TM250 engine balance problem

[Simo63]

I have a TM250 powered, period 4, road racer and I'm having a few issues with vibration.  In fact when I sit on the bike and rev it, it feels like I'm having a prostate check such is the terrible vibration.  Basically it's shaking itself apart ever time it's run.  Not good for this sort of bike (not good for any really).

The problem was initially diagnosed as an out of balance crank however the engine has been dismantled and the (already rebuilt) crank sent away to an extremely reputable and specialist engine builder (whom shall remain nameless).  A sizeable sum of money was spent doing this and the verdict was that the crank is perfectly balanced.  Reassembly was meticulous with no bashing or hammers involved  .  Crank rotation is superb and everything looks smick as far as my limited knowledge can tell.

I was thinking that the next largest rotating item is the clutch hub .. could this be a problem? 

Any ideas from the brains trust before I strip it down again?

Thanks in advance.

[John Orchard]

My two stabs would be crank or main gearbox shaft bearings or cracked frame?

[shortshifter]

Simo I keep meaning to ask is that you or your stunt double in your avatar picture?

[Simo63]

My two stabs would be crank or main gearbox shaft bearings or cracked frame?

Thanks John.  I'm confident the frame isn't cracked however will double check as that is something I haven't already checked.  The main bearings are C3 clearance so I would hope they are not the problem but it has just occurred to me that maybe one of them is a dud?  Although I did check the motor running with the ignition sidecover off to see if there was any discernable movement I guess a dud bearing is possible? 

Simo I keep meaning to ask is that you or your stunt double in your avatar picture?

Neither unfortunately 

[TM BILL]

Are the engine mounts worn ? bolts sloppy in the cases , Is the chamber well insulated from the frame ,Swinging arm bushes good and pivot secure .

[pancho]

 Was the reciprocating weight balance factor checked? Does the small end bearing have excessive clearance?
 {dont ask me about what the balance factor should be in this case but some-one on here will know.]
 Cheers pancho.

[firko]

Is the crank balanced or is it just 'true'? I've given cranks that I thought were spot on to my crank balancer and he's ended up having to throw a lot of lead into them to get the perfect balance factor. My suggestion is to have the crank/rod and piston balanced by someone who knows how to pre determine the reciprocating balance factor and balance the flywheels accordingly. There are quite a few local Sydney racers including some forum members who thought crank balancing on a two stroke was an unnecessary load of bullshit until they had one done properly by a bloke who's acknowledged as the best in the business.They're now true believers, the difference will amaze you.

[evo550]

Fly wheel not centered, taper on crank bent slightly

[oldfart]

Simon.  Hold the palm of your hand on the Clutch cover to feel the vibes and agian on Mag cover.
I have increased the size of mounting bolts that pass thru cases @ frame tabs to elimate torque flex      Ie 10mm hole using a 8mm bolt

[OverTheHill]

firko's onto it in my opinion, ie is it balanced or just trued up perfect. Motors sometimes don't like being put into a different frame for some reason also. Are you running a different frame?. I'd be hopeless giving info on balancing [too old to understand it properly] but like [think it was oldfart] said if your running 8mm bolts go to say 3/8 high quality bolts [just a whisker smaller than 10mm]. Hope you don't get caught out like i have when drilling--where the dowel [if in  middle of engine mount hole] starts turning, bugger. Was roadracing my RT1 mmm 30 years ago with no mag cover to help get rid of some heat & the rivets let go. Flywheel got to the corner before me & the vibration stopped instantly--not a good day that. Jut meaning if your running a full flywheel ignition then make sure rivets are ok, [imagine TM is small inner rotor though]. Also, i,m yamaha old school but did the TM250 run a [what do you call it?] like an old villiers--bobweight crank like a TM400 or are they a full flywheel crank, If bobweight then maybe try a TS250 crank.

[KTMaico]

Is there someone in Brisbane that can do a full crank balance for a two stroke?

Kevin

[tmman]

yes!!!!!!!!!!!!

[OverTheHill]

so far as 'balance' goes, if it was pretty ok in the original frame [providing engine mount bolts were tight] & it vibrates bad in your [if different] frame, to re-balance the crank [this is beyond my understanding], what balance factor do they use to change it to? or is it just an educated guess by them that know how to do it!! [know what works for a case like this]. Just had a brilliant thought [well--a thought anyway & might even try it]. What say you got a balancer from something like an SC500 or MX360 etc & mounted it in an external  housing & chain drove it off the crank somehow [i dunno] sprocket attached to the small ignition rotor or mag flywheel, [sounds dangerous]. Then experiment by changing the weight &/or the timing of it--bit by bit. Ok--sorry, maybe not.  Think i'll just sit back & squirt some Castrol R in the fire, cold & wet here in nz.

[Lozza]

I have a TM250 powered, period 4, road racer and I'm having a few issues with vibration. 

Sort of answering your own question, but using the TM as a road race engine has exposed where Suzuki shifted the vibrations to. Road race engines spend more time at WOT high rpm. Single cylinder engines without a balance shaft can only shift vibes up or down the rev range never eliminate them. A safe figure is 60%. Firko mate does a great job on cranks mainly due to his adding mass to the light side of the crank not drilling out the heavy side.

[slider84]

balance factor should be higher for roadrace 65 to 70 percent, more towards the 70, and yes use mallory to add weight.