Motorized bike makes a very loud bang and doesn't start...

Tom74

New Member
Mar 23, 2020
19
1
3
France
So, I have a china doll motorized bike kit on my bike. I bought it last year and I have had many many issues with it. Sometimes it was a small thing like a bad gasket or a loose nut/bolt or even stripped threads in the exhaust side of the cylinder. But one day I was riding my bike in some tall grass and I couldn't see very well where I was going so I decided to slow down and turn around but as soon as I thought about doing that I hit a tree stump and it bent and destroyed my stock exhaust. So I had to push it back home. Once I got back home I went on the internet and I bought a new exhaust pipe, it was a FlexFit Street Poo Poo exhaust pipe. Once it arrived I installed it with some other new parts like a new HP CNC intake and a new chain tensioner. Anyway so today I had put it all together and tried to start it but it wouldn't start so I took the spark plug off and then I saw that it was very oily so I wiped the oil off, put it back in and tried again. The same thing happened it didn't start... So then I looked at the chain tensioner and saw that the spring wasn't I guess tight enough or kinda weak so I put a piece of metal wire around to parts of the chain tensioner and then I tried starting it again and it made an EXTREMELY loud BANG coming from the exhaust pipe. I have tried doing many things like for example taking the exhaust pipe off and seeing if there was anything wrong with it but there didn't seem to be anything wrong with it. So anyway I have no idea what's wrong with this thing because before it used to run perfectly fine.

The only things that I know need replacing are the crank seal that's behind the magneto and possibly the other one on the drive gear too.

Here are two videos of it (not the loud bang tho) :

Thanks in advance.
 

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Oldbiscuit

Well-Known Member
Oct 3, 2020
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Your chain is much to loose. It appears from your videos that the chain is jumping the drive sprocket, good way to destroy a crank case.
 

Tom74

New Member
Mar 23, 2020
19
1
3
France
Your chain is much to loose. It appears from your videos that the chain is jumping the drive sprocket, good way to destroy a crank case.
I put a metal wire through the holes of the chain tensioner to stop it from going up and down so it has a good tensioner but then when I tried to start it up it made that loud bang.
 

Oldbiscuit

Well-Known Member
Oct 3, 2020
106
279
63
65
That chain is so loose that the top and bottom chains are touching. You need to shorten the chain first thing.