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OT - 8 hp Tecumseh motor vibration - very long post

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  • OT - 8 hp Tecumseh motor vibration - very long post

    So I am upgrading my log splitter from 5 hp, single stage pump to 8 hp 2 stage pump. Metal working has included the mill, lathe, arc welder and bandsaw. The motor that I intend to use is an 8 hp Tecumseh that was originally on a 2 stage snow blower. I don't know the history as to why the machine was scrapped and the motor salvaged - person I traded with to get the motor worked at a large local power equipment company. Once I had the 8 hp motor attached to the log splitter (bare motor, no pump), I test ran the motor. The motor has significant vibration when rev'ed up. I rev'ed the engine up to about 3K rpm (guess) by manipulating the governor link while leaning on one of the splitter's tires. The vibration from the motor to frame to tire was significant and a similar vibration on the hydraulic valve would be uncomfortable/tiring and I would be concerned that this level of vibration would fatigue the pump mounting brkt. There is one known issue being that the carb is very lean needs to be cleaned - while the engine will run without choke (serious surging), the engine does not run at a constant rpm unless it is set to 3/4 choke.

    I put an indicator on the crank shaft (output end, inboard of the key slot) and see 0.004 inch total displacement. I can grab the crank and push/pull it in the axis of the indicator and see about 0.004 inch also. The crank displacement in/out of the block is about 0.010 to 0.015 (eyeball measurement). I took the flywheel off - hoping to find a damaged or missing magnet - all three magnets for the alternator were present and looked good. There were no bent or missing fins on the flywheel. The flywheel key looked pristine - does not appear that the motor suffered a sudden stop from the auger ingesting something. I rotated the crank and visually observed the flywheel and there was no (noticeable) run out. This severity in vibration leads me to think either something is out of balance or something is bent in the crank/rod/piston/flywheel.

    Can a very lean carb cause a severe vibration on a single cylinder engine? If it were a twin I would think that only one cyl is generating power but on a single that is not the case. Your thoughts please.

    This morning I was at my favorite metal recycler (favorite because they allow "shopping"). I found the same model of engine, same displacement but w/o an alternator in a bin of lawn equipment, engine still attached to a partial snow blower. They will let me pick parts off of it, selling at scrap + mark up, the aluminum I bought today was $1/ lb. This afternoon I found out that some dufus was shaking the carb after removing it last Saturday to sling off the gas still present in the various passageways (float "bowl" removed) and he managed to sling off the needle valve! Now that dufus is intending to revisit his favorite metal recycler first thing tomorrow to get the carb off of the engine he saw today to harvest the needle valve and if lucky the carb may have an adjustable idle screw. I could get all the rotating components as spares but I am having a hard time imagining that they could be bent (w/o damaging / shearing the flywheel key - unless the key was replaced as a repair after a major crash and then the motor was replaced/scrapped because of the vibration?). With an 8 hp replacement motor at Harbor Freight selling for about $200, I have no desire to get too deep into this motor.

    Metro Detroit

  • #2
    Hard to say.... my experience is that about every 1 cylinder engine shakes at least a bit, and some like a beast.... (exceppt for chainsaw engnes, they are always well balanced, or maybe just very light) Was testing a rebuilt B&S 6FB a while back, bolted to a piece of plywood, and it would go for a dance on the concrete if I did not stand on the plywood. And I mean a jumping, spinning around dance. No good reason for that, did not have anything attached as a load and no missing fins etc. They are just hard to balance.

    That said, I have an 8 HP Kohler that is considerably better behaved, and is destined to power a generator head. It is also a slower speed engine.

    Running lean affects speed, but not vibration unless it starts missing fire, which will make it jerk around.

    Not clear exactly what sort of movement you were checking with the indicator....... 4 thou if radial is a little loose, but not totally stupid, probably over the spec.
    4357 2773 5647 3671 3645 0087 1276

    CNC machines only go through the motions

    "There's no pleasing these serpents"......Lewis Carroll

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    • #3
      Two stage snow blower, was the impeller mounted to the engine shaft? If so, the engine may have a lightweight flywheel because the impeller was part of the running mass. There are maybe 10-12 different flywheels that fit the same engine, but designed for different applications. One designed for a pure belt drive application or a pump would be your best bet.

      The picture in this Ebay listing will give you an idea of the various types.

      https://www.ebay.com/itm/283432089577
      Last edited by wierdscience; 12-30-2020, 12:23 AM.
      I just need one more tool,just one!

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      • #4
        Originally posted by wierdscience View Post
        Two stage snow blower, was the impeller mounted to the engine shaft? If so, the engine may have a lightweight flywheel because the impeller was part of the running mass. There are maybe 10-12 different flywheels that fit the same engine, but designed for different applications. One designed for a pure belt drive application or a pump would be your best bet.

        The picture in this Ebay listing will give you an idea of the various types.

        https://www.ebay.com/itm/283432089577
        wierdscience - that was an interesting link. I only expected two flywheels (with and without magnets for the alternator). But thinking about it, makes sence for things such as motors attached to generators - the generator armature is in effect a flywheel.

        Specific to my motor, dual sheave belt driven - same on the potential parts motor I found at the metal recycler.
        Metro Detroit

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        • #5
          Originally posted by aribert View Post

          wierdscience - that was an interesting link. I only expected two flywheels (with and without magnets for the alternator). But thinking about it, makes sence for things such as motors attached to generators - the generator armature is in effect a flywheel.

          Specific to my motor, dual sheave belt driven - same on the potential parts motor I found at the metal recycler.
          Probably not it then. My only other suggestion would be a governor issue, either worn linkage or out of adjustment.
          I just need one more tool,just one!

          Comment


          • #6
            Originally posted by wierdscience View Post

            ... My only other suggestion would be a governor issue, either worn linkage or out of adjustment.
            I rev'ed the engine up by manipulating the governor - could the vibration come from doing that? It does not make sense to me.
            Metro Detroit

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            • #7
              Revving the motor without a tachometer bypassing the governor is probably making it go way too fast. Invest in a touchless tach and don't run it past the recommended speed usually around 3600 RPM. Yes, clean up the jet but don't expect a vibration free engine. .004 is not bad for runout but if that is an issue I have tweaked many a lawn mower cranks with a sledge hammer.

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              • #8
                Like Mike279 mentions above use a tach as it's all too easy to over rev these engines, causing irreparable damage, they spin faster than you think.
                Good news about the carb is that replacements are dirt cheap, under 15 dollars on Amazon!

                I have one of these Snowking engines and have worked on numerous others like it and although they are good solid engines they do vibrate a LOT!
                My own snow blower has had a lot of use since acquiring it in the mid 80's. It's mounted on a very heavily built FMC Bolens snowblower. It has cracked parts of the mounting structure due to vibration. That being said I have never had to get into the internals of this engine once. It starts good, has good power, and when the carb is kept in tune and the fuel system is kept clean and fresh it has been as reliable as a brick.
                Home, down in the valley behind the Red Angus
                Bad Decisions Make Good Stories​

                Location: British Columbia

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                • #9
                  IIRC the Kohler 8 HP I mentioned is an 1800 rpm motor. If you revved it to 3000, you were probably way to fast.

                  4357 2773 5647 3671 3645 0087 1276

                  CNC machines only go through the motions

                  "There's no pleasing these serpents"......Lewis Carroll

                  Comment


                  • #10
                    It has been some years since I did much small engine work, but I know that some single cylinder engines have an internal balancing system that works with the crank. It is inside the crankcase. Could it be something about this?
                    Sarge41

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                    • #11
                      No internal balancer oh the Tecumseh 8HP Snowking engines, strictly old tech.
                      These are 3600 rpm engines although they operate and live longer @2800-3400 rpm.

                      1800 rpm generator engines are designed to function with 4-pole generators in order to maintain frequency @ 60HZ. vs 3600 rpm engines which are designed to function with 2-pole generators in order to maintain 60 HZ.

                      A nice reliable idle on small engines usually hovers around 1400-1700 rpm, although you would swear the rpm is about half of that when listening to one. Very deceiving.
                      Home, down in the valley behind the Red Angus
                      Bad Decisions Make Good Stories​

                      Location: British Columbia

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                      • #12
                        Sounds like the ignition timing isn't quite right.

                        https://www.youtube.com/watch?v=axCzcFI4kek
                        Location: Saskatoon, Saskatchewan, Canada

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                        • #13
                          That's what I would guess. Make sure the Flywheel key isn't sheared, even partially.

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                          • #14
                            Originally posted by Willy View Post
                            No internal balancer oh the Tecumseh 8HP Snowking engines, strictly old tech.
                            These are 3600 rpm engines although they operate and live longer @2800-3400 rpm.

                            1800 rpm generator engines are designed to function with 4-pole generators in order to maintain frequency @ 60HZ. vs 3600 rpm engines which are designed to function with 2-pole generators in order to maintain 60 HZ.

                            A nice reliable idle on small engines usually hovers around 1400-1700 rpm, although you would swear the rpm is about half of that when listening to one. Very deceiving.

                            They used to make a gen set that utilized a 2-71 Detroit running at 900 rpm that powered the generator.
                            Used for powering train refrigeration cars when they came in to station or something.
                            I seen a guy on youtube has one. Just running at basically a fast idle, all day long......
                            That rig ought to last 100 years running at only 900 rpm.
                            Would make a perfect house/shop backup genny.

                            -Doozer
                            DZER

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                            • #15
                              Idle or slow speed surging along with need for some choke is symptom of a clogged idle circuit in the carb and or a failed intake or carb gasket. Thoroughly clean or replace the carb, be sure incoming fuel and lines all are clean. Be sure there are no intake gasket leaks.

                              S E Michigan

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