Chain changing tension as it rotates

[STOVE]

Noticed something strange this evening while tinkering in the garage. I mounted up a bar and chain on my ms390.

The bar, chain, rim and sprocket have less than a tank on them. When I rotate the chain it gets tighter and looser within the revolution. It ranges from proper tension to go cut, to seeing 4-5 drivers. The sprocket is a rim setup from the Dukes. Chain is Dukes, bar is Oregon.

What is going on here? Am I crazy to think the china sprocket is out of round? I should have gone OEM, but I just want to troubleshoot it first

 

[Shane¹]

Is the sprocket the same pitch as the chain?

 

[jacob j.]

Is your clutch drum bearing fairly new also? How about that end of the crank - is it in good shape?

 

[EFSM]

Noticed something strange this evening while tinkering in the garage. I mounted up a bar and chain on my ms390.

The bar, chain, rim and sprocket have less than a tank on them. When I rotate the chain it gets tighter and looser within the revolution. It ranges from proper tension to go cut, to seeing 4-5 drivers. The sprocket is a rim setup from the Dukes. Chain is Dukes, bar is Oregon.

What is going on here? Am I crazy to think the china sprocket is out of round? I should have gone OEM, but I just want to troubleshoot it first

I’ve seen this and it’s usually inconsistency in the chain, but it could also be an out of round sprocket on either the clutch or bar tip.

 

[legdelimber]

Get the chain rolled around to either a loose or tight spot.
Make a sharpie mark on one spot of the rim/sprocket. Also make a mark on the chain
Keeping track of the marks, roll the chain completely around a few times and take note if the marks shift apart as you do this.
If you have two colors available, mark a tight spot with one color and a loose spot with the other colors.
If the color spots drift away from each other its most likely the rim or possibly clutch related.
Using two colors or different shape marks will make it easier to keep track of the tight and loose spots

 

[Motorka]

Check sprocket.

 

[Reloaderguy]

Tight links.

 

[STOVE]

Is the sprocket the same pitch as the chain?

Yes , both 3/8

 

[Into-Trees]

Try a different chain on the saw. I had this happen and it was the chain.

 

[Stump Shot]

@STOVE
Saw boogers jammed into the sprocket can have this effect.

 

[STOVE]

Is your clutch drum bearing fairly new also? How about that end of the crank - is it in good shape?

Bearing was in perfect shape when I disassembled. Saw was barely used. Just filthy

 

[STOVE]

Try a different chain on the saw. I had this happen and it was the chain.

I’ll toss a well used loop on this evening and see what happens.

From what I’m seeing it could be uneven stretching of the chain which could work itself out

 

[STOVE]

@STOVE
Saw boogers jammed into the sprocket can have this effect.

Won’t hurt to take it all off and blast some brake clean through the rim.

 

[HusqvarnaDave]

I had this problem and it was the chinesium sprocket

 

[Duane(Pa)]

I've seen it with no-name rims. Calipers confirmed out of round between inner spline and outer rim. SMH

 

[fossil]

Lots of good advice here.
You might also check the chain drivers to make sure there are no burrs.

 

[Junk Meister]

Yes , both 3/8

YES Both 3/8" but is one a LP/Picco?
Pull the (I assume it is) rim and chain and see if th chain sits/saddles in the rim sprocket all the way around for a proper fit.
Something easy to overlook but you still have a problem so it is time to shake up the IDEA basket.
Then do the Bar sprocket so you can X that off the check list.