How to solve/avoid problems in Chinese MX Bikes – Common issues

Hello Folks

Today I’m posting in order to help other dirt riders with problems they might have been through the same way as I did, with my new MX Bike .I think some things are common to Chinese bikes ,not all but a big number of them.

1) The first thing that got me crazy was a failure related to the Electric Start and misfiring when I opened the gas too much, this was associated with higher RPMs also.
The simple solution was to change the terminals (Bosch type) male and female that were inside a plastic connector coming from the stator to the CDI, they were sulfated thus leading to inefficient current handling. It was almost isolated as if they were covered by an insulating tape!!!
After changing the terminals y used a 3M auto-soldering tape to protect the whole socket from water and dust. This definitely solved the issue.

2) I noticed that the spark plug was a bit black, this was easily solved by turning the gas screw to the right ½ turn (it has a blue Loctite that prevents the screw to vibrate and turn, so apply a bit torque to screw in )

3) After that I installed a simple set of lights, front, rear, and brake. If you want I can upload the wiring schematic of the whole bike which is indeed the same for all china mx bikes, the only difference is between 4 and 5 contacts CDI units (I’ve them both)
Just ask for it.

4) The next step was a bit surprising,… I was climbing a hill with the lights turned on , and suddenly I heard a big explosion ..Guess what? The battery exploded into pieces, yes really exploded, it seems that the battery was not prepared for a continuous use and a lot of Hydrogen was formed and boom…too much pressure.
I changed the battery for a good brand and I never had that problem again. So watch out!!!
(Maybe the breathing holes from the battery blocked and there wasn’t enough pressure release)

5) The chain was too much tensed, I tried to make a wheelie and it broke, simple bought another and no more problem with that. (Rookie things…)

6) After a hard off road trip, I loose the nut that holds the swing arm in place, I bought another one but this time a self-locking nut (the ones that come with a plastic seals in the top that prevents them to fall)

7) A simple thing like changing the oil can become a hard time if you over tight the plastic chromed nut that holds the oil filter in place, I over tight it and the nut broke, Its really weak and it should be treated very gently. I simple bought another one but this time I toque it a little. Remember 4 strokes don’t build too much pressure on the carter as 2 strokes do.

8) I was trying to change my oil and a rusty screw coming from the protective plate under the engine broke!! This was really a nightmare ,I had to rebuilt the threads, drill a hole, hopefully I could save the threads ,this time I bought Stainless Steel screws so I avoid this in the future. Note that Stainless Steel is good but its not capable of bending that means that if you go through its limits the screw won’t bend but broke. Be careful were you use this screws, sometimes its better to see a bending screw and know that it needs to be changed and prevent a broken axle screw for example….)


9) What follows I think it was the worst thing in matters of recovery time up to the initial condition.
After another off road trip, the threads of the pushrod of the rear shock broke and the shock disassembled, as a result the bike went down hard and broke the intake manifold (made from aluminum), and the muffler melted the right side of the rear tire. (I had to ride over 30km to reach my car in order to put the bike in the trailer)

Well, finally after searching in all the net and parts stores in my country I couldn’t find the rear absorber parts which broke ,so I decided to try to rebuilt it, I lost the adjustment(nitrogen and oil),but its still running.
I bought a new tire Pirelli Mid Soft 32 (good one)
And welded the intake manifold (this was really a problem because I couldn’t find in all the internet nor parts store a replacement part,. it has a really hard curve that makes possible for the carb to go below the seat)


10) The rubber boot that goes between the air filter and the carb , got dry and broke, I had to replace it ( if some air is sucked, the air/mixture changes and your bike could run lean/rich )

11) Testing my bike after point number 9) I noticed a strange dancing from the rear, it ended up to be a broken bearing, so bad that the outer race was stuck in the rim, to take it out I had to weld and hammer it from the other side evenly, finally I could take it out, the heat from the welder (arc welding) makes the outer race to expand and as it colds it contracts and makes easier to come out)
To take out the other bearing I used a Wall Anchor ( I made my own bearing extractor),I also changed the dust seals (try to buy Japanese bearings or any other good brand, I think Americans are good too)
An interesting trick to put the new bearings is to allow them to cool 2 hr in the freezer so they contract a bit more, this way you can change them in a glimpse.

12)This time I was testing my new bearings and I decided to go to a sand mine, therefore I lowered the tire pressure and all was fine, BUT I forgotten to race the pressure after I reached the hard terrain ,I accelerated and one of the tacos of the tire blocked in a stone and the rim turned inside, as a result I cut the valve and I lost all the pressure in less than 5 seconds, hopefully I avoided hitting the ground, because it was a bit more than going through the sand but more out of control! Ha )
I bought a new Pirelli camera and I covered with 4 passes of isolating type the spokes (inside the rim) to avoid damaging the new camera, the protective plastic tape that comes with the rim was broken.

13) one thing interesting to keep in mind is that the aluminum rims are too soft to use regular tools to change the tire, avoid using too much pressure and if you can use both levers at the same time so that they apply less force to the rim (I bent the rim in some places)

14) The Positive and negative cables that went to the battery were too short and one of them cut, the problem was that the circuit was closed by other small cable and too much current through them (for example when you press the start switch) will roast them, and you will end rebuilding the whole installation, so its better to check this things before its too late.

15) Changing the brake pads could be a easier task if done correctly, if not it could be another nightmare. Open the reservoir to allow the pistons from the caliper to go back to allow room for the new pads, don’t open the bleeder nipple as I did because it can catch air and this is where the nightmare starts, I pumped the master cylinder 100 times to get pressure and no way, to solve this I had to do a whole set of things that believe me you don’t want to .(you can see my post in this forum asking for help ).Always have a transparent plastic tube attached to the nipple with a little brake fluid in it just in case it wants to catch air there will be fluid.

16) One day I turned the throttle, ok I tried to and I couldn’t…what was happening? After disassembling the whole throttle I discovered that the problem was a stuck cylinder that goes inside the carb were the throttle cable attaches that was plenty of mud and really stuck. I cleaned the carb and all turned back to normally.

18) Take care of the hose that goes between the carb and the gas tank, sometimes the clamps go off and you need to change them.

Well this is a brief summary of the things that I’ve been through with this bike, I have detailed procedures for all the things I described in this post, as well as pictures and links related to some of my posts in the forum and solutions I read outside too, if you need further assistance in something just post it or ask me I will gladly help.

This forum, help me a lot and I want to thank in some way to all the people that allow me to solve my bike issues faster.
Specially Carl and Forchetto always keep an eye on the posts and help me in some way or another.

Thanks for that. You certainly have patience...must be a Saint!. A lot of lesser people would have given up by now and dumped the bike in the back of the garage.

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Originally Posted by forchetto

Thanks for that. You certainly have patience...must be a Saint!. A lot of lesser people would have given up by now and dumped the bike in the back of the garage.

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For sure. I think the most important aspect of being a good mechanic is patience and willingness to try different solutions. Thank you for the thanks TXM but the best thing you can do for us is take good care of yourself and your bike. Ride safe and take lots of pics :thumbsup:

CC