Motorized Bicycle Huge Power Loss Help

[Yang]

66cc Engine with 8mm stud
Ported Cylinder NGK B6HS gap as 0.02
95 gasoline 25:1 mixture
Stock muffler, carburetor

A couple of days ago, my bike got snowed. And magneto went wrong with moisture, so I replaced it with a new one. But, after my Engine lost power. Before then, I can speed 35-37 miles with 44th gear. Now, it's hard to reach 10 miles. I've checked any leak from exhaust or intake, head, cylinder gaskets. No leakage or damaged gasket. I changed my whole ignition system with brand new parts but still the same low output. Cleaned the jet and carburetor, air filter, put heat resist silicon to make sure the seal. Cleaned the exhaust and tried with, without the muffler. As you expected, the same. For last, I decided to disassemble the Engine, but while I was pulling out the small gear, the thread got smoothed so I couldn't get them out. Now the only thing that left is crankcase gasket. I think this power loss caused by low compression in the crankcase or damaged bearing. So what I'm trying to do is weld the crankcase and see if there's any change.
How do you guys think about this problem? Is there anyone who solved this same issue?

 

[Frogslayer]

What's the plug look like? Is it running lean? Or is this a carb problem like a stuck float or valve or clogged fuel filter or petcock pre filter?

 

[Yang]

What's the plug look like? Is it running lean? Or is this a carb problem like a stuck float or valve or clogged fuel filter or petcock pre filter?

For plug, I can see some carbon but it's not much. still in range of OK.
When I cleaning the carb, I used a single copper wire to verify both jet and valve. And I just swapped whole fuel tank with new line/filter then I can see a bit of improvement. Nevertheless, not the same as it used to be.

 

[mark20]

66cc Engine with 8mm stud
Ported Cylinder NGK B6HS gap as 0.02
95 gasoline 25:1 mixture
Stock muffler, carburetor

A couple of days ago, my bike got snowed. And magneto went wrong with moisture, so I replaced it with a new one. But, after my Engine lost power. Before then, I can speed 35-37 miles with 44th gear. Now, it's hard to reach 10 miles. I've checked any leak from exhaust or intake, head, cylinder gaskets. No leakage or damaged gasket. I changed my whole ignition system with brand new parts but still the same low output. Cleaned the jet and carburetor, air filter, put heat resist silicon to make sure the seal. Cleaned the exhaust and tried with, without the muffler. As you expected, the same. For last, I decided to disassemble the Engine, but while I was pulling out the small gear, the thread got smoothed so I couldn't get them out. Now the only thing that left is crankcase gasket. I think this power loss caused by low compression in the crankcase or damaged bearing. So what I'm trying to do is weld the crankcase and see if there's any change.
How do you guys think about this problem? Is there anyone who solved this same issue?

which two wires did you put in the magneto? its supposed to be the black and blue, NOT the white one, as those are meant for lights only.
how easy is it to turn the engine over? any scars on the cylinder on the exhaust side? also, which way is the magnet in the engine (the one that spins) try flipping it around and see if that fixes it (it should be straight when the piston is at top dead center, if its offset, then flip it, if not, then leave it)
my guess is either low compression or a stuck piston ring (take the cylinder off and see if both rings are open, if not take some wd40 and use a pick CAREFULLY)

 

[Yang]

which two wires did you put in the magneto? its supposed to be the black and blue, NOT the white one, as those are meant for lights only.
how easy is it to turn the engine over? any scars on the cylinder on the exhaust side? also, which way is the magnet in the engine (the one that spins) try flipping it around and see if that fixes it (it should be straight when the piston is at top dead center, if its offset, then flip it, if not, then leave it)
my guess is either low compression or a stuck piston ring (take the cylinder off and see if both rings are open, if not take some wd40 and use a pick CAREFULLY)

When I got my new manito, I use my soldering iron to remove the white one and reinforce the other two, which are black and blue, blue to blue. Just tried to swap, and she won't let me start her up. For the magnet, I didn't even touch it from stock. Also, it is intact as it should be like you said. Piston ring, I was suffered due to broken rings in my first build-up, thereby that was the one thing I check first.

 

[YetiWoodz]

Check to see if anything is leaking out of the exhaust when you run it. You can check the engine surface temp with a temp gun to see if it's running lean. Normal temp is around 270⁰F. If its running lean it will be hotter. Make sure there's no leaks in the intake, feel for air leaking out. Spark plugs are cheap, you can try swapping it out just to eliminate the small chance. The only other thing I can think of and I don't think this would be it, but check the gas tank fuel filter and valve. But my guess its running too lean or rich. More likely lean.

 

[wan37]

Well not being a smart a** . I’ve been burn by the nt carb the choke is stuck on the lever is kinda stubborn at times. 1more thing remove fuel tank cap see if it better if does drill one or two small holes in cap.