KTM 300SX vibration

[wforacing]

Hey Guys,*

I am posting to see if anyone else on here has done the 300sx top end kit on their 250sx. I installed the kit last week to my 2014 KTM 250SX timbersled ST with 40 hours on it and wound up with a really bad vibration, the bike shakes so bad when you twist the throttle at idle that the foot pegs jump and the bars get a 3/8" oscillation to them. Before anyone says "its a 2 stroke they vibrate" I also have a 2014 300XC*and it is 1/8 of the vibration that this thing has, also It didn't vibrate all that bad as a 250.*

Has anyone insalled this kit before, did it drastically increase vibration?

Is the 250sx crank the exact same crank as the 300xc or is it a diffrerent crank with different balance?

Has anyone gotten a crank for a 250sx balanced to the extra weight of the 300 piston?

hoping someone has been through this before.



Thanks,



Jordan*

 

[wwillf01]

All your engine mounts tightened?... Also make sure that once tightened it is doing something and the bolt did not stretch.

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[byeatts]

All your engine mounts tightened?... Also make sure that once tightened it is doing something and the bolt did not stretch.

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motor mounts do not cause vibration, Piston rock and wear will as well as bearing condition. that series motor does vibrate even new so if its excessive the most likely culprit is piston rock.

 

[wwillf01]

I agree with what you say but if a bolt is stretched and not holding the motor tight it will vibrate...it would be like not putting the mounting bolts in...

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[wwillf01]

I agree with what you say but if a bolt is stretched and not holding the motor tight it will vibrate...it would be like not putting the mounting bolts in...

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To add to that this happened on my cr500 and I was almost crippled in my hands... But piston rock is a valid option also...

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[wforacing]

Motor mounts are tight, and the piston is brand new..... good points though!!

 

[byeatts]

Motor mounts are tight, and the piston is brand new..... good points though!!

Did you Mic cylinder for wear. what happens is people run pistons too long and it wears the front and back of cylinder from piston rock , so even after a new piston is installed the damage has already been done to the cylinder, especially important on a snow bike to keep piston fresh since its at full throttle most of the day.look at your old piston and see if there's excessive wear on skirts ,exhaust and intake sides .

 

[Shanks34]

When I did the 300sx on my bike it definitely had more vibration. Plus having the Timbersled attached creates more vibration than just running on dirt. It smoothed out to where it was tolerable while riding but on the stand idling it felt like it was gonna rattle apart.

 

[ak]

Aside from the vibration was there a noticeable power difference with the 300 kit?

 

[suva]

250sx cranks are different (lighter) than xc/xc-w cranks.

 

[capulin overdrive]

One would think they have the 300 kit figured out, and you'd be good to go?


How many hours? My latest stock 300 acted pretty weird for 3 or 4 hours, then was great after that.

 

[chumbilly1]

I would have your SX crank balanced for peak RPM by Crankworks or Falicon. I used crankworks. 300 piston is a few grams heavier than 250 and all cranks are set up the same from KTM. Although some run smoother than others. The Vibe could be intensified by lean jetting a little. TS will shed a lot of bolts if vibration is very bad.

 

[eric82930]

Im guessing the 300 sx "kit" came with a cast vertex piston? If so you might want to try a wiseco or pro-x forged piston. I normally don't like forged because they rattle so bad but they are lighter and may end up being the weight of the cast 250 piston.

Also the track kits vibrate more than a dirtbike because the swingarm pivot is so loose. I got ride of my 3rd shock and build homemade super tight swingarm bushinigs and it killed a lot of the vibs.

 

[rhino89523]

I don't have a 300, but I've been riding CR500's for 30 years. Big 2 strokes do vibrate especially without a load on it....have you ridden it? My 500 at idle in the parking lot or giving it a few revs without moving will vibrate my eyeballs and give me blurred vision....for real. Once it has a load and I'm underway maybe I'm just used to them but I think it is very smooth.

If you go the balance the crank route talk to them, on a single cylinder there is really no way to fully balance, unless those KTM's have a counterbalancer. It is just not possible to balance a single you can only adjust the Rpm range where it will run smooth it will not smooth it out completely through the whole RPM range and the guys who do the work of balancing will tell you this and can probably explain eloquently why.

As said, motormounts can wreak havok on vibes, the snobike kits transfer more vibes, crank bearings going out can create vibes as well. Good luck

 

[M5-]

My CR500 rattled a motor bolt mount right out last season, gone, then the rest of them came loose. Talk about vibrations it was insane. I didn't notice it when we were ripping in the deep but once I hit the trail out it was obvious something was wrong. Motor mounts get torqued regularly now just in case. I also lost the bottom end a few years back that rattled pretty good as well.


M5

 

[chumbilly1]

Crankworks asks you for an rpm range that you want the smoothest. I went peak RPM as thats where my lil 300 gets ran primarily. I am having wet(snowy actually) dreams about getting my BRC 500 with counterbalance shaft!