I suggest you compare a DT175 cylinder to one from a Blaster.

In terms of air boxes, have a look at the tiny low volume boxes, with very restrictive intake hoses, that were common in the 70s, and compare to one from a modern 2T trials or MX bike.

I wonder, in terms of modifying a DT175 cylinder, what do you feel is the most effective means of increasing the transfer flow?

As the idea of unsteady flow, was not well known back in the 70s, I find it surprising that the science behind resonant intake systems, was covered by Jennings? I have a copy of the book somewhere, and will have a look.

Not known about in the 1970's?  Maybe tell that to Walter Kaaden, the German engineer who is largely credited with being the first to truly understand and prove the value of gas flow and resonant tuning of two strokes going back to the early 1950's....   

Kaaden built a very successful two stroke 125 racing engine that reportedly produced over 200 BHP/liter...  that was in 1961.  Impressive even today. 

Thanks for posting that article by Kevin Cameron - the name rang a bell with me.
I bought a DT400 twin shock from the states a few years back. The previous owner was a 'Kevin Cameron'. The bike had a K&N air filter fitted on the back of the carb!!!
Someone had come off the bike because there was lots of bent parts/dents etc.
I've gone new/stock with all tuning parts on the bike (except for Rex's electronic ignition) and the bike still runs very poorly (the well-publicised problems that there isn't a clear 'fix' for). The final thing to try and get it to run better is a rebore (using an OEM piston). If that doesn't work, the bike will just have to earn its keep by looking good in the back of the garage. I'll stop waffling. My point is that Yamaha didn't ALWAYS get it right...

Sneezles has it right.  If you want modern bells and whistles, buy a modern bike.

What is the point in buying a vintage bike and trying to make it work like a modern one?  You buy a vintage bike that resonated with you in your youth.
You want to replicate the experience you had then, not change it into something different.

The nature of tuning is that of robbing Peter to pay Paul.  Increase power in one area and you diminish it somewhere else.  The only way to increase the perception of power across the entire scale is to take a long hard look at the riders waistline.
Or buy a bigger bike...

I find all this talk of generic performance mods to be not really relevant to the vintage ownership experience. 
Unless of course, you compete in a vintage class and make legal mods to get the best out of your bike to improve your position.  
 

Perhaps there is a “vintage hot rod motorcycle” magazine… that’s where the mods live and breathe..
I like the reflection of the “waistline”…
I wouldn’t want my old RT2 go like a modern bike… it would be unruly to handle… I’m simply satisfied as is.
Sneezles61

Simple and inexpensive alterations to improve performance, provide better fuel economy, and increased reliability, are things that could well be of interest to anyone who rides an older bike regularly?

If your DT is running badly and has a Rex ignition, try and change it for a known working system. The ignition is almost certainly your problem. Putting a strobe light on it, and running the bike, will tell you a fair amount.

A good book for anyone still stuck on the Idea that 70s thinking on 2T engine design is as far as it goes, should try and get hold of a copy of "The Two-Stroke Cycle Engine" by John B. Heywood and Eran Sher.

Yamfan wrote: "Simple and inexpensive alterations to improve performance, provide better fuel economy, and increased reliability, are things that could well be of interest to anyone who rides an older bike regularly?....................."

Yes, I'm sure there would be interest.  If such things truly existed outside of your imagination.  Most of what you suggest is based on pure speculation and conjecture with zero real-life experience on what these "simple" alterations actually cost in time and effort or if they are even practical.  Plus there is zero evidence that any improvement would be gained. 

Stop telling us what to do, buy or borrow a DT400 and actually show us what to do... shouldn't be hard since it's all so cheap and easy.   .

(Oh, and nobody said two stroke development hasn't advanced.  You said they didn't know anything about resonant tuning in the 70's which was completely false. )

 

Jammyyammy, in the mid-1970's pretty much every vehicle sold in the US was crap in terms of power and driveability.  The US EPA pushed strict tailpipe emissions standards as well as noise emissions standards that were extremely difficult to meet. 

Manufacturers did their best...  but even new cars and trucks made by Ford, GM, and Chrysler in that era would stumble and stall and surge.  Horsepower and fuel economy plummeted.  The larger more powerful models seemed to suffer the most problems.

The DT400 was completely killed by these rules in the USA after the 1978 model year...  they could not meet the standards at all. 

Not a good time period in history for motor vehicle performance across the board in the USA.

 

A Nibbi carb, and VForce reed block direct from China, are simple bolt on's, and will improve any 70/80s reed valve equipped bike very noticeably.

Obviously, the issues with less than ideal transfer duct area are well known, and can be rectified relatively easily.

All the above mods are tried and tested, and the transfer alterations I use have been carried out for more than 50 years.

On CDI equipped bikes a new CDI unit, will increase spark energy, and provide more power, better fuel economy and easier starting. Chinese AC units can be adapted to work on Yamaha bikes, and are cheap and easily available.

I owned a DT400 back in the 80s, and in common with the TY Mono, it seemed to suffer from limited transfer and exhaust duration, and the scavenging was poor? Those things can certainly be overcome, but then performance would be restricted by the less than ideal intake and exhaust systems.

I must be thick.

You keep trotting out these facts and statements and I am not understanding what you are saying.  A new CDI unit will increase spark output, raise the power, make for easier starting and use less fuel?

Everything else being equal, just how will it do that?  It seems miraculous to me.  A spark needs to be "sufficient" to light off the fuel.  Any more than that is just robbing power from the engine crankshaft.
Easier starting? How when most bikes start second kick from being stood a week and first kick if used that day. How can it make it easier?
Increase power.  How does it do that? Are you suggesting a fatter spark makes the fuel burn faster?
Better fuel economy.  How?  Is this miraculous spark contributing noticeably to the heat in the cylinder?

I am also very curious about your statements about the restrictive port timings on the TY250 mono.

​​​​​​We had about 8 TY250S and 10 TY250R on fleet back in the day, along with Sherco, Scorpa and Montesa that came later.
I never found the pinkys to be lacking in any sense apart from the S.O.B that landed on my shoulder and broke it.  Oh and a front end that felt a bit heavy.
They were ported to be tractable at the bottom end as any 1980s trials bike should be.  They were never intended to be motocross bikes and why would one want to alter the engine characteristics to make them so?  All you would end up with is a shit trials bike and a shit motocross bike.
I place my trust in Yamaha development engineers and development riders to make the bike perform to how they were briefed to,  not someone's unpopular opinions on the internet.
 Just buy a IT or YZ if you want top end.

Certainly my mates and work colleagues at the time, Dan Thorpe - British Expert Trials Champion and factory GasGas rider.  James Lovell, Yorkshire Centre Schoolboy Champion and also a sponsored rider, Richard Thorpe - Mr PJ1 himself, Mick Grant, Simon Hirst, or the other sponsored ride trials instructors whose first names only I can remember, ever had cause to complain about the performance.
It was the equipment they had to instruct and demonstrate on and rode it accordingly.  None of them suggested porting or any other changes.
I saw Simon - cannot recall his second name (Hirst - it came to me) - a regular invitee rider on the 1980s TV Trials show series Kickstart,  take a freshly rebuilt TY250R from my hands at the workshop door and proceed to ride it up the workshop wall, touch the rain gutter with the front wheel, flip it around and land back on both wheels.  He pronounced it to be fine.
What more do you want from a 1980s bike?

Sure, our move to Montesa 315s when they came out made it easier and were a better bike all round, but if I decided I wanted a TY mono, why would I buy one and try make it like a 315?  
I would just buy a 315 and save all the hassle.

You are just not "getting" the ethos of the average vintage bike owner and sweeping statements about making modifications away from stock that alter the characteristics of the bike and devalue its residual value prove that.

No one knows if you even posess a vintage bike, you certainly have never mentioned one.

I doubt anyone on this forum that owns a vintage bike ever reads or ever desires to read Back Street Heroes * magazine.

* For the US citizens, Back Street Heroes is a UK magazine dealing with modified bikes - chops, streetfighters, bobbers, GSXR1100 turbos ect.