Sooty spark plug on riding mower

Discussion in 'Shop Talk' started by mary,tx, Apr 16, 2005.

  1. mary,tx

    mary,tx Well-Known Member Supporter

    Dec 4, 2002
    I have a riding mower with a 2cyl B&S engine. I have been trying to fix it for some time--new plugs, new fuel filter, cleaned air filter, rebuilt carb, cleaned magneto, cleaned outside of engine, new fuel hose, etc. :bash:

    Starting the engine and driving it a very short time, it runs fine. After it warms up it runs very rough and starts smoking. This appeared to be because the governor would force the throttle wide open giving it too much gas. If I force the throttle back where it should be with my finger, it appeared to run much better, but there is no way to reach the throttle lever on the carborator while mowing. :haha:

    I noticed oil around the muffler pipe at the muffler, but no apperent external source. I pulled the plug on that side and it was still amost new. However, the other side, the plug was black and very sooty. :no: . I don't recall a problem with the old plugs when I changed them so I hadn't checked them again :rolleyes: , but that was awhile back.

    What needs to be checked for the sooty plug side? Also, the plug was very hot compared to the relatively cool plug on the other side. :confused:

    Dale (DH of Mary, Tx)
  2. Hip_Shot_Hanna

    Hip_Shot_Hanna Well-Known Member

    Apr 2, 2005
    try taking off the muffler and burning it out , set it in a can with some gas and set fire to it it will probably burn for a couple of hours while it carbonizes the gunk in the muffler ,, 2 cycle engines tend to clog up the muffler that causes too much back presure ergo rough running

  3. moopups

    moopups In Remembrance

    May 12, 2002
    In beautiful downtown Sticks, near Belleview, Fl.
    To my knowledge Briggs and Stratton do not make a 2 stroke engine, obeviously it is a two cylinder engine. Run a compression test on both clyinders with the plugs out; run it again after shoooting in some engine oil, maybe 1/4 ounce or so. If the compression builds then your rings are shot; if the compression stays the same look to the exhaust valve guide as being worn out. A temporary fix is to switch to NGK brand of plugs, in the same size, heat range, and length; but know that you are on the down side of the hill with engine wear. The NGK's may get you through this season, but its time to start saving for a rebuild or replacement engine.

    As for the governor adjustment, they do slip out of adjustement from time to time. Also make sure the choke is not loose enought to close while running; missing spring, broken linkage, loose mounting - with an air leak in the intake as a posiability also.

    The exhaust valve is steel, the guide is aluminum - the steel is allways going to win the 'who wears first' question, but there are after market guide replacements available. Knuerlling the inside of the new guide is also an option, if you can find someone old enought to remember how to do it. Its like putting in rifle lands and grooves in the guide with speciality tools, this allows oil to get to the area more freely, equalling a longer engine life.

    Also observe the engine after dark when running to see if there is any electrical leaks in the plug wire system, inadequate spark can also cause sooty plugs.
  4. mohillbilly

    mohillbilly Well-Known Member

    Mar 24, 2005
    south west Mo
    good ideas moopups, Yes, do run a commpression test on both sides, that will tell you a lot! My guess is along the lines of a valve guide, I have seen them come loose and float along wioth the valve on 2 Briggs engines. That would at least explain any white smoke

    Now, you say that it runs rough after it warms up? check the choke linkage and make sure it is open upon full warm up. Does this engine use a fuel pump? Is the float in the carb properly adjusted? Is the needle valve in the carb leaking? Is the plug wire shorted intermittandtly?

    There are lots of issues that can develope in a small engine, just as in a big one!
    My nieghbor down the road has done small engine repoair for 20 years and i have learned a little from him. so check out the things that me and moopups have discussed, I have a fe2 more ideas, but this will get you in the right diredtion.

    If you can, lisrt the engine numbers and I might be able to fill you in on a little more info.