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OT - Small engine problem...

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  • OT - Small engine problem...

    The patient at hand is a leaf shredder/chipper with a Briggs & Stratton B&S "11.50 Gross Torque" OHV Engine. It's 8 or 9 years old but is not a high hour machine, just typical homeowner use. It gets used semi-frequently, gets maintained and properly prepped for storage when it is put away for the winter, and always starts and runs great.

    I went to use it the other day and did my typical start up routine: Turn on the fuel, full choke, wide open throttle, pull the rope through a few times and the engine fires. If I'm quick enough and open the choke, it will keep running and come up to speed. Otherwise, I just open the choke, give it another pull and it starts right up.

    So I went to use it the other day and my standard routine did not start it up. Fuel was fresh. I tried various choke settings and no choke at all. Pretty much all the standard tricks. I'm not big fan of starting ether, but open the choke and gave it a small spritz as a check for fuel delivery issues. It started right up and continued to run just fine. I proceeded to shred the pile of stuff I had and it did that just fine. I stopped and restarted it several times after that with no issues and no magic starting juice required.

    Fast forward about 10 days to yesterday. I roll the machine out to do some maintenance, sharpen the chipper blade, blow out debris from the cooling fins, etc. When I am done, I try again to start it up. Same deal, will not start or fire. After trying a few things, I again spritz a bit of ether in it (no choke) and it starts right up and runs great. It slows to an idle OK, and runs right up when I open the throttle again. Chips and shreds stuff just fine.

    What gives? I am no small engine guru, but have done plenty of work on them. The behavior suggests a carb/fuel issue, but it runs perfectly after that first ether start. This is not a deal-breaker problem, but it is annoying and is an indication that something is up. Any ideas on what I should be looking at?

  • #2
    Very likely a fuel issue. No start if the carb has to supply pull-start fuel, but OK at the somewhat higher vacuum at idle. And warm start is almost always easier.

    The difference is air velocity when starting. If the idle jet is not blocked, but has something stuck in it (or one of them), then it may not give enough fuel flow or atomization for a pull-start. A not quite complete blockage might give enough fuel for idle with the higher vacuum of a full idle as compared to pull start.

    With the ether, the fuel is already in the air, so it does not have to be atomized. And it starts.

    Throttle plate might not close all the way, or an idle jet may have a partial blockage. Just seems like a carb problem. If throttle does not close completely, the idle jet is often not completely "activated".

    Long shot is that some bad gas will run the engine, but just won't start well. Doesn't sound like that, but it's possible.
    CNC machines only go through the motions


    • #3
      I would make sure it’s not leaking air into the fuel line. Next would be to check for a small vacuum leak. Loose carb mounting bolts, bad gasket etc. Also make sure the choke plate is actually moving. I’m not familiar with this engine just general troubleshooting


      • #4
        A partially plugged jet that would run ok once started would more than likely start fine with the choke on. My guess is a sticking needle valve on the float, once running the vibration shakes it loose so the fuel flows into the bowl.

        In any case, those carbs are easy to clean. Take it off, squirt every passage you can find with carb cleaner and blow them out with air. Remove the fuel bowl, float and needle first then replace after. Take note of how the float needle connects to the float before removing, that is a common area people get into trouble with when reassembling.


        • #5
          Mig welding wire - like .031 works great for making sure all the passages are open.. I have cleaned many a carb in my youth - I never had luck just blowing air through - always had to push the wire through the small passages to get them running well again.



          • #6
            Every time I "poked" the jet clear, it would block up again. Always had to pull the welch plug and wash it out, and always found grit or gunk in there.

            Yes, the choke may fix it, but not always. If the throttle plate is not closing quite right, the air over the jet may not atomize well, even though it flows. It just may not be getting into the air.

            Just having the thing start on ether seems to indicate a fuel issue. But ether is almost like cheating. The key is if it will start with the same gas if the gas gets into the air a different way.

            I get some gas into the manifold, or just soak in into the air cleaner in an area, so that you know the gas is in the air. If it won't start even then, but goes OK with ether, the gas may not be good. If it starts Ok with the gas known to be in the air, then it's likely to be a jet issue.
            CNC machines only go through the motions


            • #7
              Before we try to analyze exactly why and what is happening we need to have a bit more detail of how the engine is stored for instance.
              Is the gasoline completely drained including the float bowl? If not how long has it sat since the last time it ran without issue?
              Is the plug wet with fuel after a no-start with choke episode?
              Have you verified that the choke is indeed physically closing?
              Does use of the choke while the engine is running indicate that the engine is receiving an excessively rich mixture?

              You may get lucky with a couple of spritzes of carb and choke cleaner inserted into the main air bleed, idle air bleed orifices, and into the emulsion tube with the straw nozzle. And or a blast of air. However as often as not a persistent issue often requires a thorough cleaning leaving no stone unturned. This often requires a complete disassembly of the carburetor including the removal of pressed in aluminum welch plugs in order to gain complete access to the entirety of certain circuits.

              This will entail the purchase of a rebuild kit. As often as not a completely new carb can usually be purchased for about the same money.
              I used to rebuild a lot of small engine carburetors but have now gone to the low cost Asian replacements in lieu of rebuilding if my spritz and air blast brings no joy. Time is money and the low cost replacements have never let me down, they are that good, and cheaper than the purchase of a kit and the time spent cleaning the body and installation.

              I realize that it doesn't apply to this particular case,but for what it's worth t bares mentioning that one should never blast air into a diaphragm type carb as found on most two cycle engines, even when completely disassembled.
              Many if not most incorporate very small pressed valves with tiny membranes that can be damaged with an air blast.
              Home, down in the valley behind the Red Angus
              Bad Decisions Make Good Stories​

              Location: British Columbia


              • #8
                Originally posted by Captain K View Post
                I would make sure it’s not leaking air into the fuel line. Next would be to check for a small vacuum leak. Loose carb mounting bolts, bad gasket etc. Also make sure the choke plate is actually moving. I’m not familiar with this engine just general troubleshooting
                Yeah, that is it. Its a air leak around a fuel line or gromet. Those are a B to find. I ended up buying a full rebuild, amazon discount, 28 bucks. Diff engine but similar problems. JR


                • #9
                  Is there any kind if fuel filter or strainer in the fuel tank?
                  Location: The Black Forest in Germany

                  How to become a millionaire: Start out with 10 million and take up machining as a hobby!


                  • #10
                    New starting sequence to try. Choke on, throttle wide open, pull twice. Throttle down to idle. Pull once and often the engine will be running. Choke partly open, open throttle part way, then full open and choke off if it sounds like it is struggling.


                    • #11
                      Weak spark, change the plug. Once its running at speed spark is fine, cranking over by hand not. Or you have a bad batch of gas.
                      Retired - Journeyman Refrigeration Pipefitter - Master Electrician


                      • #12
                        I just had that problem. In my case it was caused by a slip in the throttle cable. Which meant that the travel was reduced and the choke not engaged. The choke is normally engaged at the very last end of the cable travel, so any reduction is total loss of choke.

                        Good luck. (Try the easy stuff first, but you knew that. )

                        EDIT: On re-reading the OP, it sounds like your choke is independent of the throttle. In which case, never mind what I said.
                        Last edited by Bob Engelhardt; 04-15-2022, 08:54 AM.


                        • #13
                          Yes, spark is a good point.

                          Ether will light if you look at it and think about a spark, it does not take much, a very weak spark will do. See if it will start if the gas is put in a different way, drop some down the carb, or get it on the air cleaner element. If it starts, the spark is less likely, and carb is more so. If no start, then spark or fuel quality.
                          CNC machines only go through the motions


                          • #14
                            Originally posted by J Tiers View Post
                            Yes, spark is a good point.

                            ... See if it will start if the gas is put in a different way, ....
                            A classic technique that I use is to take out the spark plug and put a few cc's of gas directly into the cylinder.


                            • #15
                              Originally posted by Bob Engelhardt View Post

                              A classic technique that I use is to take out the spark plug and put a few cc's of gas directly into the cylinder.
                              Yes. I do that too. Does not take a lot, too much and it won't burn. I usually get some on a screwdriver and shake it off in the cylinder a couple times.
                              CNC machines only go through the motions