B&S Quantum MX 4.5hp.

Model 12H802
Type 0822-01
Code 96103059

Running fine until halfway thru the lawn and it dies. Try to restart and it revs to full throttle then dies. Sounds like fuel starvation, but plenty of gas. Replaced fuel filter and rebuilt carburetor. Started it up — exact same problem. Suggestions?

Thanx, PW

I had the same problem with my first lawnmower. I took it to a lawnmower shop. The guy just try to start it and it did the same thing, replaced spark plug and walla it run like new until I finally got rid of it.
TORINO "If I don't have it, you don't need it."

Tried the plug. Does the same: starts - revs - dies.

Did you loosen the gas cap and try it? If it runs then clean the vent hole in the gas cap.
Ben Jammin" in Southern, IL.

I'm not sure if that engine has a "break-away" key on the flywheel or not. Some of those engines have a soft key for the crank to flywheel coupling so that if something is hit to hard or the blade is out of balance they will shear and throw the thing out of time. Seen some fail for no apparent reason, mine used to fail from 2 teenagers that figured out that when the key broke they were off the hook for lawn mowing till Dad fixed the mower.
Cheers John Schaben "I haven't failed, just discovered a lot of ways that don't work"

Check to make sure the blade is tight on the shaft. hh
Doubleh47

PW,

You said you rebuilt the carb.  Did you check the float level when you put it back together?  If it starts up and runs for a few minutes, the spark and key way are fine.  You are right about starving for gas.  The float level lets enough gas into the carb to start it then uses it up quickly.  Check the float and the needle valve.  that may be where you problem is.

WNY has 2 seasons: Winter and construction. Mike H.

Also,

Check the fuel line in the gas tank, if so equipped. If it has a primer bulb it does for sure. I've seen WAY to many of the fuel lines just rotted off! You can thank the ethanol blended gas for that.

Bob........54646 Central Wisconsin's most fun guy! Handy member since 3/14/'94

Landfillwiz - How do I check float level? It starts and runs just a few seconds.

Bob - Fuel line is OK. Inspected it when I replaced fuel filter.

HHaas - Will check blade tightness.

PW,

 

Is that critter equipped with a primer system, choke system or auto-choke?  The manual looks like all three are available for that model.

 

Regardless of that, I think Bob form N was on the right track -- something is preventing the bowl from filling up fast enough to keep up with the engine's consumption.  You checked the fuel line.  Next most likely spot is a restriction at the opening where the little float needle regulates the incoming flow.  The third is a restriction at the fuel tank end.  (You did try running it with the cap loose, right?)

 

I attached an MS Word document with an exploded view of one of the three available carburetors.  The place I'd suspect a restriction is where part #134 enters the carb body.  You also could find some gunk logded in the space behind part 127 or in some other internal passage.  But rule out everything else first because messing with 127 means buying a carb kit and replacing a soft plug.

 

 

Bob Hoyer ...... Bremerton, WA

PW,

I went online to find specs for the carb but you need to buy a manual from what I find.  Unless you find a local small engine mechanic who will give you the spec, maybe someone on this forum can find it for you.  the float level is measured from the top of the float to the bottom of the carb at the farthest point from the needle valve.  You need to remove the bowl from the bottom of the carb to get to the float.  I just had the opposite prblem with my rider mower.  The float level was set too high and the motor acted like the choke was on.

WNY has 2 seasons: Winter and construction. Mike H.

None of my repair manuals are new enough to give you the carb specs.  However, under troubleshooting it does list some things that may be the problem.

Fuel starvation - tank cap vent plugged

                         -partially clogged fuel filter or fuel line, possibly fuel pickup in tank.  On some carbs, a screen or fuel filter is located in the carb fuel inlet.

                        -A sticky or faulty carb inlet needle vavle, float or diaphram may be the cause of trouble.

                        -IF the FUEL system is eliminated as the source of trouble, check the electrical system.  Check for ignition spark immediately when motor dies.  Coil, condensor and points, also spark plug if no spark.  Any or all could be at fault if no spark.

                        -Check engine compression immediately when engine dies.  If little or no compression, could be sticky intake or exhaust valve, or cam followers (tappets).

 

Sorry I can't help more, but this is all my repair manuals have to say on this problem.  Try a new sparkplug.  I have seen them fail under compression but would spark when out of the engine but still grounded to it.

                        

Bart from Saskatchewan, Canada

PW,

I talked with my father on how much distance should be from the top of the float to the top of the bowl seal opposite the needle valve.  He has worked on small motors for over 50 years.  There should be 1/4" to 3/8" before the needle is seated in the valve to stop the flow of gas into the carb.  You adjust the level by bending the tab just below the needle.  I hope this helps you.

WNY has 2 seasons: Winter and construction. Mike H.

Many thanks to all for the research and helpful replies. According to the carb rebuild kit I installed a couple of weeks ago, float level is set at factory and does not adjust. Float is hard plastic so there's nothing to bend.One thing I didn't check when I had the carb apart wss if float maybe had a hole in it. Pretty sure this is a fuel starvation prob b'cus when I spray started fluid in the cylinder, the motor fires on the 2nd pull, runs a second or 2, then dies. I've run out of ability and patience on this, so it goes to the shop tomorrow. Will let u know the result.

Thanx again, PW

PW,

On the bottom of the float where the needle valve attaches to the float you will find a small metal tab.  This tab adjusts the needle valve to the seat.  If the tab is bent up too far the needle shuts off the gas too early thus preventing the gas from entering the carb.  The needle should shut the gas off when the top of the float is approximately 1/4" from the bowl seal.  If you take the carb bowl off, see how far it is from the top of the bowl seal before the gas stops flowing into the carb.  Bend the tab below the needle valve so it shuts off a 1/4" from the top of the float to bowl seal

WNY has 2 seasons: Winter and construction. Mike H.

It sounds like you have a two-stroke engine, rather than a four-stroke.  Two-strokes will run faster when the mixture is lean.  They quit running when the mixture is too lean to fire.  Four-stroke engines usually start missing and slow down, though sometimes the governor will rev up if fuel is available.  Make sure you don't have water in the gas.