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302 Airpak will strike and arc then bog?

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  • 302 Airpak will strike and arc then bog?

    serial MD170718R 230hrs have power , weld on dc will strike an arc then bogs the machine then it comes back no arc . Im leaning toward diagging the LEM first, any help is appreciated.

  • #2
    If the engine is bogging down under load, my first thought would be standard engine maintenance, not an electrical issue.


    • #3
      yeah fresh plugs filters, oil, compressor runs fine under load


      • #4
        Do you have the option for AC output? If so, you could rule out the actual DC Rectification side of things by seeing if it bogs down while stick welding in AC..

        Just because it runs fine with the compressor does not mean that the engine is able to deal with a full load, A fully saturated rotor is a lot more of a torsional load on that engine than a compressor. So basically a weak engine could still be the deal, check compression if you know the plugs are good, if you have a spark gap tester that would be ideal but you could also just ground the side electrode of a plug to the chassis, then see if it has good arc across the gap but not really the best way if you are not inclined and know what you are doing. : )

        What kind of amperage are you welding with, how much of a load?

        I would always check my O.C.V. (open circuit voltage, whats on the studs when output is on and voltage command is maxed) I would want to verify HZ on AC duplex and AC voltage itself, and always good to verify the battery is getting charged. Start there. : )


        • #5
          Before worrying about compression, time for the annual (or so) carb cleaning...


          • #6
            Pulling a plug is a lot easier than pulling, cleaning, and reassembling a Carb.

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            I personally start with easy, and if you have a valve issue or compression issue you will have very little time into the unit. Start easy, but of course that all depends on what tools you have available. My engine training has always been to verify spark, compression, and fuel delivery. in that order.

            Basically at this point trying to determine if there is an electrical fault or engine fault is the name of the game, so either load that unit down with a nice AUX load to rule out welding related components bogging anything down or start checking each system as best you can to see if it is indeed a weak engine.

            just my 2 cents, been doing this for 6 years now, I do not know everything (by far) but I know what I would do if it were in my shop, and that's to start dicing up the possible causes and ruling them out one by one. : )


            • #7
              I don't know anything about your training, but my experience has taught me that, since the elimination of points, carbs are about 95% of all small engine problems.

              Weak compression is usually most noticeable with hard starts, and rarely causes problems at high loads - that is, just about any engine with enough compression left to be normally started will run adequately at full power. At low speeds and low power the leakage from the cylinder is a lot more noticeable than at high speeds, when it doesn't have nearly as much time to leak, or at high powers, where there's so much more combustion gases that the leak makes up a minute percentage of them. At least, that's my experience.

              The engine bogging means it's losing fuel or spark, having governor problems, or having an excessive load put on it. Without some evidence that it is the case, I'd tend to rule out the last option. Even a dead shorted output usually won't cause the engine to seriously bog - after all, how often does your engine die when you stick a rod? A malfunctioning current control would be evidenced by you suddenly blowing a hole in whatever you were trying to weld, not the engine bogging. You'd need both a shorted output diode and the current control ramping up to full power, but I don't think you'd successfully strike a normal arc with a shorted diode. Every unit I've seen with a shorted diode loaded the engine up as soon as you started it, and you couldn't weld.

              A malfunctioning governor can cause bogging. The manual for that serial number looks like a carburated engine with electronic governor option. If the electronic governor actuator were failing, or the linkage was sticky or jammed (things you'd notice doing the recommended carb cleaning), it could be keeping the throttle mostly closed instead of reacting to the sudden load of striking the arc. This engine manual may be helpful if you think you're having governor issues:

              That leaves fuel and spark. A weak ignition can cause an engine that runs fine at low powers than fails at high power. An inline spark tester can help show this, or pulling a wire and using a screwdriver to see how long of an arc you can tease out while cranking. My experience has been that, with modern electronic ignitions, a weak ignition is pretty uncommon - they tend to work or not work.

              And then there's fuel. Sufficient quantity of it being delivered to the carb, clean and dry, and the carb not being dirty and gummed up. In my experience, the last one, a dirty and gummy carb, is the cause of the vast majority of small engine problems.

              Slowly loading up the generator side is a good way to diagnose problems like this. I have a half dozen little 1500W space heaters I use for test loads, and some not-exactly-ul-approved cables to plug them all in at once. To fully load up that generator you'd need a pretty good sized dummy load, with 13kW peak output. Use a meter with hz measurement, or a kill-a-watt meter, to watch engine rpm while applying a load. If the rpm is good then the engine starts missing as you increase load, dig into the carb or check spark with an inline tester. If the rpm smoothly drops but the engine doesn't miss, observe the governor linkage.


              • #8
                Or, of course, it could be some fun electrical issue only modern trailblazers get, with having way too much electronics in them and everything...


                • #9
                  Just an idea from a problem I had a few years ago with similar symptoms. Here's a copy of my old post.

                  Trailblazer surging (Update)

                  06-24-2009, 04:57 PM
                  Didn't know whether to start a new post on this or not. If not please merge this with the other post.

                  Problem SOLVED. Called Robin and talked to a service man there and he mentioned a fuel shutoff solenoid in the carb. Well after turning wrenches for 30 years I went after it.

                  I have a picture but it's with my phone and not the quality I would like, but on the end of the solenoid is a rubber tip that closes the main jet when you turn the switch off. The problem was the rubber tip had slipped off of the plunger just far enough to partially close the main jet. Pushed it back on now all is fine, for the time being. I am going to call back tomorrow and see how much that thing is. Not sure but it may still be in warranty.

                  It looks like the tip just slips on the knurled end of the shaft.
                  Attached Files
                  Tags: None
                  Trailblazer 302
                  Lincoln SP-135-T
                  Hobart Stickmate AC/DC
                  Smith torch
                  Spoolmatic 30A


                  • #10
                    Yeah i have not really messed with it since. I will try to weld in ac tomorrow when I get over to the truck its on. Let you guys know. I'm hoping to find the lem unplugged


                    • #11
                      I am with the carb group. Most likely issue is water in fuel. Once a little water gets in then bowl it doesnt climb the emulsion tube like fuel does so you get a lean condition. It also like to corrode the aluminum a little, plugging up some of the holes. Pretty easy to clean out, then add some isopropyl alcohol to the tank to absorb any remaining there, then run it out of fuel before adding more fuel.


                      • #12
                        Just tore into the carb looks good. I tried ac and DC will strike but lug out. It seems the stepper motor is not throwing up idle vs load. Weird because it goes into auto cal and settles in. Now I'm having issues with it not going into high speed via the stepper or controller.
                        Last edited by tjjd90603; 12-27-2020, 07:03 PM.


                        • #13
                          Info on the governor is in the engine manual I linked above.


                          • #14
                            It's not the style in the kohler manual. It's a 6 wire stepper module like in the service manual.


                            • #15
                              Hrmm. And that's the manual the other manual told me to read! Oh well.

                              Check for free movement of the motor and linkage, especially a sticking throttle plate.