balancing a 331

I was annoyed by the vibration of my KLX331 (was a 250), so while I had the motor apart i decided to improve the crank balance. I calculated what was required and added the extra weight by drilling the crank and balancer and pressing in tungsten carbide rods (they are twice the density of steel). I put some spot welds on the ends to make sure they stay there. here are photos of the crank and balancer.

The bike is now as smooth as it was as a 250.



Not a job for the beginner, but it can be done.


Wow! Did you use a WAG, or some fancy math? :cheers:

I put Vibranators (made by RICOR) in my handlebars (which are Renthal) with Bar end weights and Cycra Handguards. My 331 feels smoother now than when it was a a side benefit, I think I've eliminated some high-frequency road surface vibrations. It makes the bike feel much more substantial than it's weight would usually provide, when judging ride quality.

What's a WAG? No, I just did the sums, only about a page worth.

Although handle bar weights would held the tingly hands issue, the numb bum issue would have remained.

I am planning on putting a 331 in my 300. Others on this site have said that vibration would not be a problem. I hope they are right.

What's a WAG?

Wild A**ed Guess :ride:

Good for you on the math...I've been helping my 3rd grader with hers, and sometimes making mistakes, so, I better leave the big calculations to the experts! :cheers:

Difference in weight between a 331 piston and a 300 is about 30 grams, which is around 10%. If you cut down the extra material around the gudgeon pin bosses (compare the forged piston with the original cast one to see haw much extra stuff is on there) you can get it even closer. So 300 to 331 is not a big problem. The difference between 250 and 331 is 75 grams, which is nearly 30%. That's a lot of extra shaking.

I notice that the 250 and 300 share the same balance shaft, but the crank part numbers are different. I have not seen a 300 crank, but the 250 has a portion of one of the webs machined away. I was wondering if that's the difference. has anyone seen the 2 cranks side by side and can you confirm what the difference is?

I haven't seen them side-by-side. All I can say is I went from 250 to 331, and I don't detect more vibrations. Both were smooth for a single-cylinder to me. Then again, the other singles I've had were a 2-stroke 175, a 350, and two 650's. The 2-stroke was so long ago I can't recall, but, the KLX is smoother than any of the other singles I've had.

The difference between the 300 and 351 is about 30 grams also. I have some jetting issues to work out, so I can't give a seat of the pants report. I hope the tungsten carbide is compatable with the filler material.

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