Hi all,
I just got my '74 MX250-framed '75 DT250B powered machine running and I got it out to the Mojave to let her lose. The story goes that a few years ago I had a '73 MX360 motor installed in this frame and it ran very well..so well that I was kinda scared of it and felt like it was just too much motor for me. The terrain I mostly ride is pretty rough, sand wash, a bit rocky, but mostly very rough..you gotta keep your self off the seat, your elbows out, and your wits about you to keep some speed.

My concern is that my DT250 motor seems a little slow and I don't know if I am just so used to the pull of the 360 or is there is an issue with it. Low compression or carboration. It builds power pretty quick from 1st to 2nd but kinda bogs when I shift to 3rd. It does not sputter or rev like it would if the clutch slipped. It just slowly but steadily builds up power. 
Does this motor have a noticeable gap between 2nd and 3rd? I love the way it sings along in 3rd but does not accelerate very well at mid throttle.

Is it slightly worn rings or possible too small a main jet? I could not tell you what's in there now I can't remember. What should be in a 32mm carb for that motor and the type of riding I do?

Possibly it is just the nature of the beast. Just a docile trail bike motor and I am expecting to much out of it?

Any thoughts or opinions?

Thanks

The DT250 is going to be far short of the 360 no matter what you do. Torque is directly related to displacement. You could tune it into a high strung screamer like the MX but it would have even less torque.

Do you still have the MX360 motor? A DT top end would drop right on. Much milder port timing. Might be more like what you're looking for.

There is a huge difference between those two bikes’ personalities. Jeckyl & Hyde. Ford Pinto & GT500. Amish housewife and Pretty Woman.

The DT 250 is a competent trail bike. The MX 360 rocket. Frankly, the enduros are more fun for bopping around and trail riding unless you love to be wide open all the time - at least that’s my take having both bikes.

I don’t know about the gap between gears. Your timing could be slightly off?

Clogged pipe maybe? or starting to clog?
Mako is correct on the differance in the 2 bikes.

Thanks very much for the comments.  All good food for thought.  

I do still have the motor. Your idea sounds interesting. Would I need to put a 360 enduro head or could I drop my 250 head on it with the 250 piston? That probably sounds dumb but I am curious if that is possible.

I really like the idea of the a mellower 360 motor. I feel I need the power to get up nasty hills and sand with confidence and cruise down trails in the sweet spot in the 3rd gear power range. The MX just wants to accelerate down trails and it is exhausting to keep it under control.

You would need a 74 DT360 cylinder and head.  If you have the MX360 motor with the six bolt base gasket pattern it will bolt right on.  The DT and MX 360 motors use the same bore and stroke between them, the 250 is comepletely different.

The MX360 makes good power at high RPM but it is soft down low and through the midrange.  Yamaha went a little too wild with port timing in my opinion.  The DT uses more conservative port timing and makes its power at lower RPM.  With some port work it can run quite well.  The transfers need work, you can widen the exhaust port but don't raise it unless you want to trade off low-mid for top end.  The intake port is undersized and the crankcase volume is excessive.  Even if you just bolt it on and run it stock I think you would like it better than the 250 though.

There is no replacement for displacement, especially with these vintage air cooled bikes. 

Another path might be to have check with Rich (RacerClam) and send him your MX360 top end for professional porting. 

richstayloredporting.com/


,

I'm sure he could help it somewhat but you can only do so much.  The MX exhaust port is about 6mm higher than the DT, about 190 degrees duration if I remember correctly.  The only way to lower the exhaust port is to machine the cylinder base.  Then you have to raise the transfers back up and devise a method to correct the deck height. 

In the case of the DT, I like the exhaust port height right where it is.  Around 174 degrees.  Good low end, strong midrange and the power starts to roll over right around 7000.  If you sent out a DT cylinder for port work I think the end result would be much more to your liking.  Or just run it stock and see how you like it.

Much easier to start with a mild port job and tune it up than to start with a hot rod and go the other way.

One other thing you could do to tame the MX 360 motor, is to add a DT400 ignition. It would take some work, and you’d have to source one (hard to find) but adding a flywheel would likely tame it a bit, as well as give some better lugging capacity.

I have a flywheel weight on mine which helped somewhat, but I have intended for awhile to try an Enduro ignition and see if that makes it more trailable.