Seized 3 new pistons. 1 old one

[BlasterOrDeath]

I got a Blaster. Had an aftermarket carb on it. I went out and rode it. I seized it, held it WOT to long up a hill. Then well. Seized it. I'm not 100% sure but I wonder if the aftermarket carb shut fuel off. It had a wanky float that would just let gas run all over the ground. And a couple times didn't give any fuel when I was trying to start it.

I did a full teardown and replaced the crank. But the bad is I bought cheap Amazon parts.

The cylinder and piston in the kit looked ok. I gapped the rings. Put it together. Warmed it up a few 5 minute heat cycles.

Went out for the 1st ride. I made it half a mile 1/3 throttle 4th gear. The engine lost some power. I pulled the clutch in. And it made a soft clunk sound.

I coasted to a stop. Pulled the plug. It was a dark black. I'm hoping I'm running it rich for breaking it in.

Any rate. I pull the cylinder. Piston skirt was scratched all down the intake side.

Basically did this 2 more times. 3 new Chinese amazon pistons. 2 cylinders with fresh bores. 65.94mm pistons.

The last time I personally put it together. I honed the cylinder enough it should have been a little sloppy. Hoping to avoid this seize again.

I've warmed the cylinder up each time to at least 220 F using a thermal gun to measure temp.

I never got on it hard. But have tried to stay lower RPM, lower throttle.

I bought the OEM Mikuni Carb installed it with the 3rd piston I had.. Also installed a 210 jet. I'm at about 5500 feet above sea level. Checking the plug the whole time. Watching and fouling plugs.

The compression cold right after assembling the motor was 90lbs.

I haven't done a leak down test because I'm getting to much money on this to buy another tool.

1. I wonder if I can use my compression tester? Maybe I should ask when doing the leak down test am I testing the top end? Or bottom end for leaks? Or both?

2. Is there a "lay mans" way to know my piston skirt clearance is adequate? I dont have a bore scope. I did measure with calipers. But that seems like a guess at best.

3. With the cheap Amazon piston / cylinder combo can the skirt be expanding enough to cause all my seize issues as it warms up?

4. If I buy a KOR cylinder, piston kit would I still need to be concerned of skirt clearance and ring gap?

5. The blaster has a smaller muffler on it than I see on stock ones. It seems like someone just found a muffler and hung on it. Could this be causing the issues? Keep in mind I rode it 4 miles or more before the OEM Cylinder/ piston came apart. But it lost chunks off the exhaust side of the piston when it seized.

I'm concerned if its just cheap parts or something else I'm doing that I'm not aware of causing this. Any help is appreciated.

 

[acdiez]

A leak down test is actually where i would be starting. Almost sounds like you are running lean even though you upped your main jet. Leak down test is checking for air leaks that may be causing the lean condition, i.e. your engine is sucking in unregulated air than the carb can handle. Super simple to make your own at home with a cap and schrader valve for the intake side and a freeze plug (automotive or plumbing, whichever is easier for you to come by). DO NOT use a compressor to do the test, you only need 5-7 PSI and a compressor will straight up blow all your seals to the moon.

The Amazon parts being made of lesser materials may be an issue, but i am willing to bet the swelling is due to the extreme heat, which is pointing me towards the lean condition and leak down test.

If you go with KOR, which i would highly recommend if your budget allows, i would expect the same issue thats happening right now to continue until you rule out leaks. I would not go this route until you are certain everything is buttoned up, then spend the dough. I ported my cylinder and got the head upgraded from him and his work is absolutely epic.

I would not think the silencer to be the issue, if you are referring to the collector, could be. Also, make sure there are no obstructions in the collector, mice like to hang out.

Have you checked the rod play on your new crank? Doubt that is an issue on a new part, albeit Amazon, but worth a look. Enough wobble there could cause a seizure depending on how much play there is.

 

[BlasterOrDeath]

Thank you for the reply.

The cylinder is losing a small amount of air. Seems small to me and I'll redo gaskets at the reeds.

Its losing 2 psi in 8 minutes.

Is this a cause or just needs to be fixed?

Yes the silencer is the wrong one. The collector is OEM.

I check for crank tolerance to from rod to crank on the left to right movement and is was in spec.

The rod bearing seemed tight. But I didn't find a way to put a feeler gauge on it. It didn't rock or pull up and down. Seemed pretty tight.

 

[acdiez]

Thats less than what i was expecting to be honest, but yes could definitely be a cause. I would 100% fix the leak at the reeds, then recheck. Spray the case where it splits in the middle and oil seals as well just to be sure air is not escaping from those trouble spots.

Once we feel confident there arent any more air leaks, your compression #s should also go up dramatically. Would also like to point out that timing could also be an issue to check. Make sure your flywheel woodruff key is still there, not sheared, and ensure there is not play around the crankshaft.

If flywheel checks out, the leak at the reeds is remedied, and you ensure there are no other leaks from somewhere, i would be ready to try and fire her up again.

 

[RIDE-RED350r]

Once again, solid advice from Acdiez!

You've had the same problem occur with three different top ends. Your issue is most certainly an air leak at the reedvalve and/or the bottom end. Questionable Amazon parts quality aside for a moment, the rapid re-occurance of burn down with each new piston points to an air leak that hasn't yet been addressed.

A classic tell-tale sign of a burn down due to an air leak or lean condition is having the most piston damage occuring on the exhaust side. You'll see melt-down/erosion of the piston crown down to the first compression ring and heavy smearing of the skirt below the rings mostly on the exhaust side. That is the part of the piston that takes the most heat and is the first area to suffer damage from a lean condition. You'll have subsequent damage on the intake side as well but it usually isn't as severe as the exhaust side.

And as Acdiez stated, I'll reiterate that a leak down test is critical in finding your air leak