Weedeater 500 B&S Engine only runs till Prime gas used up, New carb same result

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Old 04-24-15, 12:52 PM
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Weedeater 500 B&S Engine only runs till Prime gas used up, New carb same result

I have 5 year old Weedeater 500 with a Briggs & Stratton engine. When I went to use this mower last week I encountered a problem
After priming, engine starts right up but dies as soon as priming gas used up.
At the end of last season's mowing, I ran the mower till the engine died and then tilted the mower to empty out the remaining gas .
This year after fresh oil change and gas filled, I encountered this issue immediately on attempting to start.
After trying a new sparkplug, checking the air filter, I came to the conclusion that the carb might need rebuilt. I just happened to have a new carb that I installed but surprisingly encountered the exact same problem afterwards... Engine starts immediately but dies after priming gas used up.

On searching another forum I came across a similar report from several years ago of the same issue with a Weedeater brand lawnmower, which that person indicates they were able to fix. I wanted to ask if anyone can confirm/explain if what this person indicates solved his problem: by removing the safety-lever/brake mechanism, i.e. he removed the wire-bale on the handle that you have to hold down when starting/running the mower. Aside from the safety implications, I was not aware that the wire-safety handle brake had any function other than to act as flywheel-brake to stop the running engine. Is there some other mechanism at work with the flywheel brake? I remember from the old lawnmowers where there was a Slow-to-Fast throttle control mounted on one side of the handle and that the control lever had an OFF or STOP setting that stopped the engine when you pushed it all the way to the OFF setting.
Does the flywheel-brake mechanism have any such function that might explain Why the engine won't run - though that totally ignores why the engine will start with priming gas...?

Something else I came across was that the priming gas mechanism operates differently than the normal running-gas mechanism when the engine is running normally. And it struck me that I really don't understand how these B&S carburetors work. I'm assuming it's using engine compression to create vacuum pressure in the carb when you pull the cord, to pull gas up through the long narrow tube that 'sits' inside the gas tank and then to transfer the gas to the larger diameter but shorter cylindrical tube that's covered by a wire-mesh screen. From here it looks like the gas is routed to the intake-tube for the combustion cylinder. That's the limit to how I think the carbs are working but appreciate anyone's better explanation if not complete or accurate.
Thanks,
greynold99
 
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  #2  
Old 04-24-15, 06:39 PM
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Sounds like you need a new carburetor diaphragm.
 
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Old 04-25-15, 06:04 AM
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Those carb's are so tiny I just replace the whole carb. About a $25.00 part.
 
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Old 04-25-15, 11:09 AM
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He already replaced the carb. Replacing the carb does no good if you don't replace the diaphragm. This is a push mower.
 
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Old 04-27-15, 08:40 AM
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reply to Cheese

Thanks to Cheese & Joe Caption for replying.

Yes, I installed a brand new carb with a new diaphragm right out of the box and the mower behaved the same way - runs out of priming gas and dies... but fires right up after each new prime only to die immediately.

On another forum someone mentioned that there might be something tied in with the blade-control mechanism that stops the engine when you release that wire-bail they have you hold-down attached to the handle when starting or running the mower.
I always thought it was just a flywheel-brake but I got to thinking about the old mowers that had a throttle-lever and when you pushed it to the OFF position it would choke the engine to stop or short the sparkplug wire to ground... Is it possible something like that's going on?
greynold99
 
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Old 04-27-15, 10:26 AM
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Yes, the cable that shuts the engine off may not be pulling the shut off switch very far. The switch is under a little plastic cover. Remove it so you can see it work and help the cable pull it a little further manually, then see if it runs longer.
 
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