QuestionQUESTION: I have a briggs v-twin # 407777-0121-e4. Starts fine and seems to run ok under load. That being said, the engine makes a popping/ backfire in the at idle all the way to full throttle. This gets worse once the engine is warmed up. The only time I don't really notice it is when the mower is engaged. You can feel the burst of exhaust from the muffler. When mower is disengaged, engine sounds ok for a few seconds and then the backfiring starts, as it gets worse, the governor kicks in and the engine starts hunting. Engage mower and start off and problem slowly diminishes. I have replaced air filter fuel filter, spark plugs, dissembled, soaked and rebuilt carb with briggs kit # 792455 , and gasket set. Gasket set included a metal spacer which apparently restricts carb where it bolts to intake manifold, was not on carb originally, so did not use,and removed heads cleaned out all carbon buildup and installed new head gaskets and set valves .004 on intake .006 on exhaust. Did not see any problem with carb cleaned all passages, throttle shaft was not worn. Plugs look ok tan electrode but has some carbon around outer edge. Starting to suspect ignition issue. On last chapter to this book when throttling down it backfires quite a bit, however since adjusting valve the backfire is not as frequent once it is at idle speed for about 30 seconds, throttle it up into the midrange and here we go again. Will consider an exorcist if you think it will help. Thanks Mike in Oklahoma
ANSWER: Hello Mike:
LOL. The Exorcist Sounds like a Good Plan. There have been Many a Times I have had the Same Thoughts. Either a Valve is Sticking, the Carburetor is Main Jet is Slightly Flooding with No Load(when the engine is under a load the Backfiring will Slowly Stop as the Increase Load Clears the Excess Gas from the Cylinder and the Engine will Run Properly Until the Load is Removed, then the carburetor floods again) *Check the Float* Is it Level with the Bowl Mating Surface when the Carburetor is Upside Down and Looking at the Float? You can also Try this; 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
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
Hello Again Mike:
I Forgot. A Blown Head Gasket will Cause this Problem Also. Thanks.
Respectfully
John
---------- FOLLOW-UP ----------
QUESTION: Replaced head gaskets when I removed carbon from pistons and heads old gaskets were intact. Float is level when carb is inverted, it has plastic float and no adjustment, whatever happened to good old brass floats? Carb has no adjustment except idle stop screw. I left the intakes at .004 and set exhaust to .008, but did not notice much if any difference. Also despite what the diagram for carb kit shows, it does not include transfer tube, nozzle or main jet. Tube and nozzle are plastic, may have hairline crack that I can't see. Only other jet is for high altitude application. Should I replace above mentioned parts and put in a new float? The kit supplied float valve is the cheapest looking thing plastic not brass like the old one and the rubber point is different from old one. Seems to work though, just suprised since this carb doesn't use a float valve seat. I read somewhere that a pinhole in the fuel pump diaphram could cause excess fuel in carb bowl. Blew into pump and appears ok air flow in one way and not the other, probably wouldnt tell if there was a small rupture. I know I'm grasping but it didn't run like this before. Thank you for the help Mike
AnswerHello Again Mike:
Some Other Causes for this would be the Flywheel Key being Damaged. This Throws Off the Timing of the Plug Firing. A Sticking Exhaust Valve or a Burnt or Loose Valve Seat. It Sounds like you have Eliminated the Carburetor and Head Gasket as Possible Causes Unless the Carburetor has a Crack as you Suggested. It is Rare, but I have Seen it a Couple of Times. The Newer Needle Valves use the Rubber Tip to Seat Against the Carburetor Housing Rather than the Installed Rubber Seat as the Old Carburetors Used. You can View a Breakdown/IPL of this Engine at this Site Addy, http://www.smallenginepartswarehouse.com/parts.asp and Select the Shop Online Parts Look Up. Then Select the Model Tab. Then Select the OEM and Enter the Model Numbers 407777-0121. Then Double Click on the Model in the Results Area. Now you can Select the Section of the Engine you Wish to View. This is the Breakdown I am Using for Reference. From the Description, it Sounds like a Sticking Valve or a Damaged Flywheel Key. Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.
Good Luck
Respectfully
John
