The XJ was a mistake I did mean YZ, but anyhow I've been think about your post and I think I'm going to take your advice.
The only reason I wanted a expansion chamber was more power, It runs great but sure is a pooch the 74 DT 360 I had would run circles around it.Do you have any tips to gains some more HP?

No, I don't have any tips to make it have more HP. You could maybe change the silencer out to and FMF or something to let it flow a little more. Some of the guys on here would know,EN could probably port it for you. Those bikes have plenty of torque and can pull taller gearing if you are looking for more speed.


Just my opinion but if you want more power like the 360 buy one of those in worse condition and modify it.

Depends how close to stock you want to keep it. If you keep it stock you "could" cut open the silencer and modify it for better exhaust flow. Makotosun would know what you need to do. If not keeping stock you could get a FMF silencer like mentioned by Scootern29.

You could add Boysen reeds, they give a crisp throttle response, and if the stock metal reeds get sucked into the engine, they would damage the parts. Boysen fiber reeds will just go out the exhaust.

Are you aiming for acceleration or top speed? you could change your sprocket set out going up or down 1 tooth, depending on which you are going for.
If you are not running the stock air box, you could add a pod filter for more air flow. (if taking off roading I would add an oiled layer of foam around the pod filter).

Lastly you could have your engine ported. I am not sure if Enduronut is porting at this time but a company www.kenoconnorracing.com/ does port work.

Just depends on what your budget is and how you want the bike to look. If you do make changes it will most likely require carb jetting changes and possibly ignition timming changes. I dont know much in this area.....

Thanks for the tips

The Monoshock 400 is actually a decent performer but it is rated at 7 HP LESS than the 74 DT360! 23HP compared to 30HP.

It has a larger Carb, but a very restrictive exhaust silencer. The intake is fairly complex, and it uses a different reed setup.

It is also very possible that the silencer is plugged. Easy test, pull the silencer tip off, then start it and see if it runs better with just the chamber. If so, you may gain some power by modifying the stinger or adding an aftermarket one. Make sure whatever you use is USFS approved if you intend to ride in the woods. Not because it needs to be legal, but because we don't need to get bikes blamed for starting forest fires!

Here is a pic of a slightly opened up silencer from my 78 . . .



The inlet is to the right in the above pic. The first thing the exhaust runs into is an inverted cone with a 3/8 or so opening in the small end, which sits over the top of a double cone with an air passage between the second and third cones. Once it makes it past that, it goes into a 1/2" hole that is the baffle (the part you can remove.

The above is pretty chewed up, but I cleaned things up a bit and opened up the flow so the exhaust could get out. Fortunately, the cut was made on the backside of the stinger, so my crappy welds don't show too bad.

After re-starting the bike (first kick now instead of third) it had the familiar "dead awakening" cackle I have grown so fond of on the TAT bike!

You might want to check the flow through the exhaust pipe. The 78 DT400 I am restoring had a heavy build up of carbon in the exhaust pipe and silencer restricting the exhaust flow. I cleaned them out with a blow torch. You should have seen all of the smoke. We did this a lot at the Yamaha dealer I worked at in the 70's, so 40 years later....