Hi guys, new here.

I bought an LT80 ATV for my daughter in a disassembled condition and rebuilt the engine. I used a first-oversize Namura piston and bored the original cylinder. I made intake and exhaust block-off plugs on a 3D printer and pressure-tested the engine at 0.8 bar. It held pressure for over a minute with no loss.

Then I tuned the carburetor. During tuning, the engine ran on idle and mid throttle with the wheels off the ground for about 40 minutes total. Fuel was 1:50 premix plus the oil injection system still connected.

Carb was tuned using a wideband O2 sensor welded into the exhaust. Idle AFR was lean, midrange was around 12. I installed a pilot jet two sizes larger — idle became ~14 AFR, but midrange got rich (~10). I lowered the needle one clip and midrange returned to ~12.

High-RPM AFR was around 10, so I leaned out the main jet. I installed an #88 main (stock is 92.5). This did not change AFR readings, but since I had no smaller jets, I left it like that.

Including warm-ups and tuning, the engine ran about 40 minutes total. Oil is a proven full synthetic 2-stroke oil for sport/enduro use.

First test ride was done at about –5°C (23°F). After ~300 m on flat ground, the engine stalled and could not be turned over with the starter. After cooling for about a minute, it restarted. After another 100 m it seized again, and I pushed the ATV back to the garage.

I reinstalled the stock main jet and returned the needle to the stock clip position (2), but the same thing happened again. Total of about 5 soft seizures. RPM would slowly drop, and before stopping completely it was possible to keep it barely running with some throttle. No violent lock-ups.

I noticed during test rides (using a tachometer) that the engine was running around 6300–6500 RPM, but vehicle speed increased slower than expected, so I suspected clutch slip.

The next day I disassembled the CVT. Rollers and movable face (new parts) were OK. No noticeable crankshaft bearing play on the CVT side. Centrifugal clutch shoes measured 2.5 mm, which is the minimum per the service manual. I cleaned and scuffed the shoes, cleaned the belt (it had some oil from the previous owner), and reassembled everything.

I added more oil to the fuel, ending up with about 1:25 premix plus oil injection.

After that, the engine ran without seizing, even under load (snowy uphill). I rode about 5 km with no issues.

After researching, I found two possible rebuild mistakes:

I used C3 crank bearings instead of C0, which some say is bad for small-displacement engines.

The Namura piston apparently requires more piston-to-cylinder clearance than OEM due to higher thermal expansion.

So my question is:
Should I tear the engine down and replace the bearings with C0 and rework the cylinder/piston clearance, or leave it as is since it now runs fine and just keep running extra oil?

 

I leave engine has is but bypass the oiler and premix

 

Also what is piston the cylinder wall clearance also LT80 will clime hes the pro here

 

.085 as service manual said.

 

It is entirely possible it will continue to seize with that piston as it is true a cast piston such as stick takes less clearance than the Numura.

 

Are you sure its the piston and not the rod and crankshaft wrist pin?I also would remove exhaust and intake from engine and look inside the ports see if you can see any scaring that would help you decide what could be happening

 

Any updates?