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B&S Engine starts then shuts down


carmd3's Avatar
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08-30-09, 04:38 PM   #1  
B&S Engine starts then shuts down

I have a murray lawn mower with a Briggs and stratton 6.0HP engine (model #12H882-3173-E1) that im having a problem with. I had already changed the gas and cleaned the carb and got a new filter, but i could not keep it running.
so i got the carb rebuild kit and changed everything out, but it still stalls after a few seconds, so i took the carb apart again and doubled checked that there are no obstructions. everything seems fine but it wont run. so after a lot of experimenting i found that it will run if i replace the air filter with a piece of cardboard, restricting 90% of the air, and it seems to run ok. If i keep pushing the primer i can also keep it running. why suddenly does it need such a strong vacuum to pull the gas from the carb?
Does anyone have a idea whats going on here?

 
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08-30-09, 05:30 PM   #2  
if your eng uses the same carb mine has.the nut type bolt that holds the float bowl on ,has a small fuel hole (0.020") that can get plugged,&over looked,especialy if you do not know it is there.
also,when you take the float bowl off-is there fuel in it?
with the float bowl off & fuel in the tank fuel should run out freely untill you manualy raise the float.
DO THIS OUTSIDE ,POSSIBLE FIRE CONCERN!!

 
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08-30-09, 05:49 PM   #3  
i checked that small hole on the float bolt three times and its fine. the the float in the bowl is working, plenty of gas, i went so far as to take the carb off and while holding it my hand i apllied a little compressed air from a can and watched the gas come out and all over the ground. i blew air in every oraface and there seems to be no restrictions. I checked for a clogged fuel line and tried running it with no gas cap, but no luck.
i can get it to run by severly resticting the air intake, it runs a little rough but stays running, and can throttle it fast and slow.
I.ve fixed lawn mowers in the past but this one has me stumped.

 
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08-30-09, 06:50 PM   #4  
Walk behind mowers take more abuse than any other type, People just love jamming them up under bushes and anything else....

With that said, Remove the flywheel/recoil cover and check the plastic intake pipe for cracks. If the carb is as clean as you say and the intake pipe is not cracked I would suggest a compression test.



Good Luck

 
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08-31-09, 04:00 AM   #5  
Thanks for the help. I did a quick review of the intake pipe early on and it seems fine, but i will completely remove it this time to make sure its ok. i haven't done a compresion test in years, what pressure range should the motor have?

 
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08-31-09, 06:49 AM   #6  
if low compression was at fault,it would not start to begin with. i would think it would have 80 psi

 
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08-31-09, 09:23 AM   #7  
Posted By: newtofta if low compression was at fault,it would not start to begin with.
I beg to differ, There is a difference between shooting fuel into a cylinder with a primer bulb and a cylinder PULLING the fuel in on it's own.....

 
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08-31-09, 11:52 AM   #8  
Posted By: 31YTech With that said, Remove the flywheel/recoil cover and check the plastic intake pipe for cracks. If the carb is as clean as you say and the intake pipe is not cracked I would suggest a compression test.



Good Luck
There is also an O-ring and a gasket you might check on the intake but I agree with 31Y

 
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08-31-09, 04:44 PM   #9  
Posted By: 31YTech I beg to differ, There is a difference between shooting fuel into a cylinder with a primer bulb and a cylinder PULLING the fuel in on it's own.....
ok,i was just giving my view with not having the hands on advantage.
you want to be the 'big dog' on the block,that is fine by me.

but,if you think about it ,if the eng had enough compression to 'start' it should pull enough vac. to pull fuel through the carb.

 
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08-31-09, 06:10 PM   #10  
If the intake valve is not closing fully, but close, or it's staying open too long, it can still start, but would not pull fuel into the chamber as it should. Seen this before. The fuel/air mixture starts to get drawn in as the piston moves down, but then it gets pushed back out as the piston moves up and never gets enough to start.

Having said that, I still think there is a carb problem if the o-ring and intake check out ok.


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

God bless!

 
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09-01-09, 09:35 AM   #11  
Posted By: newtofta i was just giving my view with not having the hands on advantage.
you want to be the 'big dog' on the block,that is fine by me.
Sir,
For the above reason, A better reply would have been to ask how I arrive at my theory instead of the remark you made.

I would have been more than happy to explain as Mr Cheese has below;

Posted By: cheese If the intake valve is not closing fully, but close, or it's staying open too long, it can still start, but would not pull fuel into the chamber as it should. Seen this before. The fuel/air mixture starts to get drawn in as the piston moves down, but then it gets pushed back out as the piston moves up and never gets enough to start.
And also added;

A worn intake valve guide can let a valve seat when sitting still and while under cranking speeds, But as soon as the engine fires and rpm's rise the valve rocks in the guide bad enough to stop seating. This can reduce fuel intake suction and cause the engine to die.

All the above is arrived at by hands on experience over time and not from any book.

Could this be the problem with the original poster's unit ?

Possibly....Possibly not..... The unit is in front of him not us, All we can do is speculate.

Good Day Sir.....

 
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09-01-09, 12:27 PM   #12  
do you have an air/fuel mixing screw on this thing?

could be out of adjustment from having set it before the repairs were made.

plug gap and or bad spark plug as well
my dad`s gator would run at idel but when you gave it throttle
it would die.

put in a new plug and works like brand new.
even though the old plug had pretty good spark.

 
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