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12.5 hp b&s won't start and has gas spitting through carb


jnkpojo's Avatar
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09-02-13, 02:27 PM   #1  
12.5 hp b&s won't start and has gas spitting through carb

my lawn mower engine 28r707 1140-e1 was running fine then just died, it won't start and when the engine cranks it spits gas through the back of the carb. I removed the carb and gave it a good cleaning on advice from a few people, but it still does the same thing. I'm figuring it has to do with the valves but i'm not sure. I removed the head and both valve seem to be functioning properly by rotating the engine by hand. any input would be appreciated

 
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09-02-13, 04:57 PM   #2  
If gas is spitting out of the carb, I'd be inclined to think the carb is flooding. Did you clean the needle and seat?


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09-02-13, 05:10 PM   #3  
Yeah, i actually did it twice, the gas is blowing back pretty hard. enough that it will hit me in the leg, which is about 18-24 inches away. When i'm seated on the mower trying to start it

 
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09-02-13, 09:25 PM   #4  
Does it drip gas when you just let it sit there?


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09-03-13, 02:23 AM   #5  
no it doesn't, everything looks fine

 
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09-03-13, 09:25 AM   #6  
Sounds like maybe the intake valve isn't closing properly.


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09-03-13, 10:16 AM   #7  
Is the motor an OHV (overhead valve) engine. If so, take the valve cover off and manually move the flywheel by hand (you will need to remove the spark plug) and see if you can observe any issue with the valves moving. I would then check the valve clearance of both valves to ensure that they have at least the minimum amount of clearance as per their specifications. Too little clearance might prevent one of them from closing properly

 
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09-14-13, 09:45 AM   #8  
I had same problem, same engine.
No compression, intake valve not fully closing. Set piston top dead center, then advance till piston 1/4" down from toc. File, grind, or sand valve stem for
.005 -.007 clearance (I used file and 150 grt sand paper, I set for .006). This fixed blow back thru carb.


(BTW, piston is set 1/4" dwn from TOC because cam has step on intake lobe to keep intake valve slightly open during comp cycle for easy cranking.) If you crank slowly by hand you'll not have any compression (leaks bck thru carb), must crank with starter!

Second prob, carb was flooding after cleaned - thoroughly?.
Found that float was sticking due to fuel inlet needle & sleeve was slight varnished causing needle to stick. Cleaned and polished, it worked smoothly.

Now to work on tranny prob.

 
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