Flywheel Modifications?

[PinkFloydEffect]

Are these modification to this flywheel? Came off a 250R 2 stroke engine.

So, the rusty bottom flywheel is on the motor, the one on top has a bunch of dents in it (came out of the extra parts bin):

Is this pickup shaved down on purpose? Timing? The top one is squared off.

Also, the bottom flywheel on the engine has 6 holes drilled in it:

The top spare flywheel has only 2 holes, and in a complete different place on the wheel:

^ Possibly some sort of balancing method or?

[PinkFloydEffect]

Anybody heard of this? Thoughts?? I'm thinking some sort of timing with a late spark (the shaved pickup), maybe a different/custom compression ratio would have benefited from this at one point?

[juliend]

No, just slightly different variations of the same flywheel. They are weighted individually when they are manufactured.

[William1]

The 'bumps' and partially drilled holes is how it gets balanced.

[Greg Y]

The holes and weights are for balance. Every one is unique.

[PinkFloydEffect]

Every one is unique.

Really!? Wow that is quite the job, either their old casting methods are outdated or the precision is so fine I am under estimating it.

Does that explain the smaller pickup on one? Seems it would effect the spark somehow or maybe better aerodynamics spinning in that direction?

[William1]

Flywheels have two things. Mass to smooth out the engine and magnets to energize the stator. They type (strength) of these magnets is one of the factors along with number of pole, wire gauge, number of turns and type of core that control stator output.

If by pickup, you are refering to the little (typically black) box, sometimes, there is a tab on the fly wheel that signals the box to tell the CDI to fire.

[PinkFloydEffect]

Flywheels have two things. Mass to smooth out the engine and magnets to energize the stator. They type (strength) of these magnets is one of the factors along with number of pole, wire gauge, number of turns and type of core that control stator output.

If by pickup, you are refering to the little (typically black) box, sometimes, there is a tab on the fly wheel that signals the box to tell the CDI to fire.

When you say mass you mean a load on the engine? So it is only needed for running in neutral type of thing or idling?

By pickup I was referring to the tab on the flywheel that has 1 side ground down (I think the side facing the direction it is going in) I think I am going to swap them because the rivets holding it together looks chewed up and all pushed in one direction probably from long time use. Also the backside facing the motor is all ground down almost looks like it was done on a lathe.

[William1]

No, mass means mass. When a mass (flywheel) rotates, it absorbs some engine power, when a load is applied, it releases that stored power. This effect smooths the engine at idle, makes it less prone to stall, great for trail riders. For an MX star, it not as good, it slows down the acceleration and deceleration of the engine.

Be very wary of modifying a flywheel, a mistake can trash the engine or worse, shatter and cause death.

Yes, often the tab (but not always) is what signals the engine to fire.

[PinkFloydEffect]

Interests me because I am building out a 1984 KDX 80 trail bike engine for drag racing (it's on a miniature lawn mower) so I was going to toy with the idea off this flywheel but now that I understand I might not touch my KDX flywheel.

And if the tab is causing the spark, and one end of the tab is lower than the other it SHOULD have a late spark wouldn't you think? Somewhere along the incline theres a cutoff line toward the beginning where it is too low to the flywheel for it to be triggered I would think, not by much but we are talking milliseconds here.

If the RED is the cutoff line and the GREEN is the delay then it would be an intentional late spark?

[William1]

Sometimes the tabs are merely weights, somtimes they are used to signal the sensor.

And your logic of when it fires the spark has nothing to do with the tab. Spark timing is a relationship between the sensor location relative to the crank and the point on the flywheels relationship to the crank. You can move either one to alter timing. I suggest you do not.

[PinkFloydEffect]

I got some Threebond and Permatex Ultra Slick engine assembly lube (#81950) I ran into the local motorcycle guru at the auto parts store and he said this will work fine but not to use it on the rings. I'll coat the bearings up nice with it, they are on their way out anyway.

These are the flywheels side by side, the one on the left came off the running engine, the right is my spare. The rivets are a bit chewed up on the left one:

Also the rear of the one off the engine is all ground down and "dug out"

I think I should definitely swap them out, this is the only major chunk in the spare:

They both say F3T195 71, the only difference is one says 5Y01 and the other says 4X01