Wonder if anyone has fitted a YZ85 Vforce reed block to a TY250?  Have fitted one to a TY175 (with a DT cylinder), and it worked really well.  The intake duct needs to be opened up, but its not a problem with proper porting tools. 

Sounds like 'a plan. I have a TY175 cylinder on my piston port AT1.
Funny how things have changed over the years & an early YZ80 reed block from about a 1980G model is the same as my RT2 360.

YZ85 VForce wont fit a TY175 cylinder, as the intake duct is much too small. They will fit TY250 cylinders and DT175, which both have bigger intake ducts.

If you can remove some material from the TY175 duct, it's possible to fit a TY250/RD250 reed block and that does improve running.

More than happy how mine goes for an old S@#ta. Not sure now what reed block i used but might be original. Have plenty of RD 250/350 red blocks but if they don't drop in then can't be that. Bolt hole spacings are probably the same though. I must've put a plate under the cylinder as Yam's have 5mm difference in rod length piston port to reed.
Only using an old Ag175/CT1-2-3 head but comp is probably same but not as nice shape as center plug TY. Just using what i have lol. Fix it but don't spend any mone--learnt that from my boss haha.
I did this years ago but video is from i dunno couple of years ago or more [yes i'm older now] & from when i fitted my Chinese carb. Looking at the mess on the floor i did jet the main to a safe limit & has to change threads on jet or needle jet. & lean off the needle clip a notch. Goes like a rocket.
Anyway i can't fool you lol--& can see you know your stuff & can well sort me out. I do a lot of guessing. Good to have you on board too.

Sounds like the Chinese carb works fine. Did you change from a CT3 cylinder to TY? CT cylinders have smaller exhaust ports, with different timing.

No, i went from a piston port 125 AT1 [which is what the bike is--1970.
Went to a TY175 cylinder but didn't have a head so used what i had. Essentially a CT3 head but from an AG175 farm bike of similar year.
Later AG175 Grasshopper models went more DT175 radial fining & bigger crankcase mouth from late "70's or right on 1980. Sorry wondering off the story.
Maybe i was just lucky with my carb. Got one for my honda 185 too [proper honda shape carb with butterfly choke like original & was perfect out of the box.
Doubt i could be that lucky 3 times in a row haha. Cheap to experiment though.

Later (1980 on) DT175's had a cylinder and head which looks similar to the earlier DT175MX model, but has different bolt spacing and wont fit the earlier bikes. Later cylinders and heads are available new from ebay.

In the USA, all DT175's monoshocks (known as "MX" in the UK) from 1978 to the last model we got in 1981 had the same engine with the same cylinder.

I'm fairly certain the same is true in the UK... with the engine changes in some countries happening after 1981 as far as I can tell... I've seen references as early as 1982 and in Oceania it appears the change was in 1984?

Many of the later cylinders and heads on ebay are also of dubious quality... probably should be avoided even if you have a model they fit.

The 175MX cylinders, were fitted to the cases with sleeve nuts, through holes in the fins. Heads secured with 6 studs threaded into the cylinders. The very last of the air cooled DT175s, had cylinders with flanges on the bottom, and were fitted to cases with normal nuts.

Flange bolt pattern is different to DT175MX to allow for bigger transfer ducts, and reed block housing is bigger, to accept a bigger block.

As far as I know, these were never imported to the UK, as we got water-cooled DT200s after the 175MX finished. Probably the same in the US I would guess?

The late DT175s were according to Yambits sold in Australia though. The cylinders to fit them are widely advertised on UK Ebay , so I suppose there must be a good number of late DT175s still in the UK?

I had one of the Ebay cylinders last year, thinking it was for a DT175MX, as those can be modified to fit TY175s. It was low quality and bolt centres nothing like a DT175MX. It got sent back..........

Your first post made it sound to me like the UK had a model that takes that cylinder and that it fits 1980 on?
"Later (1980 on) DT175's had a cylinder and head which looks similar to the earlier DT175MX model, but has different bolt spacing and wont fit the earlier bikes. Later cylinders and heads are available new from ebay."

 What it seems you're saying now is that the UK never had a model with that cylinder which agrees with what I have seen.  UK DT175MX was mostly like the one year only 1978 DT175 in terms of fuel tank and only coming with standard forks.  (US models got a different fuel tank and leading axle forks in 1979-81)  We also had an airbox/oil tank upgrade starting in 1980 which I believe the UK also got?  In the US, the engines were the same from 78-81 with the exception of carb tuning and 1980 also got an improved CDI.  

It's always interesting to see and hear about the models other countries got to enjoy!

Anyway, they advertise those cylinders all over ebay USA as fitting 78-81 which they really don't... though some have modified them to work.  The only bikes here they might fit would be something that someone imported themselves.  It would be very rare.  I've only seen one that someone brought from Thailand when they moved here.  It was a 1992 air cooled DT175 and looked a lot like the Australian models with drum brakes, square headlight, square gauges, and a larger and different fuel tank (fuel tank was similar to the one we got on the off-road only RT180 in the US...  which was a Brazilian model that had been stripped of lights and sold for the off road market here in the 1990's.)

1981 was the last year of the US DT175 and of most street legal two strokes over 49cc in the US...  some two stroke models larger than 49cc lasted a couple more years...  and there were a couple mid-80's exceptions like the 83-85 RZ350...  but eventually none of the larger two strokes met US EPA emissions standards for street legal vehicles here.  Last of the Yamaha street legal for the USA two stroke Enduros was the 88-91 DT50LC.