One quite pratical reason is the impossiblity of checking the compression as a method of diagnosis. Obviously, anything that the keeps an exhaust valve open during compression is going to make any check meaningless. Unlike oldfashioned systems with a manual lever for the purpose which can easily be left untouched, this one is automatic. It cannot be disengaged without taking off the camshaft cover from the head and removing it. Then the compression can be checked, but this cannot be done on the installed engine.
The deco unit is known to be of questionable stability at continuous high RPM so it is almost always removed from racing engines.
Then there is the question of weight/mass which is why many use the XT/SRX gear shown farther up.
Another very real reason is being able to change the cam timing and setting it the accurate lobe center. To do that, you need a timing gear with slots so that the gear can be positioned:
this is my aluminum gear for the roller chain I use. Timing gears from the TRX are often used because they have enuf meat to be able to mill slots.
The "Silentchain" is also not known for reliability at high RPM but this is also a question of too much tension caused by the automatic tensioner. Racing versions almost always use a manually adjustable tensioner.
None of this is relevant for stock or mildly tuned engines, but when you run these things constantly between 7000 and 9000 they become a must.
Of course lots of other stuff is also important.
Fitting a forged piston will make a diference to be sure, but a far greater one is fitting a Mikuni TM34-B65 dual carb and decent ignition box.
My (now reassembled and sold as hobby racer) blue Tour used its stock crankshaft and stock piston for over 70tkm of it life. I fit the 101mm Wiseco when the bore got too bad to take anymore. It did have a Megacycle 280-2 cam (without Deco) but used the stock springs and retainers with the original stock valves and guides which are still in there as it that 16 year old stock crank and conrod. This was the first engine I modified the flywheel assembly on. At 35tkm I removed the balancer, too. Careful porting, a halfway decent exhaust system
and a Mikuni brought this nearly stock engine to 59hp at the rearwheel. It was subsequently run like that until I sold it (over 80tkm) and is basically running like that still, albeit with a 280-4 cam, dual 38mm carbs, greatly lengthened intake manifolds.
I sold it with the Remus Cup system shown below.
Point is, don't tamper with the engine internals until everything outside has been taken care of.
With one qualification. Periodically check the primary drive and if necessary replace the key. Better yet, obtain the solid balancegear from Slipstream Tuning and replace the original which can be the cause of much grief when the engine is subject to racing like use.
Then, as said fit a TM34-B65 which can easily be mated to the OEM Airbox so well that even experts don't immediately detect it (e.g. for MOT if relevant).
Fit the Ignitech Sparker TCIP4 with a suitable curve which I can mail.
FIt a NGK Racing cable (red silcon with long straight plug) which can be screwed into the coil like the original.
Unless you are continuously underway a 35-40º+ temps, use a NKG DPR7EA or similar plug e.b. BERU 12-5DU (my favorite).
Change the final ratio to 15/43 and use the best racing chain you can buy. This will give you 2 hp for the price of the chain without even touching the engine.
Fit decent wheels!!! and use a 140/7 rear tire.
In the pictures above the blue bike has the TZR125 3,5/17 rear wheel with 130/70 tire.
My long since sold red '95 Skorpion Sport had these mods: carb + K&N +OEM airbox; better exhaust;
SZR wheels; 15/43; 8400 limiter (stock DENSO but I didn't know better back then), lightened flywheel assembly; otherwise completely stock and never opened. I never had it on a brake but it ran corrected 190km/h which NO stock Skorpion even approaches (forget the speedo!) and I used it at such speeds for stretches longer than a sprint, e.g. the 200km from Milan to Turin at a nearly continuous 180km/h equating to a nearly continuous 8000rpm.
To get a frame of reference, the same bike with stock carb and 36hp reduction (for my son) in place ran corrected 160km/h - with the the supposed 36hp.
That is how much difference those wheels make!
Like I said, do everything else first:
chassis tuning;
exterior mods to engine system;
before even considering opening the engine.
