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Question DetailsAsked on 7/28/2013

hard miss in mower

mower hard to start,got started ran poor,rebuilt carb starts on first or second pull, but has hard miss,new spark plug installed.

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1 Answer


Hard to diagnose not seeing it, but here goes with some common causes. Of course, be careful with ignition sources, and don't inhale gas or splash in eyes (or on hands, if sensitive to gas exposure):

1) check spark plug wire fits snugly on the plug - if loose, look up inside the wire "hood" that hooks onto the plug - there is a just a sheet metal loop that fits over the plug. VERY gently, crimp it very slightly smaller in diameter with a pair of pliers - you want just a hair smaller, so it fits plug snugly.

2) pull the plug wire free and ground it so mower can't start, then turn mower up (careful not to spill gas out of the tank cap) and check the blade is tightly fastened to the shaft - if it comes loose, can cause uneven running as it acts as a flywheel to keep the mower running smoothly.

3) check spark plug is snugly seated AND that it has a metal (aluminum or copper) gasket onthe plug - not all plugs come with them installed on the plug, so if that is left out you may be getting low compression.

4) check in gas tank to see if there is water or crud in the tank - clean out if there is

5) check air filter is reasonably clean - if not, clean per manufacturer instructions (usually on a sticker on outside OR inside of the air cleaner housing) and reoil if called for (not on all types)

6) check if you have a fuel filter that could be restricting flow - could be in line with the fuel line, or internal screen in the tank, or a little plastic filter on the end of fuel line inside the tank. If crudded up, clean with a clean old toothbrush or small softish bristle brush (NOT a wire brush). If you can't get at it, use a straw or piece of rubber tubing to blow though the fuel line back into the tank. If fuel filter is metal canister (enclosed) so you can't tell, pop fuel hose off outlet (side away from tank) and see if a full flow of gas comes out. If just a trickle, filter probably plugged. To check if problem is fuel line or filter, pull fuel line off inlet of filter - if flows gas, then filter is plugged. If not, fuel line or tank filter is plugged.

7) check fuel line (usually clear plastic) is not crudded up - if it is, remove (plugging fuel tank opening with a plastic cap or tapered piece of soft wood, or draining tank first) and clean out with a long pipe cleaner (preferred) or pull a small "snake" of cotton or wool (NOT synthetic fabric) through on a piece of small wire or string

8) holding the spark plug wire with insulated pliers, wiggle it while running to see if it cuts in and out - if so, wire is loose at one end or the other or has a break in the middle

9) rebuilt carb might not be properly tuned - find the main fuel jet screw - a bare metal or black screw with normally a slotted head, with a spring on its shaft before it goes into the carb, sticking out of the carb, almost always horizontally. If there are two screws, one is idle jet, one main jet - google for a picture of which is which. Here is a picture of typical appearance -

Location may look very different but screw usually looks very similar, though some brands have a knurled (rough) outer surface on the head for hand-turning. On many engines with carb on top of fuel tank, is just above the fuel tank. Should be easily accessible with a full-length screwdriver without removing anything, though may be close to and right below the air filter. With engine running, turn 1/8 turn at a time to obtain a smoother run. KEEP a written record of your turns, so you can go back to the original position if that is not the problem or you turn it so far it totally quits. It is actually a good idea to first turn it all the way in till it stops (just touches, do not turn in real hard), counting the turns as you go, then write that down somewhere for future reference - commonly either 1-1/2 or 3 turns, though varies by manufacturer. Then turn back out that number of turns and start the adjusting process - that way you can always go back to your original setting if you lose count of turns. Of course, once you get it running well, write that number of turns position also. Generally, if it has a "cough" when it misses and is maybe smoking black smoke then it is too rich, so needle needs to be turned inwards. If the miss sounds like it is trying to die or is running out of gas, turn counterclockwise for more fuel. Once it runs smooth, try at full power cutting grass. If starts missing again under full load but idles fine, probably needs to be richened up a bit - turn counterclockwise a bit more.

If these don't help you, then the next possible causes involve taking the engine housing off and checking magneto gap and wiring, condenser, etc - google your engine model for instructions on the web if you want to go that far. Not difficult, but you need some mechnaics tools, feeler gages, etc to do that and get it back together right - beyond the scope of this forum.

Answered 6 years ago by LCD

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