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Strange Choke behaviour of B&S 11.5HP engine


david888's Avatar
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10-31-13, 10:45 PM   #1  
Strange Choke behaviour of B&S 11.5HP engine

Hi all!

Let me start first by saying I am new to engines and am just starting very slowly to understand how it all works together. Therefore please excuse if I am missing something obvious.

Anyways, I have recently come into possession of a ride-on lawn mower with a Briggs and Stratton 11.5HP engine (I can look up the exact details if needed, but I assume this is more of a general engine related topic).

The mower runs absolutely perfect given its age (10+ years) and I do not really have any troubles with it. The thing that puzzles me though is the part about the choke.
If I understand it correctly then you need to choke the engine when starting from cold to allow more fuel and less air to get to the combustion chamber. Then once it runs, you slowly push the choke back in, providing more air into the mixture.

Now the thing is, with my mower it seems to be the other way around. If I choke it, i.e. close the air intake, it does not start at all. If the choke is completely open it starts immediately and runs very strongly. Also if I try (just for testing) to choke it after it has already started, the engine dies or becomes very weak.

I also verified that the air intake really gets restricted once I pull the choke by looking under the air filter. The filter itself is clean and saturated with oil - therefore looks fine to me.

Technically I do not really have a problem with the setup, as it runs fine, but I assume that this might lead to a problem in the long run as clearly something is not as it should be?

Can someone explain to me if I need to worry about it?

Thanks

 
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cheese's Avatar
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11-01-13, 01:05 AM   #2  
Being an 11.5 hp briggs, it probably has the old flo-jet carburetor which is a bit prone to leaking when the engine isn't being used. Not a lot necessarily, but enough to keep the cylinder primed with what it needs to start without the choke. Do you ever see a drip hanging from the bottom of the carburetor?


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11-01-13, 04:33 AM   #3  
I had a B&S engine on a roto tiller, and we have one on a generator on one of our fire trucks that the sticker is backwards.
If it runs and starts fine and there's no backfiring or fuel running out of the bowl when it's not running then this may be all that's wrong.

 
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11-02-13, 01:25 AM   #4  
@joecaption1: That's what I thought first as well. Therefore I opened the air filter and had a look at the actual choke plate, but it is like it should be. Meaning that if the lever is pushed in (the only state where the mower engine starts) the air intake is fully open.

@cheese: Sorry to ask, but can you point out for me how exactly I can check that? I even had to google what a carburetor actually is, so I really need it for dummies
Do you mean if I can see fuel dripping outside of the carburetor?

Also some additional info, not sure if that helps or not. If the engine is running and then I close the choke it not only gets rough but also a bit of black smoke comes out of the exhaust.

The engine ID is 28D707-0123-01
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11-02-13, 05:46 PM   #5  
Under the spot at the engine where the fuel line connects is a cup looking piece. Do you ever see a drip of gas hanging under that cup? The things that happen when you close the choke are normal when the engine is running.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

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