occassional miss

Messages
30
Location
iowa
i have noticed while mowing grass that my briggs 6hp quantum (side draft carb)will develop an occassional miss especially after 5 to 10 minutes of running.up to this point it is perfect.i am very strict on fresh fuel ,clean air filter,frquent oil changes.its just a occassional puff,puff then runs fine. i have a fresh rj19lm plug installed @.030 thou.in line fuel filtr(bgiggs 150 micron) also installed.tank vent is good.could i have a spark plug wire jumping to ground?i did apply diulectric graease inside the boot.could my coil be getting weak?by the way ,after mowing (engine hot)the engine has great compression and restarts on first pull.i fear i have an exhaust valve going bad.engine has low hours.i hope one of you pros can help me.thank you for any input.
 
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Messages
767
Location
Nebraska
Had a quantum that had a very slight miss. The flywheel key was maybe 20% sheared. Since it started and mowed OK, I never checked on it. Found it by accident a year later when cleaning under the recoil starter/cowling. I never take the topper off a mower without removing the flywheel nut and checking the key.
 
Messages
5,199
Location
Pittsburgh,PA U.S.A.
If the flywheel key shears completely the kick back while you pull the rope can just about yank your arm off. If it has the carburetor that sits on top of the fuel tank it will have the diaphragm that acts as a pump. If that diaphragm is going bad the engine will not hold one speed. What happens is that the engine runs lean because that diaphragm will not pump enough gas, when the engine runs lean the the rpms will increase, when the rpms increase the diaphragm pump will supply enough gasoline and the engine will no longer over rev because it is not lean, then the diaphragm pump will not supply enough fuel and the whole lean high rpms, high rpms enough gas, enough gas low rpms cycle keeps on repeating. the diaphragms can be purchased from any good engine shop, and many hardware stores carry them.
 
Messages
5,655
Location
Ohio
[quote=yeehaw1960]Had a quantum that had a very slight miss. The flywheel key was maybe 20% sheared. Since it started and mowed OK Last year I had the same problem with my Briggs/ Craftsman. The key was sheared in two. It wedged, shifted the flywheel and locked the flywheel from spinning. The engine worked but missed because the timing was off.
 
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