Anyone here work on engine driven welders?

[Zebu Fellenz]

Hi Everyone,

I picked up a cheap & ugly Miller Bobcat 250 last night. It's about 15 years old, has just under 1000 hours on the meter, and having lived on an open service truck its entire life in NYS it needs all the sheet metal covers replaced as they're rusted through in many spots.

I pulled the covers off and it looks clean enough, nothing looks damaged or monkeyed with. The machine starts right up, runs smooth, and the generator side seems to work perfectly. The machine does weld but the controls seem dirty and the weld output seems low unless the machine is running full out. The previous owner told me that the machine was used primarily as a generator and that it had not been used for welding work for the past few years.

I'd like to try cleaning the switches and fine control rheostat before I throw parts at it or bring it in for service but I'm not sure what would work best? Some type of contact cleaner?

https://photos.app.goo.gl/rwtBMJbopzsV656v8

 

[sberry]

The machine welds wide open throttle.

 

[Zebu Fellenz]

The machine welds wide open throttle.

I know. By "wide open" I meant 250A, engine was running wide open at every setting.

 

[ihateminimumwage]

Worked on a few. We usually used the red scuff pads and electronics cleaner to clean contacts. Also use a small piece taped to the end of a paint stir stick to polish to slip rings while it's running.

What engine does it have in it?

 

[Zebu Fellenz]

Worked on a few. We usually used the red scuff pads and electronics cleaner to clean contacts. Also use a small piece taped to the end of a paint stir stick to polish to slip rings while it's running.

What engine does it have in it?

Thanks! I believe the engine is a Kohler CH20.

 

[ihateminimumwage]

The Kohlers are good engines. Hydraulic lifters so no valve adjustment to be concerned with. Keep clean oil, filters and plugs in them and they just keep on going. Try to avoid running cheap fuel with ethanol, especially if it sits for long periods of time.

 

[mike93lx]

Contact cleaner would work. I'd pull it all apart, clean it, replace any lose/worn connections and put it back together. Should be a fairly simple setup.

Try to get a service manual for it so you know where everything goes

 

[readhead]

Every time you strike an arc at any setting it will go to WOT.

 

[Zebu Fellenz]

Every time you strike an arc at any setting it will go to WOT.

Yes, I know. I should have phrased it better. The engine is always running WOT, it was only welding correctly when set to 250A (full power).

 

[MoonRise]

Miller has a bunch of different Bobcat250 variants, even variations using the Kohler CH20 gas engine.

Step #1, RTFM.

If you don't already have the manual, go to the Miller website and download the correct iteration for your machine (based on the model AND serial number range).

From your rough description and age guess, I guessed that you have the variant with the rear-engine Kohler CH20 engine.

Like this:

https://www.millerwelds.com/files/owners-manuals/O4419A_MIL.pdf

From that manual, Page #39,

Low weld output.

Check fuse F1, and replace if open (see Section 8-4 or 9-6).
Check control settings.
Check engine speed, and adjust if necessary (see Section 8-6 or 9-5).
Service air cleaner according to engine manual.
Have Factory Authorized Service Agent check brushes, slip rings, and integrated rectifiers SR2, SR3.

Oh, wait, your machine is no good. For a small service fee, I'll do you a favor and take it off your hands.

Clean all the connections that you can see (welding/electrical connections), some electrical contact cleaner to clean the contacts, and at ~1000 hours the welder/generator brushes and slip rings are due for their second go-round of cleaning anyway (due every 500 hours).

 

[MoonRise]

btw, what were you basing your 'weld output seems low' on?

Were you running DC or AC SMAW? What electrode? Wire CV with a suitcase? GTAW attempt?

Voltmeter on your output leads to check actual welding voltage?

etc, etc.

btw, manual suggests Coarse Range dial set to 60-140 amps (1/8") and Fine Control set to 8 for welding with a 1/8" 7018 rod.

 

[Zebu Fellenz]

btw, what were you basing your 'weld output seems low' on?

Were you running DC or AC SMAW? What electrode? Wire CV with a suitcase? GTAW attempt?

Voltmeter on your output leads to check actual welding voltage?

etc, etc.

btw, manual suggests Coarse Range dial set to 60-140 amps (1/8") and Fine Control set to 8 for welding with a 1/8" 7018 rod.

1/8" 7018 rod DC. With the Coarse Range set to 80-200 the rod behaved the same way with fine control set anywhere in the 1-9 range and the machine came alive with the fine control set to 10 (definitely burned how I would expect for 200A but at all other settings behaved like 80-100A.

I didn't have my fluke handy to check voltage.

I did RTFM (S/N LF114741) and came to the conclusion that it likely needs a good cleaning & lubrication of the controls.

-Cheers

 

[byacey]

Don't use contact cleaner on the slip rings. Check to ensure the brushes aren't worn out. It's also possible that the field voltage is low, and / or bad rectifiers, if your machine has them
.
200A on a 1/8" rod should melt it down pretty fast.