Hi fellow adventurers.

I'm not sure where to post this, I don't do much forums, but I'm desperate for advice now.

My XT660Z just started giving me a strange problem. In 5th gear, it won't go past 5000rpm so my top speed is limited to 135 - 142ish km/h (around 87mph); and at the same time, I'm only getting about 14km/l (about 32mpg). This is breaking my brain. Usually, when I ride it hard on the highway at 150-160km/h (5500-6000rpm), I usually get around 18km/l. I can even get it to almost 21km/l, but I'm not that kind of rider

Have anyone experienced this with their XTZs?

Recently, the 3rd gear disintegrated, so I got that replaced by Yamaha, that came with new clutch plates. I got the valves done, cleaned the air filter, replaced the fuel pump and filters, new chain and sprockets and added more rubber to my cush rubbers, serviced the suspension, recalibrated the throttle and sensor, new tyres and got the injector cleaned. And still, this problem is still the same. The bike pulls as normal, everything below 5000rpm through all gears are awesome as usual, but not getting my usual top speed and consumption is frightening. Also, when I get a little side wind, the top speed will drop to 125km/h. There is something terribly wrong and going cross-country is going to cost me a lot more.

The history of the bike -
It's a 2014 model, I bought it brand brand new in 2015 with 0km on the clock. I put 87000km on it in the past 3 years. I won't lie, I wasn't easy on it. It was heavily loaded, did mostly offroad, I never missed an opportunity to take a jump. It has an after-market 2-1 exhaust with no other engine mods, except for the snorkel I removed. After everything I did to it after the gearbox went, it's practically a new bike. Only thing I haven't touched is pistons and rings. The mechanic gave up and said that the bike as had a great life, but it's getting tired now. That's not really good enough for me.

I will appreciate any advice.