I have a problem with a jerking motion while Im trying to weld. the wire feeds out good without the tip on. I replace the tip but I havent replaced the cup but Im using flux core wire also. What could be the problem?

Presumably the wire feed motor is running and driving the wire feed roller?

If this is correct there are a few things to check - when welding it isn't unusual for the wire to become welded to the tip. When this happens a powerful wire feed tends to keep feeding until the wire becomes impossibly tangled and knotted. Low powered wire feed units usually just begin to slip on the wire especially when 0.8 mm or larger wire is being used.

Try removing the welding tip and check if clean new wire will feed smoothly; ensure the correct roller groove is being used for the size of wire.

A welder that is used infrequently often becomes troublesome for a while. Micro-rust forms on the outer layers of wire and some of it is removed as it travels through the torch liner, sometime the liner becomes damaged and should be replaced.

Removing the tip and wire and blowing trough both ends with compressed air should remove any accumulation of dirt and dust. Cut off and discard the outer layers of wire if they have become dull. I spray a little WD-40 on the wire to keep rust at bay and help the wire feed through the liner. Ensure the wire end is neat and straight and with the torch laid straight along the floor allow fresh, clean wire to feed through the liner.

Once a few inches of wire is protruding fit a new tip of the correct size, fit the gas shroud and you are good to go.
If the burn-back rate is adjustable on your machine it should be turned to minimum.

Various things can cause the wire to weld to the tip apart from the burn-back rate - dirty or corroded substrate, holding the welding torch too close to the work, incorrect adjustment of the power/current and wire speed and welding continuously so the tip becomes overheated and if gas is being used, a gas pressure/volume that is too low or perhaps attempting to weld in a draught...

Welding with CO2 gas produces a coarser operation with more spatter and suit some machines more than others. Some machines perform better and more consistently with Argoshield, a mix of CO2 and either 5% or 10% Argon. The added expense might be worth paying.

replace the inner lining in the hose lead
it has worn a groove the the wire is jamming in

first unscrew the tiny tip at very end of torch (can be hot)!! then try to feed wire thru handle,gently shake cable as you do helps to feed wire thru cable , if successful you have allowed the amps to get too high for the feed rate (slow) and melted the tip closed , these tips are replaceable from miller and are not expensive

Hi, sometimes the wire gets stuck when feeding through at the copper welding electrode tip piece.. Try unscrewing the welding tip on the welding hand piece to make sure the wire is not getting stuck there. If the wire come through with the tip removed, then screw the tip back on over the wire. See if the wire will feed through a meter or two untill it hits the welding tip.

is this a lower end machine ? the cheaper machines take the power from the welding transformer to power the feed motor and as a result the feed isn't stable, also is the wire liner and tip in good condition if not replace them, try blowing through the liner with compressed air to clear any dust from the wire

Been welding for a while, open side, check for proper tension on the feed wheel inside side panel, hit the trigger, if feed motor continues to spin tensioner needs tightened or mig wire may be getting hung up inside main cable lead. If motor is off on off on then its probbably the switch on the trigger getting a bad good connection, hope this helps o and check welding tip on gun end, may need replaced if worn or melted, unscrews.

As long as the motor keeps turning fluidly, then the fault lies in something blocking the wire. I'd replace all the consumables in the torch (tip, tip adapter, nozzle) and most importantly the liner as it may have become clogged. Also check the rollers for a build up of metal, running them under a wire brush cleans them up well. An other reason could be simply that your wire on its roll has become tangled and is not rolling of its reel nicely.

This happens to me all the time. Wear and tear has taken its toll and everything has to be maintained for it to work. Most likely the welding tip is worn and should be replaced. Lincoln electric tips work well and I will replace mine several times during the day. Also check your tension and make sure the wire will uncoil freely from the spool. You can remove the tip and make sure the wire will feed without it. If not, your liner may be worn out. Also a welding Jell will help keep the tip from cloging. It will still happen, but hopefully not on your first weld of the day.

try changing the tip and make sure your ground connection is good on the steel and in the machine sometimes the ground wire in the machine comes loose

I would guess that you are on the wrong track. Half of the diodes going out would cause problems 60 times per second, which would be too fast to notice. Unless the diodes all shorted and you're essentially running AC now, which is possible, but unlikely. When my wire feed "pushes" like that,it's usually because the tip is burnt or dirty and not making great contact with the wire. Try replacing the copper tip that the wire feeds through. If that doesn't fix it, look for a bad connection on the wire that runs from the diodes to the torch. Good luck.