Small Engines (Lawn Mowers, etc.): BACKFIRING IN MY TRACTOR, blown head gasket, twin cylinder engines

Question
QUESTION: HI, I have a 2 cylinder Briggs and Straton tractor riding mower..lately it backfires REAL bad..and it looses power when I try to give it gas on the throttle lever. I changed the fuel filter..only thing I can see is the exhaust muffler gasket might have burnt out-the mufflers two screws were loose..I tightened them, changed spark plug..still same problem..its worse when I shift..the gas tank looks okay except some dead insects were in it..any ideas what is wrong??

ANSWER: Hello GH:

Send me the Model Numbers Off the Engine, Please. With these I can Locate the Proper Manual and Better Assist you. However, on the Twin Cylinder Engines, it can be a Sticking Valve, Blown Head Gasket, Valve Clearance needs Resetting (OHV engines), Bad Valve Seat or the Carburetor Not Metering the Gas Properly. Thanks.

Respectfully

John

PS: I am Allowed to Answer 10 Questions a Day. If you See I am Maxxed Out, then Try Submitting your Question at or Just After 9pm EST (US). My New Day Starts then. Thanks.

Respectfully

John

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QUESTION: Hi John
thanks for your fast reply..I will get the model info tomorrow..I see some smoke coming out from around the engine..the Muffler was also loose, but I tightened the hold down bolts for it..no difference, still backfires REALLY loud like a gun..I looked under valve cover..saw the 2 valves but they looked okay..perhaps a bad head gasket as you said? I will look up model and info in the morn and hopefully you can help; thanks!
ANSWER: Hello Again GH:

You will Still need to Use a Blade Feeler Gauge and Check the Valve Stem Top to Rocker Arm Clearance. When you Send me the Model Numbers I will be Able to Supply you with Further Instructions. Thanks.

Respectfully

John

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QUESTION: Hi John
i canot see any numbers on the engine..its a Murray riding mower 13 HO, 40 inch cut. OVC Serial # 7377235001E123, MODEL #40541x99C  I took the valve cover off..there are 2 valves..on one the rocker arm moves..like its loose and the rod seems loose..the other has no opening or gap..maybe its stuck? I do not know how to find TDC. I took the muffler off..its all BLACK with soot inside..jet black..i do not see any cracks in head.THe muffler has NO gasket..would that make it backfire? The gasket burnt up i think!
ANSWER: Hello Again GH:

The Lack of an Exhaust Gasket can Cause Backfire. The Murray Model Breakdown Does Not Show the Engine Model and Type Numbers. Go to this Site Addy, http://www.smallenginepartswarehouse.com/parts.asp and Select Shop Online Parts Look Up. Then Select the Model Tab. Then Select the OEM (Murray) and Enter the Model Numbers. Then Double Click on your Model in the Results Area. Then Select the Section of the Mower you Wish to View. To Locate the Engine Model Numbers: The Model, Type and Code numbers, are Stamped Directly into the Engine, are used to Identify the Engines. Engines used in Lawnmower Applications usually have these Numbers Stamped into the Blower Housing, Muffler Heat Shield or a few inches above the spark plug. Depending on the type of engine shroud used, the engine numbers may be covered. It may be necessary to remove this shroud in order to locate the Model, Type, and Code. Many horizontal shaft, overhead valve engines have these numbers stamped into the OHV rocker cover. However, on certain overhead valve powered generators, the equipment manufacturer installs a decorative chrome rocker cover OVER our cover. If your engine has a decorative cover, it may be necessary to remove it in order to locate the Model, Type, and Code. The Model Numbers will Greatly Assist me in Assisting you. Thanks.

Respectfully

John



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QUESTION: Hi john
sorry to keep asking you questions! The model # MURRAY 40541x99C  TECUMSEH engine OHV130-13 HP  40 Inch cut riding mower

I took the valve cover off..there is a mark on the crank but what do u line it up with..looks like the valves are ok..its also hard to start..takes alot of key turns..sometimes nothing happens at all when I turn on the key but then the starter finially grabs..I am not so concerned about that but I do not have any power when I open the throttle cable , and it backfires like crazy..LOUD!! HELP!
ANSWER: Hi GH:

Remove the Blower Housing (metal Engine Cover) and Turn the Flywheel by Hand Until the Magnet is Lined Up with the Coil. If Both Rocker Arms have Play, then this Should be Close Enough to TDC to Set the Valves. You can View a Breakdown/IPL of this Engine at this Site Addy, http://www.outdoordistributors.com/ and Select the T. Then Select Tecumseh. Then Select Don't have Part Numbers. Then Enter the Model Numbers. Then Select your Model and Download the File for Online Viewing. I Do Not Use the Setting Called for by the OEMs. I have Found that the Following Works Best. Remove the Valve Cover (on the Dead Cylinder for Twin Cylinder Engines). Set the Cylinder you are Setting the Valves on to Top Dead Center (Both Rocker Arms will have Play in them when the Engine is at TDC) When Setting the Valve Clearance. Now Loosen the Valve Adjuster Locks and Set the Intake Valve Clearance to .004in -.006in and the Exhaust to .008in - .010in. I have had Better Performance Out of the Mowers and Less Burnt Valves Using these Settings. Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.

Good Luck

Respectfully

John

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QUESTION: JOhn I still have some questions to ask..why do I have black soot in my muffler and spark plug?? It looks just like chimney smoke!! Is this carbon causing it to backfire or the VALVES?Also, when you adjust ONE valve (there are 2); do I have to move the crank again to TDC to adjust the other valve? Or leave it in the same spot as where I adjusted the 1st valve?
Also, do you think I have a carb problem also or is it the valves? This is a MURRAY riding tractor OVC130. I have not started it yet as I am also going to drain gas tank and put fresh gas in..does it matter what size fuel FILTER goes in the lines?? Sorry for all these questions..hope you can answer all!

ANSWER: Hi GH:

Once you Set the Crankshaft so the Magnets are At the Coil; you Set Both Valves. The Soot can be From the Valves and Backfiring  and the Carburetor. Once the Valves are Correct, then you can Tell if the Carburetor is Correct. A Carburetor Causing Backfire is Unusual, but it Does Happen. I Try to Check the Most Common Cause for a Problem First and Eliminate it and then Continue to the Next Most Common. The Backfiring Suggest the Valves or the Flywheel Key as the Most Common Causes. An Automotive In Line Filter will Work. Ask All the Question you Need to GH. No Worries. That is Why I'm Here. Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.

Good Luck

Respectfully

John

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QUESTION: Hi john thanks for all your help...FAST REPLY!!how do u tell if you have a burnt valve? They looked okay and I did what you said and changed the spark plug..set gap ..I will let you know what happens..I hope to God i have these valves set okay..the gaps were not off by much..many thanks

Answer
Hi GH:

The Valve Seat in the Head or the Valve Itself will have a Burnt Place (Black and Recessed) on the Mating Surface. The Exhaust Valve May Look Brown on the Mating Surface. Black and Recessed is Burnt. Don't Forget; if the Engine Still Backfires we have the Flywheel Key to Check Next. Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.

Good Luck

Respectfully

John