximately

> 98.5 psi.

>

> In other words, your compression is healthy and spot on! These DTs are

> somewhat gutless(some of the old timers swear “DT” stands for

> “De-Tuned”), but still a lot of fun. An expansion chamber/porting/head

> work might satisfy your want for a little more grunt. The Yamaha-Enduros

> forum is a tremendous resource for these bikes.

> https://www.yamaha-enduros.com/index.php/forums

>

> Take care and stay safe!

I owned DT 125, 175 and 250s, underpowered compared to Kawasaki and other brand enduro bikes but more suspension travel especially the late 70s early 80s models so a lot of fun on trails.

At around 16-18 hp it may seem gutless like it needs a rebuild especially if you’re used to riding other Japanese brands of 250 enduros that make more hp but that’s normal. 8k km’s is not that many kilometers so if the piston is not that scored up maybe just put a new set on rings and gaskets on it since you got it apart and run it,

If you don’t have complete air box, filter and rubber snorkel or some dummy drilled holes in it or air box then you need to re jet or replace missing or modded parts to get back lost power

Re: I picked up a Yamaha DT250 today

punkrock randy /

I had a DT125 only long enough to have it stolen. Fuckin criminals...it was beat up as F anyways

Re: I picked up a Yamaha DT250 today

Rick Bergsma /

Thanks for the Yamaha Forums link Steve , looks like I have some reading to do .

Ken , the airbox , and everything on else on the bike is original and intact , except the truck clearance light turn signals somebody cobbled on . I don't think anyone's turned a wrench on this thing ever except tires replaced at one point and a shitty Kimpex clutch lever .

Re: I picked up a Yamaha DT250 today

Rick Bergsma /

I can feel the grooves in the cylinder , not so much on the piston .

20201127_131417.jpg
20201127_130906.jpg

Re: I picked up a Yamaha DT250 today

Overpriced Parts /

Ya oversize piston and overbore is needed. Cylinder ran hot and or lack of oil,

I would let the shop determine what size overbore piston it needs, Don’t just order anyone it may need the second or third size

Re: I picked up a Yamaha DT250 today

Rick Bergsma /

Got the cylinder bored , started to reassemble it today . Here's something I never knew , the machinist drilled 4 holes in the piston to prevent soft seize . He said it's standard procedure with a forged piston .

20210101_114530.jpg

Re: I picked up a Yamaha DT250 today

Nice, I would check crank seals once you are in there, that could have caused the seize in the first place, or do leak down when you're done, but then you're back treating it apart.

I have a 74, yeah it's kinda gutless but it's fun to ride and looks so good, it's one of a very few bikes I would never sell, I've been offered 2600 for it, which is like 7x what I paid!

Re: I picked up a Yamaha DT250 today

Rick Bergsma /

> Graham Motzing Wrote:

> -------------------------------------------------------

> Nice, I would check crank seals once you are in there, that could have

> caused the seize in the first place, or do leak down when you're done,

> but then you're back treating it apart.

>

> I have a 74, yeah it's kinda gutless but it's fun to ride and looks so

> good, it's one of a very few bikes I would never sell, I've been offered

> 2600 for it, which is like 7x what I paid!

Nowhere did I say that the engine was seized , I said the machinist drilled holes in the new piston to prevent soft seize .

I figured I could probably get around $2000 for it after I get it running again. Not sure if I want to sell it yet .

Re: I picked up a Yamaha DT250 today

You didn’t say it. I bet Graham was reading your piston and cylinder. Which, depending on what your definition of “seized” is, looks like they got intimate with one another at some point.

Re: I picked up a Yamaha DT250 today

I had one once, I broke the chain and it blasted through the case and totaled it, sold it for I think 2 or 300 bucks just for the titled frame. that was a rough one.

Re: I picked up a Yamaha DT250 today

> Papa _ Wrote:

> -------------------------------------------------------

> You didn’t say it. I bet Graham was reading your piston and cylinder.

Haha yeah! In my neck of the woods we call that a seize, either way it happened for a reason.

You see these listed for crazy prices but I don't think they sell, you can get them for a couple hundred bucks pretty easy yet. The monoshock ones don't pull the tail the way that the older ones do, but they are all going to be going up in value. My buddy from Japan said they have caught on over there and a clean dual shock or DT1/2/3 can see 5 figures.

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account