What causes rod bearing failure?

doritomiata

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So I split my engine after observing an atrocious knocking sound and discovered that probably 3/4 of the needle bearing connecting the rod to the crank was destroyed and sitting in the bottom of the crankcase. I'm not concerned with repairing this engine; a new one is already on the way. My question is if any of you who are a little more versed in these engines could tell me what likely caused it to spin(?) that bearing.

Some history on the engine, I've had it for almost 2 years and ride this bike multiple times per week. I cut the skirt of the piston to match the intake port at TDC and raised the exhaust a little bit about a year ago. I've ran the stock carb with the stock jet which I believe is on the rich side, and the bike does 4-stroke pretty significantly above 6500ish rpm. I've also been running it with no air filter because it wouldn't fit on the bike, which I understand is stupid, but knowing what I know I don't think that would have caused the crank bearing to spin. The only other thing done to it was a high compression head and I ran it on 93.

I was full throttle topping out (around 6k-7k rpm) for a good couple minutes before it started knocking, and I'm not opposed to the theory that I might have just been stupid and blew the motor that way. No matter the case, if any of you could point to something that I should avoid on the next engine I would appreciate it, thanks for taking the time to read & respond.
 
I've also been running it with no air filter because it wouldn't fit on the bike, which I understand is stupid, but knowing what I know I don't think that would have caused the crank bearing to spin.
Any dirt getting into the motor due to no air filter can and will get into the bearings and cylinder etc, and destroy things for you.

The only other thing done to it was a high compression head and I ran it on 93.
Don't believe all the hype of the so-called, "off the rack" "high compression" heads available by almost everyone for sale for these either since I am sure your not running at 12,000 RPMs with a China Girl motor...87 octane Ethanol-Free gasoline is usually just fine for these motors and preferred since ethanol will eat out the motor seals as well as other components on these motors and carby's over time.

You do NOT need high octane gasoline for these things unless you are building a full blown racing engine for it.

What gas/oil ratio are you running, there should be NO LESS oil than a 32:1 mix, always mixed in a 1 gallon gas can, NEVER in the tank since it will not mix properly by just adding oil to the tank, as it must be shaken vigorously before pouring into tank each and everytime you add gas/oil mix.

Also, if your running the stock carburetor, what is your elevation where you live and where is the C clip located on the needle valve on those 5 grooves you have to choose from on it to try and eliminate that excessive 4 stroking???

Slide Needle Grooves.JPG
 
What gas/oil ratio are you running
I've always ran 32:1 but I was a tiny bit lighter on this batch, I'd guess somewhere in the 40:1 range.

what is your elevation where you live and where is the C clip located on the needle
I'm at sea level and had the C clip at the one before the richest position because I've always been afraid of running it too lean. The 4 stroking only happens at full throttle and I was under the impression that the C clip doesn't adjust the full throttle mixture, so I didn't move it down.

Another possibility is your "high compression head" was contacting the piston ever so slightly causing the bearing to prematurely fail.
That is true, I've been running that head for a really long time but likely won't be using it on the new engine. There were a couple of small pieces of the piston missing but it doesn't look like it came from contacting the head to me.

Some pics would also help us to help you in configuring how to get an air filter to fit on your bike.
I'll get some pictures of the bike with the new engine on it if I can't come up with a solution for the air filter. I had a small 3d printed one that I made but ended up taking it off because even that barely fit and started to break. I've seen intake manifolds that move the carb off to the side and wondered if that could work but haven't got one yet. It's probably worth the $7 to find out.

I guess some dirt or something could have gotten into the crank bearing, I don't know why I didn't consider that. I think the main solution for the next engine will be to get an air filter on it.

In case anyone is curious about what one of these engines after probably over 1000 miles with no air filter looks like on the inside, here are some pictures:
piston1.jpg
piston2.jpg
cyl1.jpg
cyl2.jpg


It should probably be stated that beyond mixing 32:1 and letting it warm up before revving it too far, I treated this engine pretty bad. I think it spent more of its time at 7k rpm than it did idling.
 
You modded the motor and ran high octane fuel. Probably worked great but the weak link turned out to be the big end bearing. Plus a light oil mix a couple times and no air filter added up.
Looks like you had some fun with it for quite a long time. So all in all pretty good.
 
'm at sea level and had the C clip at the one before the richest position because I've always been afraid of running it too lean. The 4 stroking only happens at full throttle and I was under the impression that the C clip doesn't adjust the full throttle mixture, so I didn't move it down.
At sea level you should be around the center groove, try that first, you may even have to move it one more notch leaner if it still four strokes on ya...refer to pic I previously posted for leaner/richer adjustment.

I've seen intake manifolds that move the carb off to the side and wondered if that could work but haven't got one yet. It's probably worth the $7 to find out.
Yes it would be worth it.

I think the main solution for the next engine will be to get an air filter on it.
Absolutely.

In case anyone is curious about what one of these engines after probably over 1000 miles with no air filter looks like on the inside, here are some pictures:
I've got one engine, a cheap Seeutek, that I put over 6,000 miles on over a 5 year span of time sitting on a shelf right now that still looks almost pristine on the inside so I know it can be done...It still runs if I ever wanted to use it again but I like the power of my Phantom 85 much better...lol.
 
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