Ty250a And Ty250c

[searay175]

I have a ty250a and a ty250c. I am wanting to tune them so the run identical if possible. The problem I have is the "A" has quick throttle response and is very "snappy". The "C" on the other hand seems slow on response and returning to idle or tickover speed.

They both have comparable compression readings, same exhaust and silencers, same UNI pod air filters, same crankcase oil. switching the carbs does not change anything. The flywheels appear the same and I understand that the timing is not adjustable.

It is possible that the "C" has a old school bigbore 300 kit in it as the heatshield was changed to "trials300" but I have yet to pull the head to confirm this. I have yet to check the reeds yet as well. Also they have the same sprockets and tires, it is even noticable in neutral. the"C" has a quick turn throttle as well. They both have #35 pilot jets.

Any Ideas?

Thanks!

[tony27]

From memory thats the way they are, the cylinders are different which makes the a model snappier

[searay175]

I can accept that. I had the C model first and thought it ran fine with tons more power that my 175 ever thought of having then came the A which I really like the best of all 3.

[tony283]

The 250A is much "snappier" and through the progression to the full width barrel the response was made much softer in the 250C/D with different porting. If you have tried changing the carbs then I very much doubt that the C model has been increased in engine size as although they will both run on a 35 pilot the bigger 300 or 320 Majesty engine requires a HUGE main jet going up from 150/160 on the standard to 290 main on the 320 engine.

Tony

[searay175]

Thanks! I never did try the A carb on the C bike as it was not correct for either bike (off a kaw kx80) but did run the C bikes carb on the A and it ran very well, although I was never into it much more than 1/2 throttle.

Any reason why Yamaha made the change to a "softer" response motor? The guys at B&J said they used to offer the 300 kit back in the day and some of the older gents from the local club claimed that it was a popular mod and also some used aheavier flywheel from a 500 what model I am not aware.

[tony283]

The Ty250A was the first bike in the Trials world to be fitted with the reed valve, and bearing in mind mine was the first pre-production bike to hit the UK shoreline in time for the 1974 Scottish. Nobody was over enthusiastic with the "snappyness" to begin with as we were all used to Montesas and Bultacos and the difference on a standard "down the hill, round the tree, up the hill type section" was very noticeable as you found yourself up the bank in a nano second and going to fast to make any turn. The later engines were much softer off the bottom end and perhaps more manageable for most clubmen. Of course now, with our classic "round the tight turn into an 18 step" the snap and zip of the 250A motor are perhaps preferred especially if you come down 1 tooth on the engine sprocket.

Tony

[feetupfun]

As well as the different porting between the A and later (B,C,D) models, the A also has a lighter flywheel.

If you lighten the flywheel from what is standard on a later model (B,C,D) motor, it ends up similar to the A model in response. You can try the A model flywheel on your C motor before making any permanent changes to the C model flywheel to try it out.

I have done quite a bit of experimenting on B and D model model motors and for me the ideal is to reduce the width of the steel band on the flywheel by about 8mm. Some riders like it even quicker response than that and run with the steel band removed completely. We very rarely ride mud here.

[searay175]

Thanks for all the input and advice! Can the flywheel ring be removed without pulling the flywheel? I have removed the 4 bolts and it is not loose. Is a puller needed to remove it?

[alan_nc]

Didn't see any mention of "Slow-Pull" vs "Fast-Pull" throttle.

If you have a Domino throttle and one is white and one is black (inner plastic piece that the cable end hooks into) it would make a big difference.

Just something else to check.

Alan

[feetupfun]

Thanks for all the input and advice! Can the flywheel ring be removed without pulling the flywheel? I have removed the 4 bolts and it is not loose. Is a puller needed to remove it?

The ring it is an interference fit on the flywheel and is also glued. It is not made to be removed and replaced easily. Removing it in one piece - say - in a press may damage the flywheel. Heating it to make it loose will damage the magnets. You could remove it safely by cutting it with a hacksaw, but then getting it to fit properly afterwards would require welding and machining. I suggest you try the lighter A model flywheel first on the C motor. It doesn't take much of a change in the weight of the ring to make a big difference to the motor response.

[searay175]

Thanks again for the advice! I think I'll leave it as is till I pick up a "a"model flywheel. I do have a quick pull on the bike in question.

On another note: they have totally different exhaust tone also. Identical exhaust 'silencer and packing. The "C" model has a low soft tone and the "A" model has a "Tang,tang,tang" note

[feetupfun]

I thought you said you had a TY250A?

[tony27]

Thats how i read it, he's trying to make the C model snappier from what i can tell