Help Needed on 11 Y/O Briggs

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  #1  
Old 05-24-08, 03:53 PM
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Help Needed on 11 Y/O Briggs

I have an older B&S on a Murray Rider. The model # is 42A707. The engine runs fine at high RPMs under load. The problem is surging at idle or mid speed with or without load. As the engine slows, the govenor will open the throttle then release. There will be a surge of fuel from the main jet then the process repeats.

I have cleaned both jets with spray cleaner. I have adjusted the air screw and at times this helps. The fuel filter is clear and after the engine has run for a while there will be very little fuel in it. This, to me, says fuel starvation but I come back to "high speed with load" being OK. Could there be a fuel pump problem? The tank is high enough to gravity feed except for the bottom 2 inches. If the pump is the problem, then could the venturi be pulling the fuel at the high speed?
 
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  #2  
Old 05-24-08, 06:53 PM
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Location: Lady Lake Florida
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Tater,
42A707 Is this the mower mod. number or the engine mod number?
If possible , check the spec. plate on the engine
hp.
mod#
spec#
Good luck, and have a great day
 
  #3  
Old 05-24-08, 07:05 PM
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Thanks for the reply. Here are the details.

This is a twin, 17hp.
Model: 42A707
Type: 1653 01
Code: 9701305B
 
  #4  
Old 05-24-08, 09:23 PM
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The surging is due to the carburetor. Don't worry about how much fuel is in the filter...unless it's bone dry, it's ok. Remove the top of the carb and clean the bowl, especially the lowest part on the left side. There is a drain plug on that side of the carb bowl. Remove this, then insert a 3/16" allen wrench (if I recall correctly) to remove the main jet. Clean this jet well, but pay even more attention to the area in the carb behind this jet. This is likely where the problem is. Get some good spray carb cleaner (I like "next dimension" available at advance auto parts...it does very well for an aerosol carb cleaner). Soak and clean the area well, and spray through this area and remove the mixture screw and spray through this area as well.
 
  #5  
Old 05-25-08, 08:33 AM
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Thanks Cheeze. I have sprayed through the mixture hole and through the bowl drain. This blasts both jets. I haven't removed the main jet. That will be next.
 
  #6  
Old 05-25-08, 12:32 PM
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Update

Cheeze,
So, whenever this mower seemed to be getting better there was still more trash to follow. I didn't see what came out and don't care. I guess the problem was behind the jet as you suggest.

It fired first try and would idle immediatly with the choke open. After a warmup I found the sweet spot on the mixture screw. I am now replacing both belts.

My next plan was to rebuild the carb. You saved me from spending that $20. Thanks.
 
  #7  
Old 05-26-08, 12:03 AM
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Glad I could help. That spot behind the jet can cause problems sometimes even after cleaning the carb unless you remove it and clean behind it like you did.
 
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