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Aerox Project Not Working


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At the start of summer my friend bought a Yamaha Aerox for £600 and it worked pretty much fine but 2 weeks after riding it, it wouldn't start. I do motor mechanics on cars and I've fixed lots of broken mopeds in the past. After it broke down he threw the air filter away and said 'It's f*cked'. I recently bought it off him for £90 and spent ages getting it started then the kick stand went through the new air box I had just bought so I asked him and he said that happened to him as well so that is why it broke down because it got all wet.

I got a sports exhaust and it was running fine i got it MOTed and used it to go to work but on the way home last Tuesday it stopped getting spark or something and the engine turned off. I stopped at the side and looked at it and didnt see anything wrong with it so I kick started it about 10 times and it started up and seemed fine so I started riding down the road again and it stopped again after about 5 seconds and this time I went to kick it and it was completely seized. I got someone to tow me home and the next day took the cylinder head and everything off. I had to hit the piston with a hammer to get it to go down through the cylinder. After taking it all apart and looking at the piston and cylinder it all looked fine I've had people that have been mechanics for 40+ years and they have said they look fine. The piston was definitely seized because I could move the crankshaft back and forth a little bit but the piston wouldn't move in the cylinder. I ordered a new 70cc sport cylinder and piston. I searched everywhere and found out it could've been running lean as I didn't re-jet it when I got the exhaust so I got a box of jets 68 - 82 or something because the stock jet is meant to be 62 from what I found but when I opened the carb I found out it had a 80 jet in it already. I bought another set of jets 90 - 108 and fitted the new 70cc cylinder and put a 92 jet in because running rich doesn't cause as much heat. I also got a new drive belt after fitting it all I started it up and it started first kick but sounded like absolute sh*t. After showing people a video of the sound they all said it sounded like a truck or tractor but when I put my phone near the exhaust it sounded like a 2 stroke. I went for a test ride and was out for about 5 minutes then went to turn around and as I slowed down and turned it stopped again and It had seized once again so I got someone to tow me home again and when I got home I pushed the kick starter down and it was nice and free so it had heat seized.

There is 2 nipples ontop of the cylinder and 2 on the side of the carb that had blanks over them and I have recently found out that it is for carb heating. I found a hose that would fit over and took the belt off because I thought it was the belt or clutch making it sound bad and it sounded better but still not the best it and it was very smoky. After running it for about 30 seconds my whole garage was full of smoke. I learnt at college last year that white smoke can be unburned fuel but I have got a brand new spark plug on it.

What could be causing it to be heat seizing? What could be causing the sound? Is there any point spending more money on it? What could be causing the smoke? (its white smoke btw but its not coolant because it doesn't smell coolanty(sweet) it does smell a bit fuely)

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NOT a mechanic...

Have you tested the bore to make sure it's round?

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The old piston and cylinder look fine no scratches or marks you wouldn’t even think that it could’ve seized

It has a new good brand 70cc sport cylinder in it and on first test ride that heat seized

 

Had new Reed valves as old ones were quite worn out. Had new cylinder, cylinder head, piston all that. New intake manifold. New air filter(and box). New spark plug. Fresh 2 stroke oil. New jet. Does anyone know what could be causing it to heat up to much

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I think your problem is crank, the crank seized not the piston that's why its in good nick. 

The smoke, white smoke is oil but that can only be 2 stroke but you haven't been far enough to burn off the assembly oil.

That would be my guess, not a massive pain to fix, would have been much simpler if you threw in a new crank and bearings while you were renewing the piston kit, crank kit is not massive money, certainly less than the sport kit if its any good. 

A sport spec 70cc capable crank is only 60 or 70 quid. A bone stock unit is about 25.

 

 

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I didn't realy want to do the crankshaft because you gotta split the engine.
The crankshaft is all moving smoothly.

When I was going to put the new kit in I took the variator  and everything off and I took the reed valves off and could see the crank shaft and bottom of piston and I could move the crankshaft round about a cm from each side or something and the piston wouldnt move so the crankshaft was all fine and everything it was very smooth the piston wouldn't move though

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OK. Have you measured the piston. 

Propperly I mean with a top middle and bottom and the barrel to see what the clearance is. 

Quality kits have the barrel bored to the piston. Cheaper units are just spec made parts put together. 

 

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It’s an airsal kit but before that it had the stock one on

 

i think it might be cold seizing which is where the piston expands quicker than the cylinder when you don’t let it warm up enough and when you put loads on the coolant is still keeping the cylinder cooler then the piston expands and seizes in

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Cold seize is only really likely on dedicated race kit, at the end of the day its a scoot that will be used day to day and running cold wont be an issue.

 

Did you fully bleed the cooling system, these scoots have a nasty habit of trapping air in the cylinder head.

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I put it all back together and put an 82 jet in which i think is still a bit big but its better Im just testing it. I left it idling for about 20 minutes and it was completely fine I couldnt test it because it was snowy and icy but going to test ride tomorrow ill tell you what happens.

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