QT50 70cc Malossi Kit Sticking?

Recently installed the treats 70cc Malossi kit on a 83 Yamaha QT50. I followed the instructions at http://qt50.net/yamaha-qt50/installing-a-top-end-kit-on-your-yamaha-qt50/

Notably, I spent time chamfering the sharp edges on the cylinder ports and then honing the cylinder with a 350 grit followed by 600. The piston moves incredibly smoothly when installed and I've shimmed the plug to ensure appropriate clearance.

However, the piston catches when bottomed out in the cylinder just about as it ready to change direction and move back up. It's baby smooth until I hit that point, after which it takes some force by hand to push it.

Inspecting the cylinder I can confirm that the piston rings have fully cleared all but one of the ports at that point. I've tried chamfering that port twice more without luck. I see no scratches on the piston or cylinder wall that would indicate any rubbing.

I'm looking for additional suggestions. A little afraid to just try firing it up before I get this ironed out... Thanks all!

Re: QT50 70cc Malossi Kit Sticking?

Dirty30 Dillon /

It is your piston skirt impacting something in the cases.

As an aside, you really shouldn't ever hone a new cylinder. It comes honed from the factory. If it was used, that's another story.

Re: QT50 70cc Malossi Kit Sticking?

Maybe the rod is hitting the cylinder skirt?

Re: QT50 70cc Malossi Kit Sticking?

> Dirty Mike Wrote:

> -------------------------------------------------------

> Maybe the rod is hitting the cylinder skirt?

That wouldn't happen at just before BDC .

I'd look for anything that might be an obstruction .

And , I sure wouldn't try starting the motor until there's smooth complete revolutions .(edited)

Re: QT50 70cc Malossi Kit Sticking?

Shave a couple mm off the bottom of the piston, I bet its hitting the cases and not the rod.

Re: QT50 70cc Malossi Kit Sticking?

Nailed it (pun intended).

Sure enough the base of the piston was impacting the case. Shaved off about a mm and I'm golden.

Thanks all. I kept staring at the poor machining on the block assuming it to be the culprit.

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