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Need help with governor theory of operation


arnoldir

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Hello All, I've searched through the archive, read all the posts about the adjustment of the governor on an early machine (1960 model 700) but have a fundametal question that I have not found the answer to. I'm fammiliar with Kohlers, and on those the governor acts to pull the throttle closed against the spring tension set up when the throttle control is moved to high speed setting. (engine wants to idle but throttle control forces RPM's) On the briggs in this model 700 it appears that the governor tries to do the opposite. The faster the engine runs, the harder the throttle is pulled open and the springs work to pull the throttle closed. (engine wants to run max RPM's but throttle control pulls RPM's down) At any throttle control position past idle, the governor just pulls the throttle shaft wide open when it is running. The scanned pages from the manual that have been linked to previous questions do not show which way the throttle shaft arm is pointing, and I'm thinking the throttle shaft might be 180 degrees out. Would the gurus please bestow the sacred briggs knowledge upon this humble tinkerer. Thanks
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The Briggs works the same way as the Kohler. The governer pulls the throttle to idle. If the throttle shaft is approximately 180 out, it's possible that you have the wrong throttle shaft, or the carb is off of an aluminum block engine. The carb on the 7 and 8 horse aluminum block engine is identical to that of the 7, 7 1/4, and 9 horse cast iron engines with the exception of the throttle shaft. Pat
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powerking_one
Roger, The govenor SYSTEM on any engine is basically a "critically damped" closed loop mechanical feedback SERVO system. The governor( or servo) on any engine is ALWAYS applying force to CLOSE the throttle butterfly when the engine is running. Governor arm force is directly proportional to RPM's. The governor arm tension spring counteracts the rotational/moment force of the the governor/arm. When the two forces balance, a steady state RPM is achieved. Change either of the two opposing forces and the engine is: A)commanded to a new lower target RPM because of a decreased spring tension, or, B)has seen an increase of spring force that overcomes the servo force letting the throttle shaft/butterfly rotate more open - increasing the engine power to maintain that higher target RPM. If the spring is too strong/or weak, the SYSTEM is no longer critically damped, resulting in RPM "hunting". The maximum travel WOT direction of the governor shaft/arm/link position must exactley COINCIDE with the WOT(wide open throttle) position of the throttle/butterfly on the carb. Sorry the explanation may be a little confusing, Tom(PK)
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Hey Tom, No problem I've got a BSME and experiance with servo and stepper systems, nice job with your techno speak description. Hey Pat, Thanks for the info on aluminum vs iron block carb differences. I've got a spare model 14 buried in the shed, so that might bail me out of this pickle. As soon as we ge another day above freezing I'll get back out there and get to the bottom of this story and post an update. Thanks guys
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Roger, don't know if the carb from a model 14 will work. The carb looks the same, but I believe it has a smaller throat than the carb used on the 19 and 23 cast or the 17XXXX or 19XXXX aluminum. The smaller throat uses a smaller butterfly on the throttle shaft. The shaft itself may be the same, but I'm not sure. Pat
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Do I correctly interpret this to mean that if an engine is "hunting" -- the RPM keeps going up and down -- that this is a symptom of weak springs on the governor, and not necessarily a carb setting or worn throttle shaft?
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Thanks again Pat, If I dig deeper into the pile I've got a model 23R6 on an old Locke Reel Mower that I can swipe the carb from. Kent, My symptoms were not hunting but a screaming engine at anything past idle. Hunting up and down while under no load sounds like a fuel flow problem. I just went through that with my snowblower (Techumsa 10hp), I did a quick drop the bowl and flush to get it running, but it now bounces from WFO to almost stalling and back. Likely has a little peice of crud acting like a flapper valve in the passage above the main jet. Someone may have a better method of troubleshooting, but I'd say put a few drops of 90wt on the throttle shaft to rule out a vacuum leak at the bushings, and if the surging responds to application of partial choke I'd say it's fuel flow.
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powerking_one
Kent, Well, the "RPM hunting" I mentioned was as it relates to the spring vs governor arm counter-force AND assuming all other engine performance affecting factors are not implicated, such as: 1. No binding in the linkage (governor shaft, link, carb throttle shaft, etc). No excessive linkage looseness in the governor arm/link/carb bellcrank interfaces. 2. Carb mixture being correct which yields a smooth stable engine performance/RPM vs butterfly position. If the mixture is incorrect and the engine doesn't "run right", the governor system will simply try to compensate for the problem(s) conditions; also resulting in RPM hunting and unstable speed operation. Anything affecting consistant mixture (tank venting, restricted fuel delivery, loose jets, dirty air filter, air leaks around throttle shaft, loose butterfly plate, etc) will cause the governor to attempt corrective yet erractic operation. Governoring, Tom(PK)
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Kent, I'm not an engineer, but it seems to me if you hold the throttle shaft with your hand, the entire governor will be over-ridden. If the engine still "hunts", the problem is not in the governor. If the engine doesn't "hunt", but does when the governor is in control, the problem is probably in the governor.
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Dutch pretty well sums it up. If all is in correct order from the governor spool to the throttle arm, to strong of a spring or spring setting will cause hunting. I have this more so on Tecumseh and Kohler where you have multiple holes on govrnor arm to install spring. If all your settings are correct and engine cannot be contolled the governor wheight. might be flipped out of the spool. Go here and it will give you a general idea what I am talking about http://www.small-engines.com/govtheory.html PS Dutch I will shadow you until your hunger is statisfied.
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Hi, Hunting may be the result of a governor problem, but most often the cause is a lean mixture in the high speed area of the carb. In the Briggs school I went to years ago, we were taught to set the governor by taking hold of the governor arm, engine dead, push the arm in the direction that opens the carb wide open. Note which way the governor shaft turns to do this. Hold the arm in the wide open position and loosen the clamp on the shaft, turn the shaft in the same direction the arm went until all slack is gone. Tighten the clamp and the governor linkage is set. This way you never have to try to remember CW or CCW etc. Works the same on Kohlers. My 2 cents worth, FREE, value accordingly. Al Eden
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  • 2 months later...
Sorry to resurrect this thread, but my engine is doing EXACTLY the same thing. When the engine is running, I have to hold the throttle shaft closed to idle....otherwise the governor will hold the throttle wide open. It appears as though it is operating in the reverse of how it should be....the springs also pull up in the same direction that the governor arm naturally wants to pull. This leads me to think that something is awhry. I recently rebuilt the engine and believe that I assembled it correctly. I have been having some other carb troubles that will be remidied by the end of the week. Once that is fixed, I will be better able to figure out what the heck is going on. Has anybody had any further ephipanys since this thread was last looked at? Spence
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Now this thread is a great example of what we need here on SimpleTractors. It's been rare lately to be challenged to think deeply and critically on basic questions and theories as related to gas powered engines and tractors.
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Here is a quick CAD drawing I did of the problem. The arm on the bottom is the actual govenor arm. The arm on the top is the idler arm. As you can see, the bottom arm is being forced up during operation (by the internal workings of the engine), acting with the springs instead of against them. This basically makes the govenor inoperable and defeats the purpose of its design. I'm going to take the pan off the engine tonight and see what I could have screwed up that would make it work in the direction opposite of its original design. Spence
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Hello Spence, Don't apologise for getting this post going again, you've reminded me to get out there and have another look at the tractor that started this topic. I have yet to determine if it is the wrong carb, or somebody got the govenor put together backwards. My initial impression was that since the motor looked like it had not been out of the frame that the likely problem was a swapped in late model carb. I will try and have a look at it tonight and see which way the governor arm wants to go when the engine fires.
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