# Briggs & Stratton 12HP Carburetor Trouble



## dabluebery

My riding lawn mower started having trouble last season. It would run fine at startup but as soon as it warms up, it floods badly.

A small engine shop rebuilt the carb and replaced the float, they are pretty confident that the carb is fine. I follow the instructions in the manual for adjusting the carb properly but it just floods out and I'm nowhere. It sounds amazing when it starts and is warming up, but gas starts pouring through the carb into the engine as soon as it's warm.

The machine is old enough that replacement parts are getting very expensive, I fear that I'll wind up junking this thing, which would be a shame. Better to put the money into a new machine?

Craftsman 38" riding mower

Briggs & Stratton Model 281707
Type 0412-01
Code 90042511

Any advice? Thanks, great stuff here.


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## 30yearTech

You might want to check the valve clearances, particularly on the intake valve. Insufficient clearance can cause an engine to run rich, although will not cause the carburetor to flood. Gas may puddle and drip out the vent, but it's not actually flooding. 

When you shut the engine down after if warms up, does gas continue to flow from the carburetor?


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## indypower

You have a flat head engine with a side draft carb. Valves are not adjustable. Check and make sure the choke is working correctly. When the shaft wears, it can cock to 1 side and wedge itself closed. I had a 1970 Monkey Wards with the same engine. Choke shaft was worn and I had to manually choke it to start it (worn so much that the auto choke would not work by itself).


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## 30yearTech

indypower said:


> You have a flat head engine with a side draft carb. Valves are not adjustable.


The valves on this engine and all flat head engines *ARE MOST CERTAINLY ADJUSTABLE!*

The difference between flat head engines and OHV engines, is that on a flat head engine as wear occurs to the valve seats and face, the clearances decrease to a point that they may not be fully seating. Particularly on the intake valve. This can cause blow by back into the carburetor, which will in turn cause fuel to be drawn up the nozzle and puddle inside the carburetor. This expelled fuel as well as a fresh portion can be drawn back on the intake stroke, causing an overly rich condition that cannot be eliminated by adjusting the carburetor. On overhead valve engine's the clearance tend to increase as the wear mostly occurs on the push rods and rocker arms.


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## dabluebery

30yearTech said:


> When you shut the engine down after if warms up, does gas continue to flow from the carburetor?


Yeah, it kinda does. The gas continues to flow into the engine and then comes out through the muffler. I've been blowing out the spark plug with an air nozzle just to get it started again because it's flooding. I just can't seem to get it running right with carb adjustments. Warms up, then floods out and dies no matter what we try.


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## 30yearTech

dabluebery said:


> Yeah, it kinda does. The gas continues to flow into the engine and then comes out through the muffler. I've been blowing out the spark plug with an air nozzle just to get it started again because it's flooding. I just can't seem to get it running right with carb adjustments. Warms up, then floods out and dies no matter what we try.


Then most likely you have a bad or pitted seat in the carburetor, the seat may need to be replaced. Another possibility is a bad float. The shop may be confident about their work, but if the carburetor is flooding, then it's not right.


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## August59

Sounds like a needle and seat problem to me. It appears that would be the cheapest place to check first anyway. Good luck


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## Duroc

*I have the same problem*

Hi Dabluebery,

I have the same engine 281707 and I'm having the same problem. The fuel flows like crazy into the engine. Did changing the needle and seat in your carburator work?

Thanks,
Duroc


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