The stored energy keeps the motor turning into the next piston firing. Without the mass keeping everything moving, the compression can be high enough to stop the motor. Kind of like turning your idle speed down low enough that the engine dies. You may need to turn the idle up with a lighter flywheel. Perhaps not on our KLX, but reportedly so on other bikes.

 

^ yea that makes sense now, probly a threshold point we haven't crossed yet on the klx... someone turn their flywheel down to nothin and let us know the resulst lol....

 

My bike's Prev Owner (guy in western PA) had the flywheel lightened... So I've been riding with a lighter flywheel all along.

I want to know what it's like with a heavier flywheel! ! ! haha..

 

I believe there's some misunderstanding between the above quoted posts. durielk is saying that if the flywheel was riveted together with the OD eccentric to the ID of the hub, then even by machining the hub ID and flywheel OD to be perfectly concentric, there's still eccentricity left within the assembled part. That's part of what I was getting at in my earlier post, too.

Other than during the firing stroke, the engine needs to make those one and a half revolutions to get to the next firing stroke; a lot of energy is absorbed through friction, compression, and accelerating the reciprocating parts, etc. If the rotating mass doesn't have enough stored energy, the motor can stall before it gets a chance to fire again. Energy is a product of mass and speed; more mass and/or more speed equate to more energy; less mass and/or less speed equate to less energy.

In a lot of cases a lighter flywheel won't affect idle speed, but it certainly can. It depends on how much energy the engine needs to make it though a cycle in relation to how much energy the flywheel must contribute to the total energy required. There may be an excess of rotating mass (aka energy storage) in an engine (such as a smooth street bike), or it may be marginal (such as a max effort drag race bike).

In a lot of cases (most cases), the idle speed of an engine is set much higher than the threshold where it may or may not stall out. This means there is an excess of energy potential to keep the engine turning between power cycles. From this point, some mass can be removed from the rotating parts and may not have an effect of the apparent idle quality, as long as there's enough mass still spinning in the engine.

Let's look at it another way. Every engine has a minimum idle speed, below which it will start to run rough and eventually stall. Yet that minimum idle speed is significantly higher than the speed we spin the engine at to get it started in the first place. Why is this? It's not because we lose spark, or air, or fuel below the minimum idle speed...after all, those are there when we kick or crank the engine over. It is because below that minimum idle speed, there is no longer enough rotating energy to consistently get the engine through the exhaust, intake, and compression cycles... And that minimum rotating energy is directly related to flywheel mass (to a more or lesser degree depending on application). So, if you take 'X' engine and turn it down to the lowest possible idle at which it will remain running, and do nothing else but remove 'flywheel' weight, it will now require a higher idle speed to stay running.

That's a long way of getting at...exactly what IDRIDR said, only he did so with much less verbage. Anyway, our bikes may, as per cpr1's experience, do nothing but benefit from a lighter flywheel. The only way to know for sure is to try it! But still, a number of 351 owners have reported needing to increase idle speed from stock; a lighter flywheel could exacerbate that issue. Like I said before, just know what you're getting into, and have at it.

 

Hi there,

here is the real deal, got my flywheel lightened and took a ride today.

WOW. it idle good like before, it is like having 13 front gears instead of 14.
they took out 1.4pounds of the flywheel . very trottle happy revs very fast.

 

Did you have cycle pro do the work ? I'm thinking about having it done on my bike once i get my 331 kit back from them and installed.

 

I had cycle pro do the job very nice and professional..

 

Thanks Robert.

 

Anybody knows the thread size on flywheel puller?
Possibly somebody can measure it?

Thanks
Mikk

 

I used the rear wheel shaft.worked like a charm.