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lawn mower rpm's going up and down

Posted by sparkync North Carolina (My Page) on
Thu, Jun 3, 10 at 20:31

I have a push mower and the motor will start ok and run, but the rpm's keep going up and down, up and down. Anybody know what might be the problem? Thanks


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RE: lawn mower rpm's going up and down

  • Posted by baymee LehighValleyPA (My Page) on
    Fri, Jun 4, 10 at 5:58

If you have a tank with the carb mounted on top, the diaphragm needs replacement. Or the jets in your engine-mounted carb have varnish in them. What type of system do you have?


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RE: lawn mower rpm's going up and down

what is the diaphragm? can i get this at the big box stores? i think i have the same setup.


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RE: lawn mower rpm's going up and down

My lawn mower is a 4.5 HP Murray 22"cut with a Briggs and Stratton engine. Also could you give me some direction on how to replace the diaphragm or get to the jets to clean them. Thanks again for your help..


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RE: lawn mower rpm's going up and down

  • Posted by baymee LehighValleyPA (My Page) on
    Fri, Jun 4, 10 at 14:36

Check back a bit later. I'll have complete instructions and pictures then.


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RE: lawn mower rpm's going up and down

  • Posted by baymee LehighValleyPA (My Page) on
    Fri, Jun 4, 10 at 15:49

3.75 Briggs carb rebuild

This is an easy DIY job to replace the diaphragm of the carburetor. A damaged diaphragm will cause engine surging, hard starting, and erratic high speed operation. You may also notice that the primer bulb isnt working properly.

This is an overview of the carburetor and these types of carb-over-tank are typical of small Briggs engines. Note the two governor springs. The carb/tank assembly is held in place by a " bolt to the left of the carb and a 3/8" bolt on the engine head to the right. Remove both bolts and pull the tank straight out with a rocking motion. If possible, the carb/tank assembly should be blown off with compressed air and all loose dirt removed. Use the proper safety equipment, especially safety glasses, before using any compressed air in any of the steps below.

In the highlighted area you will see the governor linkage still in the throttle plate of the carburetor. You will have to gently separate the carb/tank assembly from the linkage. It is a Z type linkage. It is not necessary to remove the springs or the other end of the linkage, from the governor.

Note that on the rear of the carburetor is found a white plastic ring and O ring. Sometimes these remain on the tube of the engine housing. If so, place them back into the carburetor body, as shown, before reinstalling the carb/tank to the engine.

Note the raised area of the diaphragm. This is caused by age, but more often, by ethanol fuels, which damage the diaphragms. A damaged diaphragm will cause engine surging, hard starting, and erratic high speed operation. You may also notice that the primer bulb isnt working properly.

Shown here are the dip tube, which brings up gas from the tank, in the bottom left corner. Just behind that and partially obscured, is the orifice that compressed air can be used to clear the dip tube of debris. It also leads to the primer bulb, so any air must be low pressure and the primer bulb must be held in the depressed position to prevent it from blowing out. In the center top, youll notice the jet, surrounded by a wire mesh filter. In the bottom right is a spring. Be careful to not lose the spring or forget to install it during reassembly.

Remove the old gasket and diaphragm combo from the carb body. Install a new gasket/diaphragm on the smooth surface of the tank. I always blow out the tank with compressed air to remove dirt and any water. Be aware that there may also be old gas in the tank. I always empty the tank and let it sit in the sun until all the gasoline evaporates before applying compressed air. Align the carb and lower it down on top of the gasket, trying to keep the five screw holes in the gaskets aligned. Insert the screws and partially tighten them and install all five screws. Tighten each screw until it gently bottoms out. Then turn each screw another quarter turn. It is not necessary to use more than a few inch pounds of torque on these screws. Reinstall in reverse order. When installing, be sure to engage the breather tube with the rubber piece on the carburetor.


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RE: lawn mower rpm's going up and down

Thanks a bunch for the wonderful information. I couldn't ask for more detailed pictures. I wish all my searches over the web gave this type of info. I'll try my hand at this as soon as I can... Many thanks and have a great day.


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RE: lawn mower rpm's going up and down

I saw a video in this forum a day or two back where a guy does the procedure one handed while filming with the other. Great stuff! I think most older mowers are in need of a new diaphram. The crappy fuel ruins them all eventually.


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RE: lawn mower rpm's going up and down

I found it. Let's see if this works: http://www.youtube.com/watch?v=C5nzigcNTwo&feature=related


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RE: lawn mower rpm's going up and down

  • Posted by baymee LehighValleyPA (My Page) on
    Sat, Jun 5, 10 at 12:59

Pretty good for one hand! I wonder where he's from. He has a PA anthracite coal region accent mixed with PA German.


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