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I am currently restoring a Partner 22 GBI leaf blower and facing some weird issues. I have removed the carb, cleaned it in the ultrasonic and restored everything to factory settings.

After running some tests today I am facing the following behavior. The engine starts even without choking it after 1 or 2 pulls, so I guess it gets a good amount of fuel. The engine will then idle, but the speed it idles is different on almost every start. After a couple of seconds idle RPM becomes very unstable. It revs up, starts dying down, then idle on a really really low RPM for a second or two where you'd think it will die now, then it recovers to a more or less stable RPM again for a couple of seconds before starting to surge again.

Now the weird thing is that these fluctuations are not really rhythmic but completely random (sometimes it runs fine for 10 seconds, sometimes for 2 seconds) so it does not seem like there is a bad feedback cycle. Eventually a surge happens, it slows down and then dies. Ususally the engin runs for 20-40 seconds before randomly dying down.

What am I looking at here? I am really confused, no adjustment of the mixture screw (it has no high speed needle) really did any meaningful change to the behavior above...

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  • How does it run under throttle? Also, did you check the reed valve to ensure it was in good shape? (I'm assuming it has one, as most 2-strokes do.)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Oct 12, 2022 at 21:10
  • When it runs rough and almost dies at low RPM applying some throttle helps. Generally applying some throttle makes it run smoother. Running it under full throttle keeps it alive for some 10 seconds or more before it suddenly shuts down immediately. The diaphragm is fine with regards to fuel supply, I have not disassembled the engine too much yet to check the reed valves. However when removing the carb and dumping some fuel or starter fluid into the engine it also does not work reliably. Sometimes you have to pull 10 times to have it run with starter fluid, sometimes immediately
    – Yanick Salzmann
    Commented Oct 12, 2022 at 21:23
  • So yea, might not be entirely carb related.
    – Yanick Salzmann
    Commented Oct 12, 2022 at 21:23
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    Did it run at all before the carb was re-worked? If so, was that idle also unstable?
    – George
    Commented Oct 13, 2022 at 2:12
  • Poor running may be attibuted to; carburetor, vacuum leak, ignition, worn cylinder, sticking piston rings, etc. Most small engines use magneto ignition systems. A simple compression test pull starting should result in decent resistance. A spark test, plug removed and plug wire on it with plug base grounded to the cylinder head - pull starting should show good spark. The carb gasket must not be torn or have any breaks otherwise a vacuum leak may occur to interfere with engine running. 2-stroke carbs can be difficult to repair, using crankcase vacuum to operate the diaphragm pump.
    – F Dryer
    Commented Oct 14, 2022 at 0:54

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This engine was almost driving me nuts! I had left it a couple of days and then went back looking at it. I removed the carb, inspected it for any issues including the gaskets, put it on again, started working the needle a bit and suddenly it worked perfectly and reliably.

I then went on to fix the kill switch which meant I had to remove the carb again to get to the switch. After finishing that I mounted the carb again and what do you know, it is back to the old unreliable running, maybe even worse. I was working the needle and tried removing and installing it again, making it worse and worse until it stopped starting alltogether.

Eventually I realized that one of the screws that holds in the carb seemed to vibrate quite a bit and rotate a little bit. The inner threads of the engine where the carb is screwed into were almost stripped. The rotation of the screw actually caused to sometimes "bite enough" which is when it was running nice and eventually it was rotating loose enough to create a vacuum leak which is when it started surging which in return caused it to vibrate faster until it created a seal good enough again to prevent the vacuum leak.

At least that is my understanding of what happened. At the bottom of the hole the threads were still fine, so by fabricating a longer version of the screw i was able to secure it better and now everything works fine.

So while I am not 100% sure if this is really what it was I believe this is most likely what happened.

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