Here's a few bits of info that might help you.
I actually get quite a few machines coming into the workshop with some basic faults so I thought I would put some info on here to get you going with some stuff you will be able to check and hopefully remedy your self with very basic tools.
If your machine is slowing down or spitting and running a bit uneven then the first and most obvious thing to check is your contact screw. It’s probably going to need a clean where it contacts the end of the front spring. Simple way to do this is get a double sided emery board , nail file type, and pull down the front of the armature bar so there is a gap between the spring and the contact screw, put the emery board in the gap then push the armature bar up so there is a little bit of pressure on the emery board then move the board back and forth so it cleans the contact screw and the end of the spring at the same time. Then remove the board and blow through any dirt and give it a go. Some machines with brass contacts are a lot more likely to have this fault as brass is an alloy and as such can break down under the heat caused by the arc from the contact between it and the spring. you then get a build up of carbon and this can prevent a good contact.
Keep it clean.
You could of course have problems with the springs themselves. the rear or main spring could be starting to wear out and loose its tension, this would alter the way the machine runs.
Another common cause, especially on some cheaper machines is the way that they are powder coated on the frame. if the powder coat is between the rear spring and the shelf it sits on and some of the coating is inside where your clip cord fits in then this will really affect the electrical contact. I have known machines to be very erratic like this, one minute slow then all of a sudden running very fast as power gets through to the spring. Can be nasty and really affect your work, watch out for this one.
Coils heating up ?? There’s a few possibilities on this one. Main one really is that you could have a leak within the coil winding. To be honest, a bit of heat is just not worth worrying about if your machine is behaving and doing what you want.... why worry. but if it affects the performance then it may need to be sorted. Conductivity and resistance tests will be able to tell whats going on there.
I have even seen builders use stainless bolts to hold the coils on. Stainless is non ferrous therefore non magnetic so will hamper coil performance. Even down to small things like the washers under the coils, always use ferrous metal, not brass or stainless. If you have a non ferrous frame, brass or ali etc then make sure the yoke under the coils is well connected to them.
Another cause could be too much rear spring tension so you have to put a lot of voltage into the coils to make them do what you want. Volts equal heat. As i mentioned, a bit of heat is no big deal but a lot is not good.
Pull down the front of the armature bar until it touches the front coil then check that you have a gap between the rear coil and the armature bar. You don’t want a huge gap, just a gap of some sort, even just enough for a fag paper is ok.
Some builders seem to think that the A bar should touch both coils at the same time, well thats fine but if it’s touching the rear one then there is a possibility that it is touching it first, all be it only a tad earlier, and if it does then it can seriously affect performance. It can cause heat build up, it can cause the main spring to reverberate and it will wear out quicker not to mention that it might sound like shit.
It could also be that for the power you need from your machine is such that the coils are not powerful enough. I have had this in the past where for example someone wants more power and pushes the coils to or beyond their limit and they heat up, simple fix, bigger coils, extra couple of wraps does the trick here.
If you plug in your clip cord and push your footswitch and the A bar just gets pulled down to the coils and stays there then I am sorry to say that you have a serious leak. Have you just taken your machine apart ? A very common reason for this is when someone leaves out an insulating washer from either the front of rear contact and the coils are earthing out into the frame.
Remember. The coil wires / tags must be insulated from the frame, normally using plastic or rubber washers where the fixing screws for the contact posts go through the frame.
Another quick item worth checking is when you pull down your A bar until it touches the front coil, is it past parallel with the base of the machine. Ideally, if your machine runs with spring bend in ( as opposed to say my machines or sunskin machines which use a flat real spring with an angle in the rear shelf ) then you don’t want to be pulling past level with the base so it looks like the A bar is actually pointing down slightly. Reason ? Because the spring will wear out a lot quicker as you are bending it both sides of flat and you could also suffer from more vibration in the machine. The easy remedy is to shim up the front coil a bit and re-tune the machine until you are happy.