Yamaha Aerox 50cc 2t liquid cooled 2009 keeps breaking down

Bought a yamaha aerox off a friend that has been sitting for about 4 months. They bought it for £600 and in 2 weeks it broke down turns out the side stand got bent and went through the air filter then before he noticed that he jet washed it. He didnt know anything about mechanics and couldnt really be bothered to get it fixed and pay for it so he just left it sitting. I bought it off him for £90 and got a new air filter. I also removed the exhaust and because it was so rusty from being old it snapped in half so I bought a new exhaust (Technigas Q-Tre). After kick starting it for 4 or 5 days for hours straight it suddenly started up and was working and everything but was very slow. I took it to get an MOT and it passed as had not done many miles since last MOT. After some research I found out that the exhaust had a restrictor in it to keep it going 30 by restricting the engine. I took the exhaust off and removed the restrictor and it would get to about 40 but still took ages to get there and would drop to 25-30 up hill. I rode it to work 2 days (about 20-30 minutes mostly just a straight road) on my way home on the second day the engine completely cut out (about 10 minutes after I left) and I managed to get into a little side road. I tried kick starting it a couple times and it wouldn't go then after about 10 more times it started and seemed fine again so I carried on riding it trying to be careful and just get it home but then the engine cut out again and when I went to kick start it back up it wouldn't even move because the piston was seized. I got someone to tow me home and I took the reed valve out so that I could see in the crankshaft and after removing the CVT cover too I could turn the crankshaft back and forth a little bit and it was all smooth so the piston was seized into the cylinder. I ordered a 70cc cylinder and took the old one out. There was not a single scratch or mark on the old cylinder or piston other than marks on the top of the piston from where I had to hit it with a punch and hammer to get it free from the cylinder. I put the new cylinder and piston in and got new reed valves as they were quite worn then I went for a ride and after about 10 minutes again as I slowed down it turned off and was seized up again but this time after about 5 minutes it was all free again so it had only heat seized but why? The thermostat is opening because coolant is circulating back to the radiator its not running lean. After alot of googling and watching videos i found out the 2 nipples on the top of the cylinder and 2 on the side of the carb are for 'carb heating' so that in cold weather the carb doesn't get icy or anything if some water vapour gets in. After connecting some hoses to the carb (used clear ones so I could make sure the coolant is all getting there and everything) I also put a connection on the temperature sensor wire as that had broken off. I then went for a ride and it was perfect I went on a ride for about 15 minutes and didn't have a single problem with it. The next day I went out again on a test and after about 10 minutes of riding I decided to test it in a straight line to see if it could cope constantly going instead of slowing down then speeding up so I went down the main road near me and was still not very happy with the speed only getting about 38. After about 5 minutes of riding it started choking and spluttering and losing power so I pulled over and turned it off then checked about and it seemed fine so I tried starting it and it seemed fine again so I decided to carefully ride it about 3 minutes home and had no problems with it (going about 30). I am an apprentice mechanic and have spoken to the people at work about it (who have been doing mechanics for over 40 years each) and they haven't got a clue what it could be one of them even used to do 2 strokes he doesn't know what it could be either. It still has the 12mm carb was going to get a 17.5mm one next time I get payed but could it be the coil pack?

Had a new
Drive belt, Intake manifold, Air filter (box and filter), Reed valves, Spark plug, 70cc Kit (Cylinder, gaskets, piston, cylinder head), Main jet (was 80 when i bought but meant to be 62 and worked it out to be about 74 so put a 75 in), 5gram rollers, Side stand.

Please help

 

Have you got the cooling shroud around the barrel?

 

It’s liquid cooled surely the shroud would just make it even hotter? The temperature light doesn’t come on so I don’t think it is that

 

My bad, didn't spot the liquid cooling
Have you checked to see if coolant is being pumped back to the expansion tank?
Did you bleed it properly?
Are all the hoses getting hot, not just warm, but hot......

 

I think I just found your problem..............It's your right wrist.................
Quote: "still not very happy with the speed only getting about 38."

2T pistons and barrels HAVE TO BE RUN IN for at least 300 MILES

That means keeping the revs down, keeping the speed to around 30mph, no revving the nuts off it, no seeing how fast it goes.
Just up to 30mph, back off a bit, back up to 30mph, back off again.

 

all bled up properly all hoses getting warm all getting back to expansion tank. after googling bit more about it i think its the fuel filter because thats all shitty from the old fuel that used to be in there (thats a technical term)

 

Shitty fuel filter doesn't cause a heat sieze

 

shitty fuel filter can cause it to run lean
running lean causes too much heat
to much heat causes seizure

 

You know what, I'm out. You carry on

 

what would u say it is

 

its not anything to do with the intake
its not the spark plug as that is new
its fine when demand is low but if it has high demand then it dies so the fuel filter is clearly clogged
bear in mind it hasnt been looked after i changed the brake fluid and it was black