Small Engines (Lawn Mowers, etc.)/Carburetor?


QUESTION: Hi! I have a Snapper riding mower bought in 2009. Recently replaced all the filters, changed oil, installed new spark plug and new blade. A week later I started having trouble with it. It started missing very bad and black smoke emitted from the exhaust (way too much fuel). I then pulled the carburetor off and checked it out real good and cleaned it. I realized the choke (butterfly valve) was not springing back as it should. Took the carburetor to a small engine shop and they put on a new choke spring. Thought that would fix the problem, so I reinstalled everything. I then started the engine (without choking it) and it purred like a kitten. I gradually advanced the throttle to about midway to rev the engine. It ran very good for a couple minutes, then all of a sudden it started sputtering and blowing black smoke from the exhaust, then went dead. I tried again later and the same thing happened. It's as if it is getting too much gas all of sudden by itself. I'm at a loss as to what is going on and would really appreciate some advice from anyone who may know about Briggs & Stratton engines.
12.5 HP  Model: 219907-0131-B1. Thanks for any help here. Please reply.

ANSWER: When you cleaned the carburetor, did you blow compressed air from an air compressor through all the carburetor orifices (holes)?  I'm wondering if an air passage is blocked causing the rich fuel condition.

Do you know if the inlet needle has been replaced?
Usually if there is an air LEAK, the engine will run lean.  I'm still banking on a passage in the carb being blocked preventing the correct amount of air from being routed through the carburetor.

I'd pull the carb back off and spray some carb cleaner through each orifice and follow that by blowing compressed air from an air compressor through each each passage.

Let me know if another carb cleaning helps and if the inlet needle has been replaced.

---------- FOLLOW-UP ----------

QUESTION: Hi Eric. I sprayed the cleaner in and around all the parts. I did not blow it off with compressed air, figuring it would evaporate. I do not have a compressor. Would a can of compressed air work as well? After reassembly, I moved the throttle to the idle position, then started it. Fired off immediately. I then let it sit and run for over 10 minutes and it ran great. If there was a problem with the needle, or air leak, why would it have run (idled) perfectly for over 10 minutes? I then advanced the throttle to the run position, and it ran good for a minute or so. Then it started running rough (trying to die) and emitted black smoke, as if getting way to much raw gas all of a sudden. Can you look at the needle/seat and tell if it is bad? Thanks again Eric and I look forward to hearing back from you soon.


ANSWER: I have just used spray can carb cleaner in the past but you really need an air compressor to blow out all the orifices.  Carbs these days are so sensitive a minor blockage can prevent them from working at all.

The reason it may idle fine is because there are 2 circuits in small 4 cycle engine carbs.  One is for idle and uses the circuit when the butterfly is more or less closed.  The high speed circuit becomes active when the butterfly opens, usually only has to open a small amount to activate the high speed circuit.  There could be a blockage in the high speed circuit of your carb.

Visually inspecting the viton (rubber) seat is next to impossible.  Old seats look very similar to new ones but they are usually stiffer if you try to squeeze them by hand...the viton is more brittle.  The most common symptom of a bad seat is gas leaking out the bottom of the carb or leaking into the engine oil...very common...most of service orders.

I still suspect the problem is in the carburetor but just for giggles, did you replace the spark plug?  I've seen a weird problem or two with spark plugs that seemed like carb issues.


---------- FOLLOW-UP ----------

QUESTION: I did buy and install a new spark plug set at .030. I pulled the new spark plug out today to look at it, and it was all black sooty looking. I cleaned it as well as I could and re-installed it. A mechanic looked at my carb parts today and asked me where the "main jet" was. I told him I had not seen one. This part is bronze and about the size of a match head. It evidently had fallen out when I first dismantled the float. Today, I found this very small part lying on the work bench. Now, I'm not totally sure where it is supposed to fit. The part number is 591936 at Briggs & Stratton. Anyway, I went online today and ordered a new needle and a new main jet. I guess I'll go to Home Depot and see if I can get a small compressor now. The new parts will be here middle of next week so I can start the reassembly then. I just hope by then, I have learned where this "main jet" is supposed to fit. Thanks again for your help.


Based on the part number you must have the Nikki carb.  Is this correct?  The name/brand of carb will be stamped on the carburetor.

The main jet should go right inside the center of the main fuel tube in the center of the carburetor.

Was there an o-ring on the old jet you found or is there in o-ring inside the carb?  Something has to hold the jet in place.  The bowel gasket may actually hold the jet is place.

Did you take a look at the service manual and verify with carb you have?  Use the URL below and refer to section 4.

Since you know a part was missing I would not spend the money on an air compressor just yet.  FYI, Harbor Freight have some inexpensive air compressors.  I bought a $40 compressor from Harbor Freight for cleaning carbs at home.  It's not a big compressor, takes up very little space, and it is cheap but for occasional use I'm sure I'll get my $40 worth.

Let me know which carb you have.


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Eric A. Jones


Lawnmower Repair . Certified Master Service Technician from B&S. Have 23 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.


Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. Continued working on mowers part time as I completed 20 years of military. Retired from the military on a Friday and continued in the lawn and garden industry the next Monday.

MAS Aerospace Operations BA Mathematics AAS Electronic Communications AAS Electronic Technology

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