GT500/T500 crank problems

[jabcb]

Working on the T500 cafe build.

Got the top end off the GT500 motor with just a little grief & the cylinders look pretty good.
But when we spin the crank it feels like there is a rough spot.

Have a spare T500 motor that doesn’t have a top end.
We spun it for comparision & it doesn’t have the rough spot.

What typically goes wrong with these cranks that causes the rough spot?

[Jimroid]

I have seen varnished nasty old gas make perfectly good cranks very clunky. And I purchased a perfectly good motor for almost nothing that was locked completely solid due to varnished gas on crank bearings. (2 strokes) Worth looking at that.

[Alan H]

Fill the crankcases with diesel and turn the crank steadily - lots. Drain & repeat. Then get some oil into all the bearings and try it. What have you got to lose?
It's worked on two GT550 engines for me.

[jabcb]

Thanks for the tips.

Don’t know the GT500 motor history. The PO got the bike as part of the package when he bought two nice Yamaha RDs.
I suspect the prior PO was doing a build and thought the GT500 motor was good.
The frame & battery box were painted. There is a little extra fresh grease on the lower triple tree bearing. The gas tank is newly painted.

It has standard pistons & only slight cylinder scoring, so I’m expecting to use 1st over pistons.
Was already planning on new crank seals.

I was mostly worried about the GT500 crank bearings & possibly even the crank itself being shot.

[Vintageman]

If the engine case bolts are not torqued in the correct order/amount it can cause the crank to behave as you say as one possibility.

[jabcb]

If the engine case bolts are not torqued in the correct order/amount it can cause the crank to behave as you say as one possibility.

Interesting. Do you know why?

We removed the cylinders & pistons. Got the rough spot when we spun the crank by moving the connecting rods. We haven’t removed the cover so Its still connected to the clutch+tranny.

[jabcb]

Might have run into a bit of good luck.

Pulled the alternator rotor off this morning. (The GT500 has PEI ignition and a permanent magnet rotor that surrounds the stator.)
There was some rusty crud on the inside of the rotor that will clean up easily.
The crank rough spot seems to be gone. So I think it was the rusty crud rubbing against one of the stator coils.

Just to be sure I’ll have the crank shop flush, lube & check the crank bearings.