Which Juki do you have? The semi-industrial Juki's (98Q, 2000, 2010Qi, etc.) have a circuit board attached at the top. And they have oil holes at the top, so you don't need to take the top off. They also have 2 upper tensions on the outside of the machine, that can be adjusted.
The domestic Jukis are pretty easy to work with. They actually have service panels that come off the top and bottom.
It could be a piece of something in the upper tension. I know everyone thinks that if the stitch isn't right, it's the tension. Usually it's not. It could be a bad needle, bad thread, burr on the hook or needle plate, piece of crud on the race, timing is a little off, etc.
I hope you're able to get it sewing.