When I lower the needle it slides to the centre

On my Brother, after you select your pattern, select adjust. At the bottom of the adjust screen you will see 2 boxes inside a box. Select that box & choose where you want the start needle position - middle or lower left. Then press the curly arrow to return to the adjust screen

Not enough weight on the arm, adjust the weight on the arm end. Can also be a build up of dust on the needle.

You may have already got it fixed or just don't use needle #1. If you have hit a hoop it will cause this. Take the front cover and top off the machine. At the top of the 4 needle bars there is a screw. Look to see if the screw on needle #1 is lower than the rest. You should be able to compare with the other 3 needle bars. Loosen this screw and slide it back up even with the others. This lets the needle bar go lower and hopefully this will fix your problem.

for the two upper needles you use the "L" shaped tool that came with the serger. All you do is stick it in the hole for the needle you need to change. Upper leftish one is for the one on the left, lower rightish one is for the right side needle. That loosens the neelde and it slides down. Put new needle in, slide it up, put the L tool back in the hole and tighten.

Air to fuel ratio is important to starting and running your bike. A lean condition is when you have too much air for the fuel, this makes HEAT. A rich condition is too much fuel for the air, this FOULS plugs. An easy way to adjust this is to change the clip position on your main jet needle. THis is accessed from the top of the carb. It is mounted in the center of your slide. Try raising your needle position, by LOWERING the C-clip one notch.(lower the clip to raise the needle)(raise the clip to lower the needle) See if it runs better and cooler. if its better, but not perfect, try another notch lower, to raise the needle. Raise needle = richer Lower needle=leaner/hotter. Also check to be sure you have correct gap and heat range for your plug. 0.5-0.6mm gap

The slides are the bits inside the carbs that "slide" up and down when you work the throttle. The needle hangs in the centre of the slide on a clip, the needle will have about 5 grooves machined in it, The needle is small about 3mm in diameter and it is tapered towards the bottom. if you move the clip one groove down, the needle will sit higher and let more fuel through and visa versa. to get to these you have to unscew the tops of the carbs, its a little hard to explain the whole process by writing it down and there will be some small parts to fly off and lose, please make sure the throttle snaps back to idle properly before you test ride it

This is a timing issue and for an overlocker, really beyond the scope of anyone not trained or familiar, it is a 3D ballet between the loopers and needles and needs to be finely balanced to exacting tolerances I'm afraid to say.........the cost of replacement of each looper is around the price of a service......so..... Strongly recommend you to take it to a service centre for adjustments.......you will break more than a needle if you persist.

I have found with any serger I have used that you have to cut the looper (bottom) threads before re threading the needle threads. You then rethread the loopers.
If you don't do this, the needle threads keep breaking. Hope this helps.

First switch off your machine and lower the needle. Try to pull your needle bar to the right and to the left. There should be a little bit of resistance but you should be able to do it. If it is tight your machine needs a service and from a qualified tecnician.

If you are able to pull it, then switch your machine on, and check if the machine initialize and the needle move to the left and stops in the centre. If not your step motor is faulty.

Hope this help