Vibration and Big Bore YZ

NWDirt · Nov 28, 2006

When you buy a wiseco piston for a larger than stock bore, is the weight of the replacement piston the same as the stock one or.....? Are all big bores prone to vibrate? Should I have weighed out the old and new parts before I put it back together?

2001 YZ 250 Bored to 68.5 on diameter, very noticeable vibration.

Thanks,

Dave

bruno670 · Nov 29, 2006

The weight difference will not make it vibrate. The most likely cause is you have a loose motor mount(s) . It would be best to make sure you torque them to the proper specs. Make sure that you have no gaps between the frame and the motor. (head and upper head stay)

jason33 · Nov 29, 2006

check the main berrings

NWDirt · Nov 29, 2006

Main bearings are good.

It is also much harder to kick over. It was fine before the bore, but I wanted more bottom end so I had it bored and ported. It wasnt very hard to kick over in comparison but I cant say for fact the bike didn't need a top end.

There is either a new input force that is causing the entire bike to vibrate (rotational mass has increased, hence the piston weight question) or something is less rigid than it was before.

I will check the bottom mount but I am still curious if anyone else had this happen after boring.

Dave

KXtrailrider · Nov 29, 2006

My 99 KX250 has been bored to 74mm by Eric Gorr and the head modified for pump gas and it dosen't vibrate or kick over hard at all. I also have a 13oz fww on it, still no extra vibration. Did you have your head redone for the 68.5 bore? Anyway other than that all I can say is check the main bearings, motor mounts, and make sure the flywheel is on right and tight. Good luck Ed

NWDirt · Nov 30, 2006

68.5 bore and port and head work is by eric gore and the head was done as well. I am meticulous and even double checked the part numbers on the parts he sent me. I have read so many good things about his porting and boring that i believe it has to be some weird thing, but I got to tell you it freaks you out when you fire it up and your hands end up hurting from vibration. It also kicks over just like a four stroke. very hard to go over TDC. Its a workout and wont doesn't start easily.

I also have an 11 oz FW weight on it, but it is rock solid. I checked the mounts last night and they are all tight.

Thanks for the replies!

Dave

letsride24-7 · Nov 30, 2006

How was the end gap on the rings?? Not sure if that would make it vibrate. But could maybe make it harder to kick. I know on the big bore kits i installed some of the rings need to be filed down.

jason33 · Nov 30, 2006

bigger bore- more air in the cyl
it should be harder to kick - but not bad-
fly weights slow down the spining mass of the motor- there not going to reduce the vibrations- as a harmonic balencer would
was there work done to the crank??- it might be out of balance-

Vibration and Big Bore YZ

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