Hi,

I have tao motor raptor / hunter 125cc rebranded atv, and after I got the stator fixed by getting new one, we were able to drive with this atv for about week. Last monday my son was driving it, full speed down the hill, drop the gas off and made a hard breaking with both front and rear breaks. After atv stopped, son tried to give gas to drive forward but engine had shut down. Well, he tried to start it, but nope. It did not fire up. He pushed the atv back to garage, and we started to investigate that what is going on. I was thinking, that ok, it might be the stator again, maybe I was not carefull enough putting the new one in, but it turned out that it was still working. Measurements were giving correct readings. Then I took the plug out, and checked it. Was not wet, gave spark.
After going trough the threads here, I started to think that ok, it might be that what if in/out valves had something to do with this. I tried to start the engine again, and noticed that then the gas in full, I can feel the pressure coming out of the air filter. That I think is not a good sign...
And when closing the gas, I can feel air coming out from the exhaust when trying to start. That lead me to think that could this be something to do with the timing.
I opened those caps that hide valves, also the flywheel side cap to rotate it and check the timing mark. Timing mark on the spot, and checked the valves and the clearances. They were little tight as in should be .003mm and ex .005mm, but I was not able to get the .005mm in ex. It is really hard to see those, as engine is still in place so atvs covers block most of the visibility.
Anyways, I will to more investigations about the timing and valves..

But my question about this is that have anyone had similar issue? Meaning that when you hit breaks at full speed, and engine shuts down totally? Revs are high, but there is still gearbox (this has auto with D, N, and R options) which should take the heat in this case. Only thing that I can think of here is that somehow the timing has gone totally wrong and it's because of this full speed breaking.

 

0.003mm is tiny, I doubt if you could get a feeler gauge that thin. First thing to do is fit a new plug, if that doesn't work, clean the carb out, no altering settings, remove the float bowl and jets, then blow through them and the holes they came from, out with compressed air. If that doesn't cure it, check compression, less than 90psi and the bike has problems, burnt valve, bent valve, seizure, or just wear on the bore or rings.

 

Yep,
​​​​​​That sure is small gap. I did drain the carb, still need to take it to table and check it properly.
I'll check the timing first, after that take the carb to table. I hope that the head gasket is not the case...
Need to check the compression, already checked it with finger check. Ok, that does not tell anything except that there is at least some pressure.

 

What I was implying is 0.003mm is wrong, I would guess that should be 0.003 inches, which is 0.08mm.

 

About those valve clearances: Tao Motor user Manual states that in should be .003 mm and ex should be .005 mm. I do have measuring plates which can help me to set those but I am not convinced that I should use those. I think I will go with .005 mm for in and .007 mm for ex.
I rotated flywheel to catch the tdc and checked the valves, ex valve is not showing any indications that there is any gap so I have strong feeling that my ex valve is now open all the time. I get strong smell of gasoline when trying to start, so I think the gasoline is pushed to exhaust pipe.
Plug is new NGK, less than 20 hours used, gives clear and good spark when not put in. Strange that it does not get wet during start.

I did took the carb out, checked the needle and float, cleaned the jets and holes with air, there was no signs of dirt. Gaskets were in good shape.
Well, I need to continue investigating what is causing this. Maybe next I take the head and cyll off, check if the gaskets are ok.

 

I took the stator side cover out and also the head side plate that covers the top end. Timing is in place, the tdc markings match in both, the flywheel and in the gearwheel.

I also took the carb out again, and cleaned it with air. Float chamber was still clean, I was thinking that did I miss something last time.

I try to borrow a proper pressure meter to see if the valves or the gaskets are leaking, as now it starts to look like a pressure issue.

 

Oh well...
No need to check the pressure. I inspected the input valve, and found out that rocker arm is touching directly to the spring. Valve tap is visible through the rocker arm hole, where adjustment **** normally sits. Valve is sitting way too low. So, the valve lock pieces are now somewhere in the head, or in worst case, inside the cylinder.
I have no idea what could cause this, except that maybe there has been a contact with piston and the valve.
I am taking this atv to repair shop tomorrow and they can investigate what has gone wrong with the head and check if there are other damages to rocker arm etc.

I put the picture here where adjustment knub is still in place :-)

 

Hi, just had a chat with service company about our atv. They just got new parts and start to assemble it back together. The Head needed a full replacement as the input valve was not ok, it has taken a hit from the piston and bent. So did the actual valve hole in the head, that's why they are replacing the whole head, as there were cracks. The piston is also new, it was beyond repair. The lower part of the cyl seemed to be ok, they did check it.

The service guy said that he has not seen this kind of stuff before (with the engine so low hours in use), so it's really hard to say what has caused this. Maybe there was an issue with the head already when we bought the atv, but they cannot be sure about this. Normally these chinese ats should endure normal driving and with easy maintenance, they give no issues at all except normal wearing. But, when you have less that 80 hours with the engine and head gives up, that's not normal.

 

"Dropping a valve" is not too uncommon, I have had a few engines that have done it. Over revving can cause it, but a mechanical problem can also be the cause. I had a Honda 420 that wore the top of the valve down to the collet groove. With low hours on your machine, it could be faulty collets, or one not seated properly, or the groove in the stem not machined right.

 

Oh well... We got the atv back 1 week ago, then had a summer road trip so not had time to drive or check the atv, just started it and drove few meters. Now that we are home, kid started to drive it gently. I told that we need to be carefull to let the new parts fit in slowly. So no full throtle. But after starting it, you could hear a clear bang noice when engine is cold, and when you give throttle a bit. And it did it again, several times. After engine has warmed, not so much. So, I took a look what the heck is causing this banging. First I checked that what is the air/gas mixture, but it has not been touched at all. Still I wanted to try if it has something to do with this, but just as I was making small adjustment and put some throttle, I noticed that this bang comes from the cylinder and it pushes a small smoke puff out from the head gasket. Then I observed more, there is clear indication that something is leaking from the gasket.
Need to contact the service again to get more information. Since they replaced the head and piston, I assume that they did change the gasket too. If not, then it is clearly a service failure and they need to fix this. I sure gives headache to own chinede atv :-)