Engine Cannot stay running, easy to turn engine with clutch in

Last weekend, I was riding my bike like always and all of a sudden, I heard a weird pop sound and felt a quick jerk. I thought maybe my clutch internal got messed up. After stopping and shutting down the engine, I started to pedal around to see what is wrong. The clutch still works fine and engages and disengages like normal, but I do notice that when the clutch is in, I can turn the engine easier than before. It is as if I unscrewed the spark plug slightly, allowing easier compression. But the spark plug is on tight and all cylinder seals are still intact. Yet the engine turn rather easily with the clutch in. I tried to start the engine again and while it starts up, it lacks power and as soon as I idle for a couple of seconds, the engine dies. If I kept the RPMs high enough, the engine will run, but at a very weakened state. Nothing is leaking.

I rode it back home and checked the clutch plate and everything appeared normal. I took off the cylinder head and examined the inside and I noticed my piston has a small part of the top edge nicked in a couple places, like a piece of it was taken out. I also noticed a large vertical shallow groove gouged into the cylinder wall. The piston itself moves freely and doesn't bind or get caught on anything, but I am guessing my engine is dead. Would shallow groove in the cylinder wall cause easier compression and present with problems such as easier turning of motor when off and weakened power and inability to idle? Should I buy a replacement piston and cylinder or is it better to buy a whole new engine?

Greg58 said:

It sounds like a broken ring, or a part of the piston got hung on the side of the piston. But the good news is you can replace the piston,rings and jug fairly easy. around 50$ should cover parts needed, be shur to take the engine out of the frame and wash out any remaining debris with carb cleaner or wd40 spray. Just make certain all small pieces are out.

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What about the vertical gouge on the cylinder wall? Should I replace that too?

I didn't pay attention to the rings on the piston, but I will check it out tomorrow. As for the head gaskets, I replaced both the upper metal gasket and the lower paper gasket with new ones and it still is easy to turn the engine while the clutch is in (no compression). Could the exhaust pipe have anything to do with no compression? The carb is brand new and has a filter on there, so I doubt anything could have gotten in through there and cause that big of a gouge in the cylinder wall. The spark plug is tightened down good and isn't leaking any gas/oil from there either. If a piston ring was broken, would that lead to a situation where the engine is less compression? Is the gouge in the cylinder wall affecting compression in anyway? Is it something that I need to replace ASAP?

Ok. I took the cylinder apart and here are some pics and my findings:

Perry was right. The top piston ring got fused to the piston head in one area and snapped at that point. This point also coincides with the gouge on the cylinder wall. The piston head is also badly damage on the top edge. The crank needle bearing also broke into pieces. Looks like I have to replace all these things. Is there an easy way of cleaning out the crankcase without having to split it in half? Can I just spray a bunch of brake cleaner into it and let the liquid carry debris out? Splitting the engine in half requires me to pull gears (and deal with woodruff keys) and break crank seals and the such. I live in california, so getting a new engine is near impossible as many online retailers don't ship to CA due to carb laws. I got lucky with a craigslist seller that isn't selling kits anymore.

How do I know which side of the piston is suppose to face the front? How does that make a difference? isn't the piston symmetrical?

Here is the pic of the crank shaft. It's not really close up enough to see the details. I would have to check it out tomorrow. I didn't pay attention to the smoothness of the inside. Didn't seem like any major flaws. Would minor grooves or imperfections be bad for the needle bearing? What does 'mic' mean?

GearNut said:

Mic is an abbreviation for micrometer. A measuring tool that measures down to 4 places past the decimal point, 0.0001 of an inch or centimeter (some even go down to 5 places).
You need to check the connecting rod race for any deep scratches as well as any out of roundness as either condition will spell impending doom for a new bearing.

You can always just take your chances and install a new bearing and run it until it blows again. Just keep in mind as you ponder this option that any of the above mentioned issues will reduce the lifespan of said bearing.

FWIW, that bearing failing is what caused all this mess for you anyways.

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Is the connecting rod race the part shown in the previous pictures I posted? The part where the piston head connects to the crank shaft?

I took a closer today at the top part of the crank rod and while I don't have a mic, it feels smooth with no gouge or any visible signs of scratches or such. A simple one-eye-closed examine from the side yielded no signs of gross uneven roundness. I know this is not nearly as accurate as with a mic, but i'll give the upper engine rebuild a try.

BGF on ebay sells cylinder and piston and piston ring sets for 66cc/80cc, so I figure it should work. The arrow on the piston head should point towards the front of rear of bike?

This is so retarded. I got the replacement cylinder and piston and it turn out the piston is the wrong type. It will hit the top of the cylinder head before it can reach TDC. I called the various online places (bikeberry, etc) and they don't carry the piston I originally had (Raw 80CC kit). Since I live in Cali, I can't get a new engine (which I would have done in a heartbeat to save all this headache). Someone mentioned to me to place extra top head gaskets to lengthen the piston travel distance. Is there any adverse effect from using multiple metal top head gaskets together in this fashion?

I am not sure if that picture is of the actual product, but if it is, it's the wrong type. Here is my what piston looks like. The wrist pin hole is offset much higher than the one I bought and the one on RAW's website, if it is correct. And to double check, my engine does have the RAW name plate affixed to it. Could I just put extra top head metal gaskets to compensate for the new piston?