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  1. #1

    Join Date
    Apr 2019
    Posts
    15
    Location
    Hayes, VA
    Mower
    Lawn Boy 8403 and John Deere 14SB

    8403 running weird

    My 8403 has been running weird lately. When I start it, it runs good, but slowly loses RPM, then slowly regains its speed. I know the carb is good because I just cleaned it, including the jets and passages. I thought it might be a plugged muffler and clogged exhaust port, but I just replaced the muffler with an NOS cast one, and I cleaned the exhaust port while I had it off. I'm wondering if it needs a new ring and piston. It's a 1984 model, so it''s possible that 35 years of use has taken its toll. Did the F Series engine cylinders have nikasil? On that note, has anyone ever used a ring and piston kit from Ebay, or is there a better one out there?
    Lawn Boy 8403
    John Deere 14SB
    John Deere 116
    Stihl KM 90 R Kombi System
    Stihl HS 81 R Hedge Trimmer
    Stihl MS 260 Pro
    Still MS 261C

  2. #2

    Join Date
    Jul 2013
    Posts
    728
    Location
    Michigan
    Mower
    Kubota, Lawnboys (5), Toro/Suzuki's (2), Honda HR17

    Re: 8403 running weird

    Hello,

    No Nikasil.

    Have you done a compression test? Cleaning the exhaust port should be done with a wooden dowel as anything else may damage the cylinder wall, piston or rings. The loose particles of carbon can cause cylinder damage too.

    AS far as I know the cylinder jug and piston are not available anymore (although eBay may have NOS parts).

    I think one guy on this forum, put a new piston in a F engine and the compression wasn't raised substantially.

    Regards

    Jeff
    Last edited by jp1961; 07-14-2019 at 11:07 AM.

  3. #3
    FuzzyDriver's Avatar
    Join Date
    May 2016
    Posts
    14
    Location
    Michigan
    Mower
    LawnBoy

    Re: 8403 running weird

    Any chance the governor vane is rubbing on the shroud? Does yours have just a gasket between the engine and carburetor, or two gaskets and a heat shield?

  4. #4

    Join Date
    Apr 2019
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    15
    Location
    Hayes, VA
    Mower
    Lawn Boy 8403 and John Deere 14SB

    Re: 8403 running weird

    Sorry for the late reply. I kind of forgot about this post with everything that's been going on this week. I haven't had a chance to do a compression test, but I will tonight before I mow. I was beginning to suspect the governor vane. I thought about trimming a little off the top just in case it was rubbing the shroud. I know it swings freely with the shroud removed. I don't remember seeing a heat shield between the carb and cylinder. I'll have to look tonight. I guess the engine heat could affect it, but I never considered it because the whole carb is plastic.
    Lawn Boy 8403
    John Deere 14SB
    John Deere 116
    Stihl KM 90 R Kombi System
    Stihl HS 81 R Hedge Trimmer
    Stihl MS 260 Pro
    Still MS 261C

  5. #5
    FuzzyDriver's Avatar
    Join Date
    May 2016
    Posts
    14
    Location
    Michigan
    Mower
    LawnBoy

    Re: 8403 running weird

    Some Lawn-Boys have the heat shield and extra gasket and some just have a single gasket. It's not the heat that caused me to mention it, it's the vane. When they added the extra gasket and heat shield, it moved the carburetor about 1/8" farther away from the engine, which causes the vane to get very close to the shroud base (where it comes up through the hole). I was getting weird engine speed issues because of the contact between the shroud base and vane. I filed the edge of the shroud base to ease the rubbing, but it could still contact the shroud base even when I filed it completely away. In fact, it made it worse because once the horizontal edge was filed away the vane could contact the vertical part of the shroud base (rather than just the horizontal edge) when the flywheel blew on it (I could see the vane rub marks on the shroud base). Perhaps this only happens because the wear on the hole where the governor shaft goes through the carburetor allows the vane to lean into the shroud base.

  6. #6

    Join Date
    Apr 2019
    Posts
    15
    Location
    Hayes, VA
    Mower
    Lawn Boy 8403 and John Deere 14SB

    Re: 8403 running weird

    So I did a compression test, and it has 140 psi of compression. I don't know if that is good or not, I haven't checked. I printed a bunch of technical manual pages today, so I'll start messing with it. It ran the same way tonight - very low RPM while idling, smoking a lot while idling, a little faster while turning the blade, and struggling while cutting. That lasted for about 20-25 minutes, then it decided to speed up about 1000 RPM, and ran that way for a few minutes until I got into thicker grass. Then it went right back to running slow. I'm wondering if the carb needs adjusted. I've never been this close to sea level, so I never considered that as a factor. I'm only 33 feet ASL. You'd think it would run leaner, but it smokes quite a bit at idle and has a noticeable exhaust plume while cutting. The plug also looks wet and oily, but that could have been caused by the previously clogged muffler. I'm only running a 40:1 ratio with synthetic oil.
    Lawn Boy 8403
    John Deere 14SB
    John Deere 116
    Stihl KM 90 R Kombi System
    Stihl HS 81 R Hedge Trimmer
    Stihl MS 260 Pro
    Still MS 261C

  7. #7

    Join Date
    Jul 2013
    Posts
    728
    Location
    Michigan
    Mower
    Kubota, Lawnboys (5), Toro/Suzuki's (2), Honda HR17

    Re: 8403 running weird

    Hello,

    140 PSI compression is very good.

    You could try seeing if the carb to engine mating surface hasn't warped (a common issue with the plastic body carb). If it has you should be able to sand it flat.

    Seems like she's running too rich. Try closing the idle mixture screw in until the engine falters, then back it out a 1/4 turn or so.

    Definitely seems like its a carb/governor issue, any excessive play in the governor shaft to the carb?

    Jeff
    Last edited by jp1961; 07-20-2019 at 09:36 AM.

  8. #8

    Join Date
    Apr 2019
    Posts
    15
    Location
    Hayes, VA
    Mower
    Lawn Boy 8403 and John Deere 14SB

    Re: 8403 running weird

    I finally got a chance to work on my 8403, and I got it running much better. I bought a Champion CJ14 plug and gapped it at .035. I then bottomed the carb jet on its seat, backed it out two turns, and fine tuned it from there. I also adjusted the plastic knob under the air vane to get its idle speed up a bit. I might be slightly lean, so I think another 1/8-1/4 turn richer should get it sorted out. There was no visible smoke in the exhaust. I don't have a tach, but my ears tell me I'm really close to optimum RPM. A tach is on the short list of things to buy. It didn't bog at all while cutting, only slowing slightly and settling into a nice, even hum in tall grass. That doesn't mean I didn't have any issues, though. My BBC was dragging at idle, then after engaging it, it wouldn't release after letting go of the handle. It got to the point once where I could smell it and see small wisps of smoke coming up through the actuator slot in the deck. I figured as long as I had it engaged it would be fine, so I mowed with it anyway. Of course, the problem never arose again once I did that. I'll have to look into it to find a cause.
    Lawn Boy 8403
    John Deere 14SB
    John Deere 116
    Stihl KM 90 R Kombi System
    Stihl HS 81 R Hedge Trimmer
    Stihl MS 260 Pro
    Still MS 261C

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