So as Piston starts to descend down we get pressurization of the crankcase and air is forced back into the intake as well as the transfers. The windows in the Piston are okay, but as previous need to be raised. The windows disappear below the intake port before we hit bdc that's why I believe need raising at least 1/4 inch or more. Do at your own risk going too far. We can also get a greater volume of gas back through the intake to the boost port by cutting a third hole into the Piston skirt. At bdc it is fully open. Also saves from having to go to far in opening the original 2 piston windows. Lots of people cut a series of 2 holes or more instead of one. If Boyesen ports are installed extra piston window or windows may help with feeding them better. Boyesens are just holes drilled from the intake to the rear transfers ports. Also that hole or series of holes in the Piston skirt really helps to cool the Piston crown, gets that stagnant air moving, that may not have otherwise contributed to power. I hear desert racers have done this mod on big bore 500's for up to 2 hp increase.

Just my thoughts again. As the Piston comes down air is being forced through the transfer ports. One thing I did was round the squared off section of the case along the oil drip way just above the main bearings. But not much as that provides strength to the bearing journal, and I did not know how far to go. There is also a mismatch between the outer cylinder wall transfer tunnel and the case. On all 4 cylinders that I have had, all had the same mismatch- about 1 mm the case over lapped the cylinder. I sanded the case, but seemed like the gasket surface became narrow although it still had the same gasket clamping area. A narrower gasket offers less strength to blowouts. So I do not recommend chasing that, it's not much anyhow. Alot of builders will shorten the cylinder sleave above the entry way to the main transfer tunnel. I believe it starts the fuel /air sooner into the port. But since the intake side of the sleave provides thrust support for the Piston they leave the sleave alone above the secondaries I am guessing. I did not do that at the time when I ported my cylinder. I rounded part of the divider between the port tunnels. Most of the divider was rounded as cast from the factory, but say a small part that is machined flat with the gasket surface.

Hey guys, will a mx400 cylinder fit a dt 400, looks like the mx head has more area, larger reed blocks?

There is a pipe in the works for my 78. It may be a month and a half out, God willing. It may add up to 10 horsepower and 1500 more spread of power (low and high), all on the stock ports, reeds.

Yes

This is a MX top end on a 75 DT bottom

The carburetor joints are different
The one in my hand is a 75 DT and does not bolt up.
The MX does not work with the 75, 76 air box.
I do not know if a 77 or 78 joint bolts to the MX cylinder, so you could run a smaller carb?
The MX 38mm carb drinks the gas like a sailor drinks rum!!

Rob, the 77-78 DT400 has a smaller bolt pattern on the reed valve manifold than the earlier model. It might be the same as the MX400? I can measure if you want to know.

No joke about that! maybe 10 or 15 mpg if you run it really hard! but that power though!

you can call that 38 mm Mikuni a fuel leak rather than a carburetor.

either I am getting the venturi divider Rich offers or a 38 mm 4 vent Kehin AirStriker.

No thank you Mark, I appreciate the offer. I'm gonna leave that one set up the way it is. Keeps the arms stretched out ; )

Get you some more spark energy. The system on the 78 400 seems like it could use some improvement. More spark = more power generally. I would like to try out that Rex Speed Shop dual coil set up. I already have their stand alone cdi box I got years ago. Worked great.

I am lightly restoring / modding a '78 DT 400. Its pipe is trashed. So I have to ask, who is doing your pipe? Thanks in advance.
David