QuestionQUESTION: I have an older model Craftsman lawnmower with a Kohler MV18-58556 engine. A couple weeks ago, after a stint with backfiring but otherwise running fine, it decided to quit. It will turn over but will only backfire as opposed to starting. I replaced the fuel lines, the fuel pump works fine. The carburetor is clean and free from build-up. I replaced the spark plugs and the spark is good at both plugs. The keyway is intact. I am getting good compression and vacuum on both cylinders. I pulled the heads any way to see if a valve is stuck and both sets of valves are working as advertised. When attempting a start the motor will turn over but I get backfiring from both the carburetor and the muffler. What am I missing here and what should I check/try next?
ANSWER: Hello Chuck:
Check the Valve Clearances. This is the Gap Between the Valve Stem and Tappet. The Following are the Specifications for this Engine. The Intake Valve/s should have .003in - .006in Clearance and the Exhaust Valve/s should have the Following: For Engine with Serial # 1816500646 and Earlier, .016in - .019in. and With Serial # 1816500656 and Later, .011in - .014in. You can View a Breakdown of the Engine at this Site Addy; http://www.kohlerplus.com/login.asp and Select Enter as Guest. Now Select the Thumbnail of the Engine Catalog you Wish to View. Select the Magnum Thumbnail. Select the M18 Thumbnail. At the Bottom of the Page Select the Service Manual M18-20 SM TP-2204-B. Download this PDF File for Online Viewing of this Engine. From your Description it Sounds like the Valve Clearance or the Ign Coil is Causing this Problem. I Usually Check and Reset the Valve Clearance and then Replace the Ign Coil if the Engine Still Backfires. The Ign Coil is Firing, but it is Firing too Early/Late and this Causes the Backfire. This Type of Problem with the Ign Coil is Rare, but it Does Happen. The Following is a General Instruction File for Setting the Valve Clearance on Non OHV Engines. **Remove the Carburetor and the Crankcase Breather/Vent and the Valve (Stem) Cover so you have Access to the Valve Stems, Springs and Keepers. Set 1 Cylinder to TDC on the Compression Stroke. Using a Blade Style Feeler Gauge, Check the Clearances. The Intake should be (as Specified Earlier) and Exhaust (as Specified Earlier) If the Feeler Gauge will Not Slide Between the Valve Tip and the Tappet, then you Must Reset the Clearances. **You Will Require a Small Engine Valve Spring Compressor to Remove the Springs and Keepers from the Valve Stems.** Some Engines Use a Valve Keeper that is like Automotive Keepers (2 Cone Shaped Keepers that Set in the Valve Retainer/Rotocoil Assembly)and the Other Style is a Slotted Retainer that is Moved (once the Valve Spring is Compressed) to the Larger Opening in the Retainer and the Retainer Slips Over the Valve Stem Tip for the Valve to be Removed. Remove the Heads. Set 1 Cylinder so the Piston is at TDC on the Compression Stroke And Check the Clearances Between the Valve Stem Tip and the Tappet. You have to Lightly Grind the Valve Stem Tip and Place the Valve Back Into the Engine (you can Hold the Valve Seated by Hand) and Check the Clearance Until you Reach the Required Clearances. Once the Valve Clearances are Reached, Re-Install the Valves on that Cylinder and Turn the Other Cylinder to TDC on the Compression Stroke and Set that Cylinders Valve Clearances. Once that Cylinder is Set, then Re-Assemble the Engine.** If you Hit a Snag or this Does Not Correct the Problem, I am Here if You Require More Assistance. Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. 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 8pm EST (US). My New Day Starts then. Thanks.
Respectfully
John
---------- FOLLOW-UP ----------
QUESTION: John, After finally getting the time, and parts, to try your suggestions, I've completed them. The spacing between the tappets and valves were fine. I have replaced the ignition module and I'm still getting the backfiring, although the firing is happening a little more consistently than previously. As I was attempting the starts with all the backfiring leading my neighbors to believe I was in a firefight in my backyard, I noticed some differences in the backfires while manipulating the choke. If I had the choke close to full on, the backfiring was small pops through the carburetor. If I leaned out the mixture that's when I got the extremely loud (I had to go get hearing protection), gunshot-like backfires from the exhaust.
Any further suggestions on what I could try next?
Thanks for the quick response on the last question, unfortunately I wasn't able to find the time to work on the mower near as quickly. By the way, have a great Father's Day. God bless.
AnswerHello Again Chuck.
Thanks for the Good Words and Happy Fathers Day to You Also. Did you Check the Flywheel Key? If the Flywheel Key is Sheared or Damaged, this will Happen. It is also Possible the Carburetor is the Cause, but the Symptoms you Described Make it Rare for the Carburetor to be the First Item to Check. I Try to Eliminate the Most Common Cause to the Least Common Cause Using the Description of the Problem. No Worries about the Length of Time that has Passed. I Know it May take Longer for Some to Perform the Diagnostics or Repairs and Contact me. I Try to Respond within 12 Hours of 8PM EST USA Time. Sometimes I May have to Do More Research on a Problem or have Phone Connection Problems, but Usually it is Within 12 Hours. Lately with All the Rain, it has been Taking me About 14 to 18 Hours to Answer. Darn Phone line Keeps Going Out. LOL. Gotta Get Satellite. The Following is the Instruction File I Made for Correcting this Problem. It is the Basics and if you have a Question, Please Ask. Any Input to Make this File Better is Appreciated. ***Disconnect the Coil Wire from the Spark Plug. Remove the Blower Housing (metal engine cover). Remove the Flywheel Nut or Starter Cup (Depends on the Model). Now Attach a Steering Wheel Puller to the Flywheel Using the Threaded Holes in the Flywheel. Tighten the Puller. Now Hit the Puller with a Hammer. Repeat this Procedure Until the Flywheel Pops Up or can be Lifted Off the Crankshaft. On Some Models you Can Not Use a Puller and Must Use a Pry Bar and a Pipe Cap that is Slightly Larger than the Crankshaft. Place the Pipe Cap Over the Crankshaft to Protect it. Place the Pry Bar Under the Thick Section of the Flywheel and Pull Up Against the Flywheel. **BE CAREFUL DO NOT PUT A HOLE IN THE BLOCK UNDER THE FLYWHEEL** Now Hit the Pipe Cap that is on the Crankshaft (covered with the Pipe Cap). Repeat the Procedure Until the Flywheel can be Lifted Off the Crankshaft. *** Now Replace the Key Between the Flywheel and Crankshaft and Reverse the Procedure. Be Sure you have the Flywheel Torqued to the Proper Torque. You can Locate the Proper Torque for the Flywheel Nut or Starter Clutch at this Site Addy; http://outdoorpowerinfo.com/engine_specs/kohler_head_torques.asp and Select Your Engine Model from the List Provided. In Case the Flywheel Key is Good, then here is the Carburetor Cleaning File. The Following is a General Instruction File for Carburetor Cleaning. Even though the Carburetor Looks Clean, the Internal Passages May be Restricted with Varnish that Gas Causes to Build Up over Time. I Suggest you Make Sure the Gas is Free Flowing to the Carburetor Inlet Port. If the Gas is Free Flowing to the Inlet Port, then **Remove, Disassemble and Soak the Carburetor Overnight in Carburetor Cleaner (I Use Gunk). I have Found that Most People can Use a Breakdown/IPL and Disassemble the Carburetor Enough for Proper Cleaning. You Only have to Remove the Bowl (if Applicable), Float Pin (if Applicable), Float (if Applicable), Needle Valve and Needle Valve Seat if Applicable. This is a Flat O-Ring Located in the Inlet Port the Float Needle Rests in. Use a Strong Wire with a Slight Bend to Remove the Old Seat. Sometimes a Small Pocket Screw Driver can be Use Also. The New Seat will have a Slightly Beveled Side and a Flat Side. The Flat Side goes Towards the Carburetor Body when Inserted into the Inlet Port. Spray the Seat with Spray Lubricant to make Installation Easier. The End of a Drill Bit that is Slightly Small than the Seat can be Use as an Insertion Tool. Wear Gloves or Use a Rag when Handling the Sharpened End of the Drill Bit. and Any Adjustment Screws that Go Into the Carburetor Body. The Welch Plugs Do Not have to be Removed. Then Soak the Carburetor Body and Parts in a 1 Gallon Can of Gunk Carburetor Cleaner Overnight. The Can has a Parts Tray Inside it for the Small Parts. Once the Carburetor and Parts are in the Cleaner, you can Replace the Lid for Safety and to Prevent Accidental Spillage. Then Blow Out the Passages with Compressed Air.*** (Do Not Use High Pressure Air for the Zama Carburetors, they have Check Valves for the Primer and these are Usually Blown Out of the Carburetor if Not Careful. Allow the Zama Carburetor to Set on a Drip Pan and Dry)***. Install New Parts if Required.** If the Carburetor has Air Mixture Screws, then Let me Know and I can Give you that Instruction File. You can View a Breakdown of the Engine at this Site Addy; http://www.kohlerplus.com/login.asp and Select Enter as Guest. Now Select the Thumbnail of the Engine Catalog you Wish to View. If you Hit a Snag or this Does Not Correct the Problem, I am Here if You Require More Assistance. Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. 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 8pm EST (US). My New Day Starts then. Thanks.
Respectfully
John
