View Full Version : Dimensions for 84 250r expansion chamber

01-08-2023, 12:24 AM

This is my first post. I just acquired a basket case 84 atc 250r. I am a long way from getting it running, and really don't have any money to spend on it. Since I have more time than money, I would like to build a custom expansion chamber for it. It has the stock one in good shape, but my final plans are to make the bike run a little better than stock, so I am planning on a little more compression, 34mm carb, aftermarket reeds, mild cylinder porting. Once all those things are done, I think the expansion chamber will be an area where large gains will be possible. After doing some reading, it is clear that I am not anywhere near ready to design my expansion chamber from scratch. I think even if I read up on it, and used the calculators online, I would still not land on the right combo. I would rather spend my time out in the shop welding away, even if its a failure, it keeps me busy.

The rareness of the air cooled 250r makes the price just too much right now. I also considered buying a cheaper more readily available pipe for something else that has better cone dimensions and cutting it apart and routing it through the trike frame. Anyone have any ideas on that?

Does anyone have any dimensions for an expansion chamber that was on a build similar to mine? Anyone have the dimensions from a DG pipe? I heard the DG pipes are better than stock, but not ideal.

Thanks for any help

01-11-2023, 02:43 PM
Unless you are very good at math and have a grasp on harmonics with associated equations or have a computer program that allows you to dimension a pipe I would highly advise you stay away from building a pipe.

If its not running, why go aftermarket? Get it running well as a stock machine. You will save money in the long run. Clean up the OEM carb, pull the cylinder and see if it needs a bore and forego the porting at this time.

01-11-2023, 11:08 PM
The cylinder is trashed and at 72mm already. Its gonna be a while until I am ready to buy the stuff to get it going. I was just thinking of tinkering with the pipe as a project to mess with while funds are saving up. I don't have the know how to design a pipe.

To get it running I need a tank, carb (someone put a 38mm off a cr250 on it), a connecting rod (bearing is froze up), cylinder, piston, gaskets, and a seat so easily close to $1000 worth of parts. Two kids in college right now, so its gonna be a while. I just hoped if someone on here knew how to design a pipe, or had an old pipe that worked well, that I could copy that design.