I had this motor bored, new piston & rings, and put it in on May 24. And in just a little over 2 months, the motor was backfiring when I tried to start it. But after choking the life out of it, I could get it to start. So I figured I would tear into it to figure out what was going on. I did notice a drop in compression, but didn't think nothing of it because the whole top end is new. So I put a compression tester on it, and without the carb on it, I turned a 30lbs psi. So I torn down into it, and I found out that one side of the rings were worn down more than the other. Notice in the lower right corner. Here is a pic of the mess This is the exhaust side of the motor, but the intake side is ok. But on the piston notice that the intake side has a scuff mark right in the middle, but the exhaust side that the rings are worn so bad on, it's ok. I don't get it. Especially after a top end rebuild two months ago. Has anyone ever seen anything like this before??

 

400fn sport,

It would help to see what the inside of the cylinder looked like. It does look like someone bored it out of round, though. That could explain the uneven wear on the rings.

 

It does kinda look like a offset bore, You might want to flip for a new liner and start over with a stock bore just to be safe, Did you have the machine work done locally or did you send it out??

 

Stupid question did you end gap the rings when the motor was built? Is the rings are to tight that will happen(they need room to expand when the motor gets hot).

 

The inside of the cylinder is ok. Just a little scuff mark where the piston was rubbing. I took the camera back to my friends, so I can't get a pic of it. The work was done locally. And they do a lot of work for the local dealers around here. And yes I test fitted the rings, and checked the gap, and it was good to go. I'm going to see if I can find a micrometer to check the bore, and the piston. A buddy of mine said it could have been an odd shaped piston. He told me to check it to see if there is any uneven spots. So I'll hopefully be back with some #'s later.

 

Well, with my crappy micrometer, I got the best measurements I could get. But the cylinder bore does look to be offset. From the intake to exhaust, it measured 3.28125". From side to side it was 3.27345". These are rough measurements. But it don't seem that much of a difference, but I guess it is enough to cost me more $$. After I get it fixed the only other investment will be a .75 cent for sale sign.

 

Hey Curt!!

I wasn't trying to say that I didn't enjoy riding with you guys........it was a blast. I just want to know about as many riding places as possible.

No wonder you have been fouling plugs..........with that low of compression. What do you plan on doing? Are you going to be able to ride this weekend? Gary was talking something about hide-n-seek, and I was just wondering if you've heard anything on that.

If you need any help working on that bike, just call me....Gary has my number. I brought my tools with me.

 

My guess is the ports wern't cleaned up (with a file) after the cylinder was bored, check for a sharpe edge on the port where the ring wear is.