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OT - Tractor Engine Problem !!!

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  • OT - Tractor Engine Problem !!!

    Know this isn't a machining problem but maybe someone has an insight to this problem. I have a Craftsman lawn tractor with a Briggs 21.5HP Intek engine. I was doing the snow blowing today and after a while the engine starts backfiring only on a heavy load. Seemed odd to me as I have never had an issue before. I thought that maybe something got wet but looked under hood and everything looked dry. The thing seemed to be just fine as long as I didn't try to over load the blower, I did get the rest of the blowing done but this just didn't add up to me. I service this tractor in spring and fall every year and also I us StaBil in the gas year round to help with any fuel problems. It does seem to loose a bit of power when it starts the back firing but guess that would make since. Just thought that maybe someone has run into this so as I don't start replacing parts that aren't needed. Looked everything over but can't get it to act up just running with out the heavy load. Any ideas !!!!! Thanks

  • #2
    I have no experience with that model but backfiring makes me think vacuum leak.


    • #3
      or intake valve getting held open slightly... or the beginning of one burning,,,

      lots of possibilities --- got a gurls 4wd actuator to fix but will check back later...


      • #4
        Vacuum leak maybe but why only under heavy load??

        Sticky valve ? Also why only under heavy load ? If a valve is burnt it wouldn't change from light to heavy loads would it?

        I don't have any issue as long as the engine doesn't get any heavy load. If just taking smaller paths of snow it is just fine. Didn't have any issue when doing the lawn in summer but of loads aren't as bad doing grass over heavy wet snow. Right that there is a lot of possible problems but wish there was an easy answer sometimes.


        • #5
          Check the coil. It may be bad. My BILs B&G mud motor would do the same thing. Rebuilt carb,twice. All new fuel components, etc. Turns out is was a bad coil.


          • #6
            Maybe the muffler is getting clogged up, not letting enough flow through? The 2 stroke Lawnboy mowers used to have that problem.


            • #7
              Probably short of fuel. Ice in fuel line or carb? Plugged or iced filter? Float level too low? Fuel tank vent iced?


              • #8
                Check your valve clearances. Backfiring is a valve opening when combustion is occurring. I would think a heavy load would have an affect. Your engine is consumer grade, Vanguard is their industrial top of the line. I have a cheap piece o crap push mower with a cheap Briggs engine. When it hits a heavy load, it starts to knock loudly.
                Also check your timing / points if applicable.
                Last edited by RB211; 12-29-2015, 01:24 PM.


                • #9
                  +1 for valve clearance,also is the tractor having any trouble maintaining a charge?
                  I just need one more tool,just one!


                  • #10
                    Thanks for the all the replies, this is not a 2 cycle it is a twin v, over head valve motor. One of the coils could be bad but wouldn't that show up also without a heavy load? Tractor runs good other wise, only acts up with heavy load.

                    Doesn't seem to have a problem holding a charge, starts and runs good. This problem just started today. Like I said before only acts up under heavy load, like when the gouvenor wants to increase power for the load if that makes sense. I just thought of that now thinking of how to explain the problem. There could be an issue with the fuel I guess but will have to look into that for sure. Maybe the fuel filter is going bad,that could be causing fuel issue under the heavy load. Thanks again for the help. !!!! This engine is electronic ingnition.
                    Last edited by Old School; 12-29-2015, 05:04 PM. Reason: added to post


                    • #11
                      above sound good... It would be worth a post to


                      Lots of knowledge there.


                      • #12
                        My money is on a fuel delivery issue. Heavy loads and cold weather will make this issue more pronounced.
                        A fuel filter that is dirty is definitely a possible source of the problem but don't overlook dirt and or corrosion inside the carb as another source of inadequate fuel delivery. Also make sure you don't have an icing problem since most lawn and garden equipment does not have warm air supplied to the carb like a snowblower does.

                        I believe your B&S engine is equipped with a 2 barrel Nikki carburetor. If so and you decide to go into it yourself, I've included a link to the disassembly, repair and cleaning of these units at the bottom of this post.

                        This past season I have gone thru three of this units with similar complaints as yours and found that after a careful going over that they once again performed like new. In two of these cases the carbs appeared spotless internally but in fact were not. Be very diligent in the cleaning process as it only takes a small pepper sized particle to cause an issue.

                        Home, down in the valley behind the Red Angus
                        Bad Decisions Make Good Stories


                        • #13
                          Great post Willy,

                          well i got the gurls 4WD fixed - little "halo" around one of the solder connections in the internal relay of the 4wd module - just needed to be reheated and melted back together, got paid then she took me to lunch and then we went shopping at a second hand place,

                          lot's of good stuff in Willy's post first paragraph esp. the icing problems and if you live in a humid place (like where O.S. lives)

                          O.S. --- engines are funny - load can both effect a boderline intake valve and create backfire through the intake simply due to more pressure getting past it,,, load can also effect a borderline coil as different effective compression ratios have a direct effect on how a spark can bridge the gap of a spark plug.

                          it's all one big can of soup - strain out the most obvious and easiest --- quick check for the carb icing is next time it's doing it then shut er down and pop the air cleaner real quick - get a look at the venturi --- if it's starting to shut down you will see it --- this is where ice collects as it's a low pressure area and this reduces temperatures...

                          there's also another ear-mark with this problem --- the engine will consume fuel like crazy - it pulls way more on the fuel jetting when the venturi starts to get restricted...

                          keep us posted


                          • #14
                            Yes, not ruling out a weak coil as a likely cause either. A fuel filter and an hour or so spent making sure the carb is clean and receiving adequate fuel costs little to nothing, while once your purchase a new coil you are committed. Mind you measuring primary and secondary resistance and checking the plug wires eliminates a lot of guess work.

                            I have run into situations in the past where inadequate fuel flow was suspect under load and all it took was a quick spritz of gas vapor from a hand powered spray bottle down the intake while under load to confirm this.
                            As you know it's all in the diagnostic steps. As often as not I sometimes omit these important little steps in my response here because I take too many of these little things for granted.
                            Like some of the full time and pro machinists here that do so many little things while setting up a procedure that they don't even think about. Yet the rest of us mere mortals have to stay focused because this is not our specialty.

                            I know I once tried writing down all of the little steps I take for something that I did daily for a pre-trip inspection, this as a guideline for someone new. Well after three pages I gave up trying to explain all of the little nuances of what should be a 5-10 min process. I'll be the first to admit that I'm a lousy teacher.
                            Home, down in the valley behind the Red Angus
                            Bad Decisions Make Good Stories


                            • #15
                              yeah coil is about my last choice - I just brought it up cuz O.S. mentioned he did not think one could be load related and if borderline it sure can --- but not a very likely choice

                              another thing people need to keep in mind is allot of these engines do not have "thermostats" they are just full bore cooling from the fan/flywheel ---- in frigid temps the engines can actually cool too good and never reach operating temps - this can play hell with full bore running and can mimic lean conditions due to the fuel not being enriched for the operating temps that the engine is seeing - super cold engines want at least partial choke ESP. under full load,,,

                              im not aware of the set up on these - I just know the little singles that are on allot of things are full bore wide open cooling --- think about 0 degree F blasting past aluminum cooling fins,,, not good...

                              there we just gave him a couple very simple things to check out - either at the flick of a wrist (applying more choke when its stumbling) or popping off the air cleaner to get a look at the venturi for icing. the best diagnostic procedures are the free ones...