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  1. #1

    Join Date
    Sep 2011
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    MTD - Yard Machines

    B&S 500 series motor won't stay running

    I have a Bolens push mower with a Briggs & Stratton 500 series 158cc L head 4 stroke motor.

    Last used it about a month ago and it started right up and ran great. Yesterday I pulled the starter cord and it would fire but then die after about 3 seconds.

    Good spark, clean plug.
    Dirty air filter. Cleaned it and tried it with and without it. No change.
    Drained the gas and added new gas. No change.
    Verified that at least when I push the primer bulb gas moves freely.

    When I start it if I immediately start priming and keep doing it steadily it stays running. It revs up very quickly to its normal idle and rapidly starts to die and I can just hit the bulb again in time to have it rev up again before it dies.

    I'm going to buy a tune-up kit today just in case but between good spark and still not running with the filter off and fuel able to move when I press the primer I have a feeling the kit isn't going to solve the problem. It seems like maybe a clogged jet. I have no problem opening a motor up and have all the tools but would like a good place to start on this one.

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  3. #2
    Edwards saw service's Avatar
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    Jul 2011
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    John Deere

    The problem with your mower is definitely fuel issues. To help you further can you tell me what kind of carb is on there? Is it the float type carb or a vacu jet carb that's mounted right on the tank?

    Edwards Saw Service in Glen Mills PA

  4. #3

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    Re: B&S 500 series motor won't stay running

    It is mounted directly above the tank. The air filter bolts straight down through the carb and the jet that is fed by the primer bulb is clearly visible and the gas is gravity fed down a hole. I don't have it in front of me right now but I don't think it is set up as though it would have a float.

  5. #4
    Edwards saw service's Avatar
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    John Deere

    What you have then is a vacu jet carb. Take the tank and carb off of the mower by taking off the two bolts and the throttle linkage. Then drain the tank and take the carb off of the tank inspect the diaphragm and gasket for cracks or holes and to make sure it's not stiff an that the spring inside the carb is in good shape. If not a diaphragm and gasket kit is easy to obtain for less than 10 bucks. Clean out all holes in the carb and tank. Reassemble if it still won't start the carb is at fault and a new one runs around 20 bucks. Remember if you run the mower without the air filter you must still thread the bolt in that holds the air filter and cover on for the engine to run properly.

    Edwards Saw Service in Glen Mills PA

  6. #5

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    Re: B&S 500 series motor won't stay running

    Thanks I'll try that this evening. Already went across the street and bought new oil, spark plug, and filter. Cheap as they were why not.

    Any idea what the compression should be?

  7. #6
    Edwards saw service's Avatar
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    Due to an automatic compression release mechanism in most Briggs 500 series engines maximum compression isn't obtained until the engine is running at about 3200 rpms but for compression tester results it should be about 50 to 70 psi.

    Edwards Saw Service in Glen Mills PA

  8. #7

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    Re: B&S 500 series motor won't stay running

    Quote Originally Posted by Edwards saw service View Post
    What you have then is a vacu jet carb. Take the tank and carb off of the mower by taking off the two bolts and the throttle linkage.
    For whatever reason I pulled the cord after I got the linkage off to see what effect no linkage had and suddenly it was clear that the bolts serve a second purpose.. I'm sure it's fine.

    Should the springs attached to the linkage be taut? They were practically hanging free but could get tension if I moved the linkage enough.

    The rubber boot that attaches the carb to the metal tube was in good shape.

    Quote Originally Posted by Edwards saw service View Post
    Then drain the tank and take the carb off of the tank inspect the diaphragm and gasket for cracks or holes and to make sure it's not stiff an that the spring inside the carb is in good shape. If not a diaphragm and gasket kit is easy to obtain for less than 10 bucks.
    Tank and carb came off easily. When you say diaphragm are you talking about the thin rubber that was tightly pressed against the gasket, or is that a rubber coating that had come free of the gasket? It was tightly fitted against the gasket material and I peeled it off cleanly, but then again the gasket was pretty stuck to the carb body and it too peeled off cleanly and with about the same resistance. If I have that right then both were in good shape. Image below.

    The spring still works fine.

    Quote Originally Posted by Edwards saw service View Post
    Clean out all holes in the carb and tank. Reassemble if it still won't start the carb is at fault and a new one runs around 20 bucks. Remember if you run the mower without the air filter you must still thread the bolt in that holds the air filter and cover on for the engine to run properly.
    Edwards Saw Service in Glen Mills PA
    I sprayed everything out with carb cleaner and then blew it all out with compressed air but didn't get around to reassembling last night so we will see this evening. Other than some debris that I think was in there as a result of blowing everything off, and some black specs that I guess were carbon from the earlier head bolt incident which I saw blow out into the carb when I pulled the cord, it looked pretty clean but of course I won't know if i did get some gunk out until I test it.
    Attached Images Attached Images

  9. #8

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    Re: B&S 500 series motor won't stay running

    Looks like it was the diaphragm. Good.

    Lowes.com says my local store has the gasket/diaphragm kit in stock for $2.98 so I'll be picking that up today.

  10. #9
    Edwards saw service's Avatar
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    The spring should be taught it works as an automatic choke when the engine starts it released the choke loosening the spring and opening the butterfly in the carb. Make sure the spring is connected and in good shape. The thin rubber piece is the diaphragm it looks beat. Replace it as well as the gasket (the paper piece that lays on top of the diaphragm.)

    Edwards Saw Service in Glen Mills PA

  11. #10

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    Re: B&S 500 series motor won't stay running

    I'm not sure if the spring gets taut when I start it but it is definitely loose when the motor is at rest. Should it be taut at rest? If so it has plenty of length in the straightened parts to adjust it.

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