First off, check all your connections on your leads. A loose or poorly made connection on welding lead are usually the culprits. Connections or at your electrode holder- known as the "stinger"- and your work clamp (which is what AWS now calls the 'ground clamp') can heat up if there is not a good connection. This adds resistance and an inconsistent arc can result.
Next, check your stinger (electrode holder). Make sure the jaws of the electrode holder are clean of weld spatter, and gripping the electrodes properly. Cleaning the vee grooves in the electrode holder with a triangular file to get rid of weld spatter and oxidation and the film of oxides from the burnt off flux helps a lot.
Lastly, I'd check the primary current supply. Make sure the wiring to your welder is properly sized and has good connections.
If you do look inside the welder, look for the obvious: more loose connections. I am not sure how the old Idealarc 250's work, but I imagine they consist of a transformer with a tap changer and a rectifier. I know the old Idealarc 250's have a crank on the front to set the heat. This should work a sliding contact.
Check this mechanism and the sliding contact to be sure it is making up fully. It may have worn, become burnt from arcing, or lost spring tension to hold it against the contact surfaces.
Beyond that, I would question the condition of your electrodes. Using old electrode that's been laying around in unsealed containers for any length of time can also cause erratic welds. Moisture gets into the flux on the electrodes and the flux starts to crumble or break down. When you try to weld with rod in this condition, you can wind up effectively trying to weld with bare wire, then the heat dries out the flux above that section of rod and the weld improves, or, the flux continues to crack off. This leads to erratic welds with spatter, stuck electrodes, and when a bead is run, it is sloppy and usually has a lot of porosity in it.
For a rod like 1/8" 6011, the rule of thumb for figuring your heat, welding in the flat position, is 1 amp per 0.001" of rod diameter. So, for a 1/8" rod diameter, 125 amps would be a ballpark figure. As you try a test weld on scrap, you will find that the machine may tend to run hotter or colder than the indicated amperage setting, so you adjust your heat until the rod "runs" to your satisfaction. The other factors in determining the heat (amperage setting) are the thickness of the work and the rod itself. Different manufacturers of the same designation of electrode will produce electrodes that run slightly differently. The first thing in this area of investigation is to check the condition of the electrode. If you bought a used old machine and the person who sold it handed you a bunch of welding electrode in opened containers, this could be the problem. A person pays good money for good electrode, but if stored improperly, it deteriorates and becomes no good. The flux coating on electrodes absorbs moisture from the ambient atmosphere as soon as the containers are unsealed/opened. Within a couple of hours, if you were welding on ASME or AWS Code work using low hydrogen electrode, you'd have to get the electrode into an oven or rod heater for this reason. 6011 is a "homeowner" or "hobby" type of rod. It is a fast-freeze/penetrating electrode designed for use on AC as well as DC current. It is the AC approximation of E 6010. E 6011 will have a gray flux containing cellulose, iron powder, limestone and sodium silicate amongst other things. This gray flux is not too different from the flux on E 7018, and as such, will absorb moisture from the atmosphere and deteriorate over time.
I was in my buddy's shop a few weeks ago, and a young mechanic wanted to become a welder. He had the right touch and natural instincts. My buddy has some old electrode that had been laying in opened containers for years. He also had a really good Lincoln "square wave" power supply for a welder. The young fellow was running beads on scrap. His rod position, travel angle and manipulation of the rod were text book, but his welds looked like they were shot with birdshot for porosity and there was plenty of spatter and some sticking of the electrodes when he'd go to strike his arc. He got a can of fresh E 7018, and his welds were immediately improved to the point that when he'd run his beads, the flux would curl off in a single piece on its own. His welds look like the "knocked over row of dimes". I am working him up to taking his plate tests in the 3 G (vertical position). A simple thing like some good electrode made the difference.
As I said earlier, start with the simple things and check the connections on the leads, then inside the welder. Also check how your "work clamp" is made up to the work. If you do not grind a clean spot, contact with the work clamp becomes a hit-or-miss proposition. At one point, it would seem you cannot even strike an arc, or you can strike an arc but it extinguishes and you stick the electrode. You cuss, fuss, and wonder what's wrong. The answer often is that the work clamp has poor electrical contact with the work. Trying to make good contact thru rust, mill scale, paint, grease, or who-knows-what coating on the work will produce some very erratic welding. If you strike an arc and get it going and the work clamp is making poor contact, there can be a localized arcing at the jaw of the work clamp and the result is the welding arc becomes erratic and extinguishes. You wiggle the work clamp, strike your arc and all seems OK for a bit, and then the problem resumes. The answer is the arcing at the work clamp jaw has made poor conductivity and it gets hot and resistance increases and the arc becomes really hard to maintain.
I've been around welding for over 45 years, and the last 12 have been as a Certified Welding Inspector. I've been around a lot of field stick welding, and once took a welding test on plate, SMAW using the Idealarc 250 machine you have. The Power Authority figured that if I were a CWI and were going to be testing and teaching the mechanics and contractors' crafts, I ought to brush up on my welding skills. The result was they sent me to an off-site welding school for 5 days. It was fun. I was put into a test booth with an Idealarc 250 and told to run some weld and get comfortable with the machine. Great running machine. An instructor/examiner came by and saw how I was welding. He asked me to run one set of plates in the 1 G position, then the instructor/examiner asked me to run a set in 3G, and he did the guided bend test on them. I passed. As I recall, I ran one set with a backing strap using E 7018, and then for s--ts and giggles, I ran a second set open root since we had time to kill. I ran the root pass on the open root weld using E 6010 (the "old" 6010 with the red flux). This is run downhill. It is a pipeliner's rod. It is a real fast freeze "digging" electrode and great for field work with poor fitups and dirt and rust such as you'd encounter if you were repairing farm equipment or similar. I then ran my hot passes and cover pass using E 7018, uphill. I ran hot and steady with that Idealarc 250 and it ran a smooth steady arc. About the funniest part of it was the examiner/instructor asked me to run vertical welds. He looked over my shoulder, liked what he saw and told me to run him up a set of test plates. He left me alone in the test booth. I was burning rod when I felt the back of one of my calves getting kind of warm. When I picked up my shield to get another stick of electrode, I smelled burning fabric. I had set the frayed hem at the bottom of the leg of my jeans on fire. I doused it with a bottle of drinking water and got back to welding. Later on, the people in that welding school all laughed and showed me a large steel water tub. They said at least once a week, someone would run madly down the aisles and plunge his feet, boots and all, into the tub. The reason was wearing low shoes and getting hot slag down them. I always wear a rawhide welding coat, welding gloves with gauntlet cuffs, a welder's flameproof "beanie" under my shield, and wear high-top boots for that reason.
I've seen Idealarc power supplies (as "welding machines" are supposed to now be called) that looked like they'd been through a war that ran good weld. Thos emachines are workhorses and can take a lot of abuse and neglect and keep on running good weld. Start with the obvious and simplest before you blame the internals of the machine. As I said, I'd also look at the power supply to the welder. Make sure line size is right for the amperage and that all connections are properly made. If you use a plug and receptacle, check the contact surfaces to be sure they are making up properly and not burnt or oxidized.
