DT400 engine into a DT250 chassis: WILL IT FIT?

[Catbird]

Seems to be a game I'm playing a lot lately, "Will It Fit?"

Only I've gotten smart and started asking you knowlegeable folks before breaking out the hammer and the torch. I've found it saves a lot of headache.

So here's my question.
As it turns out, (who'd'a thunk?) the 247cc Yamaha DT engine feels like it's a little bit gutless when I'm trying to keep up with a Buell and a Harley FXD. I am going to try decking a cylinder head first to increase compression, and doing some aggressive porting to raise the powerband. But if these fixes don't give the little bike enough guts to be practical on the highway (enough to do 55-60 right now without really beating on it too hard, so I'm not too far off) my next thought comes from my hillbilly musclecar lineage.

Just jack a bigger engine in thar an' it'll do!

My question then, is as to whether I could fit the engine and pipe from a DT400 into a DT250 frame without any serious modification.

 

[makotosun]

Yes. Same years will interchange. 400 has a CDI ignition, so you will need the electrics too, though you can run the 250 ignition in the 400.




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[hillsy]

I've been told by a DT collector that the 250 and 400 share the same bottom end, just have different cylinders / bore.


So yes - the engines should be a bolt-in swap.

 

[crazypj]

I've had both (at the same time) but, only the MX versions
Motors look identical, extra one or two fins on 400 (I forget, it was 21 yrs ago)
Fit a DT250 aftermarket pipe on 400, little bit of porting remove airbox restrictors and it will run with any on the full on (early 80's) MX bikes (and it will out accelerate a KX420 ;D )
Had to change gearing from 18/43 to 17/47 and it would still do 110 (but, only 9mpg when revving a bit )

 

[Catbird]

Yeah, this *actually* isn't supposed to be an off-road application, I'm turning one of these into a dedicated road-only cafe.
Also, my father tells me that they share the same piston, the 400 basically just has a longer crankshaft and a different connecting rod. Is that true?

 

[crazypj]

I think the 250 is 70x64?
The 400 is 85x70
The 400 piston is same as watercraft/outboard
I bored mine 0.5mm (cylinder was worn after a year of me beating on it)
Replacement came in 'Yamaha Marine' packaging
http://www.bikez.com/motorcycles/yamaha_dt_400_mx_1978.php (mine was Blue, 250 was White ;D )
Great motor, left crankcase is weak spot, if you drop it it can tear gearshift out of case (I busted 3 cases while playing off-road)
I actually broke a lot of pieces, tore footrest off frame, shattered top spring mount on rear shock, 'folded' both rims after hitting a 8" rock ledge at about 60mph (it looked smooth, 6" of snow will do that ;D )
It was great fun though

 

[makotosun]

A bit more - you do need to keep the year consistent. The 75-76 models (B&C) all share the same bottom end bolt pattern and the 77-79 models (D, E & F) share bottom end bolt patterns. OK - they didn't import a '79 400, but there was a 250 . . .

The 77-79 400 used an improved CDI setup from the 75-76 models. As far as our tests have gone, it is believed that the late model flywheel/stator setups for the 250-36-400 models are all backwards compatible for bolt up. I am currently running a '76 CDI setup on a '71 RT1 bike (pictured in my signature line). Seems to work fine, but I am still working out the wrinkles. There are some wiring challenges in adding the CDI setup to an early bike and vice versa. If you can swap out the entire wiring harness it will be easier. Simpler to just put the points setup on the 400, but you lose a few benefits of the CDI.

Which year are you working on again?

If you need technical help on the bike (just don't tell them it is a CAFE ;D) go to www.yamahaenduro.com which is the best knowledge source for those bikes. They are enduro buffs (I am a moderator there) but there are a few of us who like more than the pure restorations.

_

 

[bradj]

Pm me if you need a 400 motor i got a 77

 

[rd350 survivor]

if you get the 250 running right it will probably have enough power to hurt you.

 

[Catbird]

This is truth.

Of course, it was able to hurt me running *wrong*. XD