DT250 top end+porting?

[zillaguy89]

OK well I got bored today so I decided to tear into the top end on the old DT. SO i get the head and jug off, and I noticed the piston was awful loose on the connecting rod. Sure enough, the wrispin was worn out in the center where the bearing goes. Let the piston slap around enough till it wore the cylinder out of round a little. Other than that, nothing broken or terrible looking. The top of the piston and head were covered with carbon buildup though, the head gasket was more than likely blown on it, making it run super-rich. Oh, and on top of that, the piston I took out is a yamaha piston with no oversize...this thing is on its original bore still. Not anymore but not bad for a 74

Anyways, here's my big question for you in-depth 2 stroke mechanics. An aftermarket pipe would be impossible to get for this thing. I think a set of YZ250 reeds form the proper year might work for me. Or maybe put something on this factory cage. And I was also wondering if I could port the cylinder, and if anyone knew any specs on doing so. I'm just doing this out of sheer curiosity, if anyone can give me a hand I might try my hand at porting this old tank and make it surprise some people. Without an availiable aftermarket pipe I'm thinking I'd probably be wasting my time. The expansion chamber on this pipe is real long and skinny, so I'm guessing its made for bottom end power.

The porting stuff isn't something I have to do, I just wanted to throw this out there for sh*ts and grins. Always lookin to make something faster, even if its 31 years old

[Kane]

If you know a boneyard or shop that sells the older Clymer manuals look for #M415 with a reprint date of no later than 1982 titled Yamaha 250-400cc-piston port- 1968 to 1976. It gives the diagrams for porting the cylinder and modifying the piston for some very nice improvements. Later printings do not include these digrams. There are also sections on tranny mods, ignition mods, suspension mods, blah blah.

You can fit an early MX or YZ pipe with little modification, greatly increasing the useable power. The MX pipe is a drop-in. You can also find old Bassini and Hooker pipes for the early Yamahas on Ebay very frequently, I picked up a few for my older MX's.

I just bought a new scanner but have yet to hook it up. It's one of those pain inna ass all-in-ones. If I get motivated and find myself with the time I'll scan a few things for you.

[flotek]

if its a production 2 stroke it can be ported to make more power ,id be willing to bet the port layout on a 74 dt 250 is very crude all wrong and in dire need of correction to make a strong powerband. it probably only makes 8 hp stock back in 1974 lol theres no reason a 250cc single cylinder 2 stroke shouldnt be making 40+ hp of coarse your bike will never be able to hit that high a number due to its basic design and pipe ,but you can probably double the stock power by porting it anyways,try it
i just ported a ts 100 and it felt like it went up to a ts 200.

[zillaguy89]

Originally Posted by eviltwin

if its a production 2 stroke it can be ported to make more power ,id be willing to bet the port layout on a 74 dt 250 is very crude all wrong and in dire need of correction to make a strong powerband. it probably only makes 8 hp stock back in 1974 lol theres no reason a 250cc single cylinder 2 stroke shouldnt be making 40+ hp of coarse your bike will never be able to hit that high a number due to its basic design and pipe ,but you can probably double the stock power by porting it anyways,try it
i just ported a ts 100 and it felt like it went up to a ts 200.

yeah the ports look pretty crude to me. The main intake port is wierd looking, its shaped like an upside down T. The transfers (looking at the bottom of the jug) are machined off totally flat, thats gotta be killing flow. The exhaust port seems way too small...I just don't know enough about porting to know where to cut and how much to cut out.

[zillaguy89]

Originally Posted by Kane

If you know a boneyard or shop that sells the older Clymer manuals look for #M415 with a reprint date of no later than 1982 titled Yamaha 250-400cc-piston port- 1968 to 1976. It gives the diagrams for porting the cylinder and modifying the piston for some very nice improvements. Later printings do not include these digrams. There are also sections on tranny mods, ignition mods, suspension mods, blah blah.

You can fit an early MX or YZ pipe with little modification, greatly increasing the useable power. The MX pipe is a drop-in. You can also find old Bassini and Hooker pipes for the early Yamahas on Ebay very frequently, I picked up a few for my older MX's.

I just bought a new scanner but have yet to hook it up. It's one of those pain inna ass all-in-ones. If I get motivated and find myself with the time I'll scan a few things for you.

Wow, thanks Kane. I know your the old-school kinda guy, I was hoping you'd have some good news for me. =) I have a clymre manual, but it must be a newer one than your talking about. It covers those models but has nothing about mods. I'll start searching on flea-bay. Thanks for the help.

[zillaguy89]

OK I checked fleabay, no manual or pipes as of right now, but I did bid on a head from a 74 MX250, that should bump the compression up a little for me, right?

[flotek]

damn dude(Mike) ...i wouldnt be bidding on stuff like that, if your unsure about what it swaps with or compressions of various 30 yr old heads.it would be cheaper and wiser to just cut your stocker and port the jug im sure . ill give you an idea what to do or id do it for you for really cheap ,dont waste your money on stuff that could not work or run worse.

[zillaguy89]

its only 15 bucks, if it doesn;t work I toss it on the scrap heap...haha its not like i'm spending hundreds of dollars on something I'm not totally sure about. I'm 95%positive it bumps the compression a little. ANd i'm 100% positive it will bolt on. All the old yamaha 250's from that era are of the same basic desgin, the only real differences between the motors are the trans gears, the stators/cdi's.

[370banshee]

just put that old thign back together and ride it.... if you really want it ported.. i'll rasie the exhaust duration and transfers for a a bit... and widen the transfers alot...... actualy i depends on what you want...... how bout we use an old out fo frame drag pipe and we give it alot of exhaust duration..... now that would be a col enduro haha not really..