Hi everyone,

I am planning to modify my DT175MX ('81) a little, here's the setup I am going for:

- BigOne expansion chamber + silencer (already fitted)
- 28mm Mikuni flat slide (VM28-418, already fitted) 
- allowing more air coming through the airbox (bought a cover that already had holes in it - so I am not destroying a stock one) 
- VForce 3 reed valve with glass fibre reeds
- cylinder porting 

I've got questions especially on the last point: I've never done porting by myself, but I am really keen to give it a try. Of course I will clean all the ports and try to improve the flow, for example by opening the boost port fully. Other than that I don't really have an idea what to do, so I would like to ask if anybody has done porting a DT cylinder and could give me some hints. I don't want anything crazy to squeeze the last bit of power out of it, just a gentle porting to fit the power band of the BigOne exhaust a bit more, which clearly likes higher rpms than the stock pipe. I think I would not touch the intake window itself, but would like to do some work on the exhaust and transfer ports. 

I got myself a Clymer "How to" performance manual for Yamaha piston port models to '76 and found some port maps in it for 175cc enduro, motocross and motocross special. Is that something I can use as a guide? 

Really appreciate any help! Thanks! 

Cheers,
David

"Mikuni flat slide (VM28-418, already fitted) "
VM's are round slides.

The exhaust pipe should be enough.
Leave the porting alone.

Yes, except for the 28mm from the TM series.
I don't know why they chose to label it as VM28.

Cauz they do not know.......

I don't have time to answer in detail right now but here is some info from one some of my previous posts here that you might find helpful for your projects.

While it will almost certainly run afterwards, messing with porting can very quickly give you less power than stock.  Do your research first and have the right tools, especially for the transfers!  Most people should probably consult an experienced professional with proven results on that engine and let them do the work.  Removing obvious casting flaws (which is rarely needed on a Yamaha) is ok, but altering port heights and sizes can really make a mess.  Just one mm change in porting heights can make a few HP difference, and if you're guessing, usually you end up with less power, not more.

One trick I often used was to analyze the pipe design first and then match the porting to optimize the pipe design. V Force is a waste of money on these old bikes in my experience, stock reeds work great. Every time I tried VForce, power went down, not up.

If you are "going wild" on the exhaust port, compression will be too low.  Cheapest fix for the monoshock DT175 is to fit a 77-79 IT175 head.  (Head bolts right on, IT175 cylinder will NOT)

I used Gordon Jenning's "Two Stroke Tuner's Handbook" to build my race engines... copy in the Tech Library here or on the Internet... still have the "real" copy I got in 1975.

A. Graham Bell also wrote some good books... still in print... some are probably available for download somewhere on the Internet

.......be careful of ring locator pin placement... using an IT or YZ piston can sometimes result in the ring gap passing over a transfer port if the pin spacing isn't right... very bad if a ring end catches in the port opening and it probably would eventually. Ring gap locations for various pistons marked in photo below. 



Good luck with your project.

Thanks Mark for your helpful answer!

I am quite aware of messing with porting can result in worse performance than stock, that's why I would only clean and optimize the ports itself, but not touching the windows as long as I don't know what I am doing. That's exactly why I asked here, because I was hoping somebody might have some experience porting these DTs and could give me some numbers to work with.

Regarding the VForce I made different experiences. First of all I didn't buy the original ones, this piece of plastic is something you can buy cheap in all conscience. I got mine for 20€ each and replaced the stiff carbon reeds with glass fibre and put it on my RD350LC. It works really really well. So I thought this should also work on singles. But I noticed on the DT you HAVE to work on the intake, not only because it wouldn't fit, but also because of it's shorter length into the port, leading the gas flow against a hump before entering the barrel.

I've already found the handbook you are talking about, really nice! I will definitely take this to heart

About the piston: I found a local dealer in my town who bought lots of TKRJ pistons, when visiting I saw lots of them for the DT175MX on his shelves. Definitely going to get a few of them

Edit: May I ask you how you analyze the pipes design for porting? My attempt would be to find out the power band the exhaust likes, but with these old rev counters it's more guess work  

You use a tape measure and a calculator along with the exhaust port timing.  Or there are pipe design programs on the web.  Basically you "reverse engineer".

With porting, bigger passages are not always better.