i bought this 9yrs old Yamaha SV Max- 125 few months back from a friend, my scooter stops after riding it 20mnts continuously, i am facing this problem from past 3months,
i bring it to several scooter shops, some said needs to change CDI, even after replacing CDI i am facing same problem, change the spark plug, still facing same problem,
the mechanic check all the parts of my scooter and after 2 days finally he said needs to rebuild the engine or some cylinder boring (搪缸 引擊; 磨令古), i didn’t get his chinese, he ask me this will cost a lot , i don’t know what is the exact problem, so i am confused now should i let him rebuild the engine and what is the guarantee that scooter will run perfectly after fixing up the engine, is really engine got some problem or might be some other problem? plz help me
[quote=“Nikhil”]i bought this 9yrs old Yamaha SV Max- 125 few months back from a friend, my scooter stops after riding it 20mnts continuously, I am facing this problem from past 3months.
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“Scooter stops” isn’t enough information, I’m afraid. It isn’t even a fault condition. Scooters are meant to stop. [Oops : See EDIT below]
Assuming you mean, “scooter won’t go” (or start, not quite the same thing) and assuming the fault is static or reproducible in nature (intermittant faults are trickier) then your alternatives are:-
(a) Investigate it yourself, following a logical approach.
(b) put yourself at the mercy of “the professionals”
Though I’m certainly no expert in either, I don’t personally like approach (b) though it is very popular and can work out OK.
For (a) you can find lots of fault-finding guides on the internet (Dan’s motorcycle maintenance course comes to mind) which will explain it better than I can, especially with my grading deadline approaching, but the basics are:-
Will the engine turn over (eg on the starter motor or kickstart?)
Have you got a spark? - Don’t test this without earthing the sparkplug or you might fry the CDI if it isn’t already fried.
Is fuel reaching your cylinder?
Have you got compression? (You can get a “feel” for this on the kickstart, though measuring it requires buying a guage).
Even if you don’t diagnose or fix it you’ll learn something and limit the pro’s scope for invention.
EDIT: Sorry, I just realised that “after riding it 20mnts continuously” did NOT mean you’d been riding it for 20 months 
While the “basics” above (except, probably, the first one, though still worth checking) still apply, that kind of problem is often fuel starvation, perhaps due to a vacuum developing in the fuel system due to a blocked breather. If it runs with the fuel cap off, then the tank is developing negative pressure as the fuel is used up.
Could also be a sticky or partially blocked float valve in the carb, or a blocked fuel filter. It runs on the contents of the carb float bowl, stops when it empties, then slow leakage allows it to refill and it’ll run again.
Engine rebuild is NOT particularly indicated by this symptom, but an engine rebuild is quite a nice earner for the black hand.