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Mower only runs with air box removed

Posted by inneedofrepair none (My Page) on
Mon, May 30, 11 at 21:04

Craftsman 917.378770 self-propelled mower
Briggs 7.0 hp, OHV, serial 128602 0215 E1 020405 FA
carb: 214661, primer, no choke

I rebuilt the carb with Briggs OEM parts after soaking it for two days in carb cleaner then boiling it. I also sprayed it real good with aerosol carb cleaner. I adjusted valve lash. I replaced the spark plug. I replaced the fuel cap. I replaced the primer bulb. I removed the air filter while diagnosing. I flushed the fuel tank, fuel line and replaced the fuel filter.

With the air box installed, I can prime the engine and it will run for three seconds and die. If I remove the air box, it runs fine. Note: This is with no air filter or filter cover on the air box and without the air box breather line connected to the air box.

If the air box is removed and I place my finger over the prime hole in the carb, it still runs fine.

What gives?


Follow-Up Postings:

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RE: Mower only runs with air box removed

Double check the airbox gasket part # as OHVs tend not to use the standard gasket.


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RE: Mower only runs with air box removed

They did not send me a gasket in the kit so I made one myself. I thought perhaps the gasket might be clogging up one of the small holes in the carb face so I held my finger over each hole individually with the mower running and it ran fine. That leads me to think the gasket is fine.


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RE: Mower only runs with air box removed

I kept thinking about what you said about the gasket and so I removed the gasket completely and the mower ran fine. What is the difference between OHV gaskets and L-Head gaskets for this carb?


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RE: Mower only runs with air box removed

I would have to lay them side to side to know. I'm sure that there are slight differences. I think there are 2 different OHV styles. Use the OEM Briggs and you will be happier.


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RE: Mower only runs with air box removed

I would look for an air leak that is created when the airbox is added. Is the carb secured? Is the carb gasket good?


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RE: Mower only runs with air box removed

I finally figured it out thanks to your mention of the gasket. The gasket that I made was just a tiny bit smaller than the carb inlet and was just enough to cause a disruption to smooth air flow.

Thank you.


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