I have a Craftsman pressure washer powered by a B&S model: 09P702-0124-F1 engine. Bought used. Engine surges when not under load, smooths out when you pull the PW's handle to spray. Shutting the engine off will usually result in a backfire thru the exhaust. This appears to me to be a lean running condition, which I have confirmed by inspecting the spark plug. The insulator is almost white [lean mixture] instead of light brown. Reviewing the page at:
 

 
and comparing the call out numbers [#] from the carb parts diagram, 1 of the following: the  [724] 697478 Retainer Seal,  [617] 270344S O-Ring Seal or the [623] 799581 SEAL O RING (Carburetor Spacer) is missing from mine, I think. I am assuming that 724 is the intake "seal" o-ring and that 623 is the white, plastic ring that snaps in on top of 724 to retain it. The white plastic ring sits flush with end of the carb when installed over the "seal" o-ring [724], as I see it. It appears that 617 is missing. But does 617 fit deeper into the intake as shown?



I have already disassembled the carb and blown it out with no resolution. Is there actually 1 o-ring missing? Which number & where how does it install, along with the other 2 rings taking up all the available space? Or was this carb known for mixture problems & replacement is the best option?
 
Thanks for any guidance you can provide.
 

Part #117, 118 are usually the issue that clogs up with crap and fuel doesn't flow.
 
I've cleaned MANY of them and have not had any other issues with them.  
 
Other than that, being plastic, they don't corrode or varnish up. 
 
I haven't had to pull those intake manifold gaskets apart, so I'd just 
put back together with the parts listed in the diagram..
 
It sounds like someone's been in there, dinking about. 
 
And no, I never had an issue running lean or surging.  
**A missing seal, after the carb, WOULD cause that issue.
 
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Thanks for the post, I'll disassemble and be very careful to blow thru 117 this time.