Capacitors over time degrade, and in a CDI ignition system, this leads to the charge being provided to the ignition coil reducing, which will mean a weaker spark. Bikes will still run, with a weak spark, but will work much better if spark intensity returns to what it was when new.

The TY Mono's used by the Hamilton team back in the day, were fitted with cylinders modified in various different ways. Perhaps the most popular were those modified by Stan Stephens, which have been widely copied ever since? The Kato intake chop, was also popular, but isnt comparable to one of Stan's cylinders.

The factory TY Mono, ridden by Thierry Girard to win the French round of the world trials championship in 1988, used a YZ495 rod, had factory suspension front and rear, exhaust back box made in the UK, modified air intake, and minor alterations to the cylinder.

The TY Mono can be improved enormously by the Stan Stephens porting, and a Kato type air box, but ideally the rather poor TK carb needs to be changed for a VM Mikuni.

A rider who won the air cooled mono class in the British classic trials championship, several times, used a bike modified in that way.

Suspension on the Mono isn't great, though, and an effective rear shock is needed if you are really serious about the best possible set up.

If you want to keep your older bike stock, that's fine, but it seems silly to suggest modifications don't work, simply because they weren't carried out when the bikes were new, or mentioned in 2T tuning books written in the 70s?

A CDI can certainly fail.  Not often in my experience, but it has happened.  When it does, it will manifest itself in not working at all if the caps have blown their tops, or it will develop a misfire, or as you say a poor spark.

​​​​If one fails, of course one will have to replace it. 

However to suggest replacing one that is working just fine and suggesting it will make all the differences you mentioned is a silly waste of money.

Everything else you have written just reinforces what I already said. 
You have no concept of the ethos of the average vintage bike owner.  He is not a high level trials rider competing at factory level,  he does not demand top level performance with a factory mechanic riding with him in the van and he is not looking to campaign his bike for one season and pass it on.

He wants a bike he can ride anywhere, at any time that is dependable and durable.  He wants to hand it down to his sons when he croaks and he wants to have reliable information and the known correct parts available on hand if it goes wrong to get it repaired quickly, cheaply and be durable.

A member on here takes your advice.  He carries out your mods. 

His bike is up for sale at $3500.  A knowledgeable buyer wanting an original bike comes along for a look.

Hmm.  Aftermarket reed block.  Deduct $250 for the effort of findng and fitting a new one.
             Nibbi carb.  Gonna be hard to find a genuine one.  Deduct $500 for swap meet petrol, time and purchase.
             Non standard CDI.  That is going to be another $500.
             Non standard exhaust.  Jeeze that is loud.  $750 for a quality replacement that is quiet.
             Non standard pipe residual devaluation.  $500.

We are already totalling $2500 in the buyers mind to put it back to stock on a $3500 bike.

The seller happily mentions the ported cylinder...

The buyer looks at the seller, closes one eye, ponders a second and says "OK, I"ll leave it.  Goodbye.

That is the reality of vintage bike ownership that you do not get.
             
 

Yam-fan wrote: ".............................If you want to keep your older bike stock, that's fine, but it seems silly to suggest modifications don't work, simply because they weren't carried out when the bikes were new, or mentioned in 2T tuning books written in the 70s? "

What seems "silly" is that you keep posting about generic modifications you've never tried for bikes you don't own.  Everything you mention is allegedly inexpensive and easy. And you promise fantastic results for little to no effort or cost. 

The not-so-silly part are the lies and manipulations posted to support your claims.  And then your attempts to twist the facts of the matter.  For example, you mentioned Kevin Cameron and intake resonance theory he briefly wrote about and copied into a 2014 post you linked to...  You alluded to this being "modern theory" when it absolutely was not.  Factual proof was given by mentioning a book written on two stroke tuning back in the 1970's.  So now you're twisting that all around and turning it into a another lie that some of us somehow believe anything that happened after the 1970's is not legitimate or something?   This is clearly "troll behavior". 

I know for a fact these bikes can be improved upon having done and seen it.  For example, someone like member Rich (Racerclam) can and has very successfully modified these engines with porting, reeds, carbs, etc.  Enduronut has been known for his legendary "Big Sig" modifications.   There are dozens of others.  Some of the members here worked with aftermarket company Powerdynamo (now VAPE) to develop modern CDI kits for these bikes.  None of this was or is "cheap and easy".  It took time and hard work.  And in the case of Powerdynamo specifically, there is now a proven "bolt on" modern replacement ignition and charging system for these bikes for those who need or desire one. 

Jennings actually does mention the modifications you're "promoting" in his 1973 Two Stroke Tuner's Handbook.  He also gives excellent advice in the "Fundamentals"  opening.  The below statement was as true then as it is now: 

"..............Another mistake commonly made, sometimes even by those who have enjoyed some success in modifying two-stroke engines, is to believe in a kind of mechanistic magic. Bigger carburetors, higher compression ratios, altered port timings and expansion chambers often do bring an improvement in power output, but more and bigger is not magically, instantly better. All must work in concert with the basic engine, directed toward the particular application, before they constitute a genuine improvement. You cannot treat them as a voodoo incantation, hoping that if you mutter the right phrases and stir the chicken entrails in the prescribed manner, your mild-mannered, all-purpose chuffer will be transformed into a hyper-horsepower fire-breather. With a lot of luck,you might get that result; the chances heavily are that you won't.............." (emphasis added)

If you have some cheap, easy, magical way to achieve "more power, better economy, and increased reliability", prove it.  The time has come to put up or shut up. 

Yam-fan, opinions are one thing.  Trolling behavior is another.  The trolling behavior you're engaging in needs to stop as it is not welcome here.
 

I took the 400 for a couple more rides. It's running really good. No flat spots, starts cold, starts hot, it idles, but best of all my surging is very minimal, like a two stroke should. I find that if I use the torque of the 400 in the higher gears and get off the throttle I get zero deceleration surging. The bike is pleasure to ride. The plug looks excellent as well. I ordered a Pod filter with a 15 degree offset. I'm not sure I'll go this way permanently and am hoping it will bolt on. Probably going to modify an air box and run a short section of flexible hose into the box after it gets to cold to ride. I haven't really gotten on the bike full throttle to see what it will do given I'm braking in a rebuild. It may never happen, I'm 64 years old. It's an interesting and good running bimble machine to my liking.

I do have another issue. I cant get the lever for my front brake adjusted. It's to soft. I have new brake shoes and have tried both an aftermarket cable that I thought had to much stretch and an OEM cable. Still to soft. It stops good, but feels bad. It's been a while since I've had a non hydraulic front brake, but me AT3 feels much better. New shoes, I flipped the cams over that actuate the brake pads, tried adjusting and have wrestled with moving the lever on the brake drum up a notch, haven't gotten it yet. Any experience?

Also, got some weird replies to my original post. I'm just a back yard mechanic looking for that type of information. 
(I'm also a pretty good back yard mechanic)  I'm not entering the ISDT, just bimbling about a little.
 

The brakes will firm up a little as the shoes bed in.  I recall some shops used to have a specialist machine to shape the new shoes to the drums to get the best braking action from day one.

However, a cable operated drum brake is always going to feel like you are squashing an orange under the lever due to the construction of the cable outer.  It is a long coil.  Pull on the brake and the coils press harder against each other, taking up a minute amount of slack between each one.  Times that slack by maybe 1500 coils in the length of the cable, and you have a lot of squashing going on.

I feel it keenly, since I only ever owned disc braked bikes for maybe the last 40 years.

Apologies for the thread hijacks Bassani. 

Are you using Factory Yamaha shoes?  They seem to fit/work best in my experience but other brands are okay too.  I had trouble with EBC shoes being undersize for the 175's. 

Something else you could try a backyard method Scootern29 mentioned here...  get some self adhesive sandpaper and line the drum with it.  Do not ride the bike this way.  You spin the wheel while dragging the brake to gently arc the shoes to the drum. 

Places like Race Tech in Southern California offer professional shoe arcing but it's not cheap last time I checked.

All the mods I have mentioned on here, are things I have carried out myself, and I know that they work. Some have been copied from other people, and those are things which have been used successfully for more than 50 years.

The fact that capacitors degrade over time, is well known, and the manual for the Japanese TIG welding set I own, suggests that the smoothing caps should be changed after a certain amount of time.

I first came across the big difference a new CDI makes, many years ago, when I changed the one fitted to a Fantic I used to ride in British championship events, and was quite surprised at how much better the bike ran, with a new CDI on it.

A simple bolt on mod, such as a carb or improved reed block, can easily be removed, so any bike upgraded in that way can be easily returned to standard form if required.

I noticed a post on here, detailing some modifications carried out on a TY175 cylinder. The very restrictive transfer ducting, was improved by adding an extra port at the rear of the cylinder. That looked to me like it would be very effective, but seemed quite involved and probably costly?

Gordon Blair is probably the best known exponent of unsteady flow theory, related to 2T engines, but his material is very technical, and not that easy to understand. Camerons short piece on induction systems is far more accessible, and I think easy to understand for most. Making and fitting resonant air boxes, to TY's and Fantic's, confirms that what Cameron outlines is spot on, and for bikes whose stock intake systems, are poor, the gains to be expected are even greater.

The TY175 motor essentially uses a cylinder which is very close to a DT175, with the only major differences being, that the TY has a smaller reed cavity, and the diameter of the cylinder fins is reduced on the lower part of the cylinder.

I have used both TY and DT cylinders, with the DT ones being preferable, as the fin area is greater, and the bigger reed cavity means a V-Force YZ85 reed cage can be fitted, without breaking through the reed cavity walls.

In terms of factory bikes, the only TYMono to win a world championship round, was other than factory suspension, some basic cylinder mods, and a YZ495 rod, pretty much stock.

The UK, Hamiliton team bikes, had more heavily altered cylinders, and Kato air boxes, but other than those things were pretty much standard, other than different back boxes, which were available commercially at the time.

The stock is best types wont like this, but getting woven brake linings, of the type used on things like fork lifts, bonded to Yamaha brake shoes, is a very simple way to improve brakes on any sort of off-road or dual-purpose bike. With a modified front brake cable (the outer is shortened to reduce stiction), and woven linings front brake isnt that far from a disc. 

British Championship events eh? Perhaps I know you.  Do tell us more.  I am sure we are all far more interested in your anecdotes and pics of your campaigns.

Once again, you are telling tales about esoterical modifications when your target audience are clearly not interested.

We all have tales of mods, I know and saw most of the mods, moving away from stock in the GasGas factory bike engine that competed in the Scottish Six Day Trial around the 2009 mark.  Indeed I myself built the very wheels that carried that bike for the event.
Those mods were not generic as you seem to propose.  They were very specific to the riders style of riding.

I also spannered and tuned a Cagiva Mito for a team which won the Auto66 club northern road racing championship and blew the Elvington 125 Supersport lap record out of the water around the 1991 season.

However, it is not my style to bore others with my past glories.  There are others on here that I happily accept know and have forgotten very much more about Yamaha Enduro bikes and two stroke tuning than I ever will know.
That is fine, everyone has their specialism.  They have theirs, I have mine.

As far as Vintage Yamaha Enduros are concerned, you are in very exalted company.  I keep my mouth shut, lest I embarrass myself.  Perhaps there is a lesson in that somewhere?

I keep repeating myself.  This is the audience you are trying to preach to.  Not a thread of Apico lycra in sight.

So better brakes, more fuel mileage, extra power, and increased reliability, are all things that don't interest owners of older bikes?

Seems not.  Would you agree?  What good are better brakes with skinny knobbly tyres, long travel matchstick forks and straw thin frame top tubes?