[Prass]

Hi, I had a similar problem with my Splendor where one road side mechanic overfilled engine oil. he put 1L in place of 750 ml. the bike started losing revs at higher rpms. this was corrected when the oil was drained and refilled.
Well i dont have a technical explanation behind that.

[PhrozenFire]

Hey guys, update for everyone. Took my bike to Balaji as suggested by Ajabiz below. Balaji seemed to be a knowledgeable guy, and he speaks very good English! He rode the bike around for a few hundred meters and immediately said the problem was due to loss of compression caused by a worn out piston and piston rings. I did suggest to him some of the issues mentioned on this thread, but he point by point ruled them all out. The piston and piston rings will need to be replaced, as will be the head gasket. Also will need to get a reboring job done. So it was what I was suspecting all along. Total cost, including labour, will be around 2-2.5k. Will keep everyone updated if there are more developments.

[windsurfer]

If this is case bike should be sluggish in all RPMs. I had the same problem and did what you are about to. But in my case bike was not dying at 3.5k .

[BIKeINSTEIN]

Originally Posted by PhrozenFire

Hey guys, update for everyone. Took my bike to Balaji as suggested by Ajabiz below. Balaji seemed to be a knowledgeable guy, and he speaks very good English! He rode the bike around for a few hundred meters and immediately said the problem was due to loss of compression caused by a worn out piston and piston rings. I did suggest to him some of the issues mentioned on this thread, but he point by point ruled them all out. The piston and piston rings will need to be replaced, as will be the head gasket. Also will need to get a reboring job done. So it was what I was suspecting all along. Total cost, including labour, will be around 2-2.5k. Will keep everyone updated if there are more developments.

Weird.
I have never heard worn out rings to cause audible Phheenshh-Phheenshh from the cylinder as revs/compression builds up and loss in compression to limit rpm to 3.5K only.

In fact, when i read your post where it said the bike suddenly was revving ok, it is also characteristic of head gasket leakage due to loose head block. worn gasket or worn out studs/bolts/nuts.
Fuel+Oil leaking from head attract dust and create greasy mix near leakage.
Cool engine contracts and reduces or closes the small gap.
Bike behaves normal till block/head/bolt expand after getting hot enough and then again phheenshh-phheenshh.

Get a second opinion imho.

If you can, fit single piston ring in the bore, use piston to even it out and then check end gap of piston ring against light if you have an idea or with a feeler guage.
Also a compression check done using a pressure guage.

I will suggest-
Check head studs for slippage- i.e see if they are tightening well or f any of the nuts slip or the head bolts slip.
If they do, u have a culprit- replace bolt/s or nut/s as mentioned in previous post.
If not, use a new gasket meant for RX135/Z and get head refitted tight enough.
Ride around hard and see if that loss in compression occurs.

Following the above makes sure you try to locate and rectify the issue with minimal expenses and maybe avoid a wrong diagnosis (deliberate at times to make money) and save unnecessary repairs and expenses too.

Another stupid suggesion-
Have you checked your fuel cock and the breather hole for the tank.
Clean both and make sure fuel cock allows free flow. :P