Craftsman/B&S Motor Popes and Backfires

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  #1  
Old 06-02-07, 09:21 AM
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Craftsman/B&S Motor Popes and Backfires

I have a Craftsman LT 1000 Riding Mower, 18HP B&S Motor. Model Number 917273392.

The lawnmower starts fine, but when it runs, rather fast idle or slow - and when mowing, the motor sort of "pops", it has a secondary pop like a backfire, but not nearly as loud, however, after using the mower for 15-20 minutes the mower backfires (loud and clear) one-two times. The mower seems to run ok other than the "pops" and seems to have plenty of power, doesn't seem to affect the performance - however I am concerned with potential damage. I have replaced the plug, the old plug was normal looking indicated good compression/timing etc, replaced fuel filter (and gas), and cleaned air filter.

I'm now wandering if it's a valve issue? any ideas?
 
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Old 06-02-07, 06:00 PM
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Sounds like one or more valve seats came loose thus preventing a valve from closing. I had a similar issue with a Briggs 18 and there was nothing I could do aside from replacing the block.
 
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Old 06-02-07, 06:24 PM
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It may be a terminology difference but I'd say you have a sputter related to the carburetor in need of an adjustment. However, the carburetor is not adjustable which means you would need to remove and clean the carburetor and possibly recondition it with a Briggs rebuild kit. Bear in mind that this will be difficult to pinpoint the exact trouble you have since it is an issue that would be helpful to hear the engine running.
 
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Old 06-03-07, 09:53 AM
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It may be the carb, but before sctrain spends money cleaning and reconditioning the carb, he might as well pop off the heads and take a look.
 
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Old 06-05-07, 02:13 AM
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Any update for us sctrain? As far as the possible valve issue, are you sure you don't lose any power, at all, when the popping does show up? If, indeed, not, then it should not be a valve issue and should be either a carburetor issue or an ignition related problem.
 
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Old 06-05-07, 07:29 PM
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No Answer yet...

I stopped short of taking the head off, after working on the Carb....at a certain point I can get the mower to run smooth, but throttle up or down from that point and right back to square 1...but I'm thinking if it runs smooth at a point...I wouldn't think the valve would be bad...I will try to reproduce the smooth run....and try to figure out exactly what I did to get it....
 
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