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😆 ....sharp intake of breath.... did you just 'dis' a Stihl ? 😨

Well, saw is fixed with the new carb so its no longer going in the skip - thats a good thing. As for the above - I agree with what youve said. Ref the video of it: - Most saws die on choke with over rich mixture (not all, but most). - Didnt revup properly once throttIe was opened, think it was just spooling down from there with out any more combustion. I dont think it had a full metering chamber of fuel (after runniny on choke), I'd guess its pulling in fuel and air into the metering chamber while on choke and thats why it kept running (on choke) and didnt die (over rich). Its just that on choke theres such a high vacuum on the fuel circuit that even with an air leak there is still enough vacuum to pull some fuel though. Either that or the diaphragm's too stiff or wrong version and just not operating the metering arm. But anyway all somewhat irrelevant now.

The fact it wasnt running right since the last bad repair hasnt stopped the guy from using it - judging by the chain imprint (wear) in the spur gear teeth. No sense of mechnical sympathy = bigger repair bill.

I presume it had the rails with it ? Works ok once rails are set up on a straight plank, just a bit of effort to get to that stage.

Might be the picture, dunno, but that metering arm looks a tad low ? Quite a jaunty angle too, I'd be tempted to tweek it up a bit and then bend the contact face so its parallel to adjacent carb recess, maybe Im a bit OCD ??? 🙄

Always nice to be completely sure about its itegrity and tick it off the list of suspects. Onwards.....

You dont need the purge line gubbins. I'd argue youre better without and have deleted it many times on my saws. Just put a short bit of hose on the carb (in place of the purge hose) and blank the end of short hose. Blank off he purge line too. So long as that oneway valve passes fuel without restriction your now good to go. Regarding the main jet/one way valve .... small parallel punch and hammer.....tap it through into the centre of the carb. I hold the carb in vice jaws with the choke in closed posn, bit of something soft to protect the faces of the carb. Keep the punch in the midldle of the jet. The jet will pop out into centre of carb, you can retrieve and inspect jet for its one way operation. Also have a look at the drillings in the carb and the groove around the jet and see how far the jet should be installed. Just tap it back into place when you're ready, it alignes itself easily with a little care. Theres no rocket science here.

I agree with adw, the check valve (red circe) is to stop the purge bulb from sucking air through the various jets - its so much easier to suck air than fuel. Where's the fuel feed to the low speed circuit ? Is that through the same check valve (red circle) as high speed circuit ? Also agree the brass main jet looks odd, looks like its pressed in too far. Does it protrude a long way into the carb throat?

How many times do you hear a lawn mower running out of fuel, starting to hunt up and down and think to yourself just stop it and refuel. But no, they keep going until it completely dies, then after a few minutes there's the sound of a pull cord being pulled over and over.

Yes. The carb is for my toy chinese top handle which was in need of help when I got it. It was cheap as chips so after making it work and being utterly upset about how crap it was in standard trim its been though some changes. Runs great although I might have gone a tad too far with inlet timng. After that I was pursuaded by a Bellhopper video that increasing the carb size was a benefit and as part of that I moved the check valve opening towards the edge of the venturi where air flow shouId be higher. It all works fine although in all honesty I cant tell any difference in the before and after carb mods. But yes, the check valve survived being opened up, shortened some and swaged back together.

Yes, although the little retainer disc with its castlations faces inwards towards flap disc so less surface area to stick to.... if sticky is the issue....

Wonder cure probably not, cant fix broken. But carb cleaner isn't either, useful but not a cure all. I thought 'super' unleaded had some nasties in it that helped clean ? Add in a running engine ie vibration and heat plus exercise for the carb - you never know. Could be completely wrong here, its completely subjective, often feels like an engine thats not been used for a while goes better on day 2 of use. Might be softening of metering diaphragm, change in the weather or sumtin ? 😒

Point taken. My last input on check valves then, its gonna change your thinking though ! Picture below is a check valve in bits, its from a chinese carb, I was messing about with it at the time. But they are all the same. The point is .... there is no spring (or equiv) holding anything closed. The little flapper disc moves open or closed with the breeze. Like a reed valve but with zero bias.

Yes I agree it does divide the carb. I'd add this: At low speed (low throttle openings) the check valve closes off the high speed circuit completely, engine is fed from low speed circuit only. At high speed both the high speed and low speed circuits feed fuel to engine. I suppose a test for this might be to some how get the engine runnIng at a decent lick and close the low spoed screw. Look for a high no load speed.... Im gonna have to disagree with you 😨 with what happens if the check valve gets stuck open. While the throttle is closed there is ambient pressure at the high speed opening in the carb inlet port so no means of pulling fuel into the carb inlet. I suppose there might be some vacuum in the inlet port if the air filter was monsterously blocked.... just like the choke does. Edit: Actually rather than worrying about what we think is giong on with presures and all that, just have a look at the check valve in the flesh as it were. It only checks the flow from reveresing. It does not stop the flow in the forward direction, that is to say fuel flow in forward direction is the same if the check valve is operating correctly or if it is stuck open.

I've seen both ie some carbs where the hi speed screw controls all the flow and in others there is a fixed jet to control some percentage of flow and the hi screw controls an additional flow. I suspect earlier carbs fall into the "all the flow" category but dont know that for sure. The boring reality is you need to investigate each carb to fully understand it .... unless you already have experience with the specifc type of carb (I don't). Because it fluffs and will not recover makes me think that the diaphragm is not opening the metering valve yet it must to some extent for it to continue running when the throttle id held full open. Yes, metering valve must be opening at full open throttle, t'is the only way fuel can get from tank to metering camber. The metering chamber and flexible metering diaphragm should behave like a (small) reservoir of fuel able to cope with transitional behaviour - clearly not in this case. I removed the Hi jet screw today and pushed a plastic tube in, when I blew I could not get a good seal but I saw bubbles moving into the purge bulb so the air must have passed through the check valve and into the metering reservoir. I dont think it will go through the check valve, it goes through the jets or drillings into metering chamber. The check valve is on the other leg of t b e tee junction. To test further I'd suggest opening the metering chamber, blank off these hi speed drillings (or jets) with bluetack and try again, blow and suck through the hi speed screw hole and see if you can feel a difference in flow ie check valve working. Tell me... do both saws have purge fittings ? Does the purge pipe fit snuggly on the bad carb? Any air leaks here with allow the carb to suck air into the metering chamber... Similarly if the purge bulb is split. So here's the thing.... you dont need the carb pump, the check valve or the metering function to be operating 100% for the saw to run on choke or at fully open throttle - the magnitude of the vacuum in the carb inlet port will suck fuel through. At idle and low throttle, the depression in the carb inlet port is small, you need all the above to work well. Any air leak into the metering chamber eg, check valve leaking, purge bulb circuit Ieaking, acceleration circuit Ieaking, diaphragm or gasket leaking etc then air will be drawn into meter chamber in prefernce to creating a depression here which you need to open the metering valve.

"Most faulty check valves I have seen are stuck open...." airline pressure into the high speed screw hole has that effect on the one way valve. Carb cleaner ok, airline not so much. "....run very rich at low revs..... " I dont think thats right, definitely not right at idle. Check valve exists to stop air going through the the high speed circuit and into the metering chamber. Logic: at closed throttle and low revs, there's (nearly) ambient pressure where the high speed circuit enters the carb - its before the closed throttle. The metering chamber is at slight depression due to low speed circuit sucking fuel. Air wants to flow from high go low pressure, so metering chamber would fill with air, not useful. Further bad consequence is metering chamber cant get sucked into depression to move metering diaphragm / open the metering valve. ".....but the saw WILL run flat out" Oh yes ! Gloriously, given the shite part throttle behaviour. But im not convinced the problem is the check valve either. Do the pipe over the hi speed screw hole test like adw and spud described. Sounds like issue is getting fuel into metering chamber - could be check valve screwing thing up - but also the usual suspects: the wee fuel pump or the metering side ie lever height or diaphragm....

Ah-huh, consistant then. Well, definitely gettting fuel to the metering side of the carb while its on choke so its moving the diaphragm, operating the metering lever and opening the valve. Coming off choke you'd expect it to run for a few sconds while it uses up the fuel left in the metering chamber... doesnt sound like it does. On the good carb, how many turns out on the mixture screws? Here's the running on choke page, same guide....

Couple of things, Zama isnt the same as Walbro but the principles are the same. In the pictures above the air flow is from right to left ..... chinese innit. The check valve is required to work for idle conditions. It stops air from going the wrong way through the high speed nozzle / jet / check valve what ever you want to call it. With out the check valve, air from the intake in the carb would be drawn the wrong way through the check valve and high speed circuit, into the metering chamber due to the suction fom the idle circuit. Engine would run lean at idle if it ran at all. Check valve should be open for wide open throttle, its possible for it to be stuck closed, would send the engine lean as soon as throttle was opened. In your opening post, did the original saw idle ok or was that only on choke too (you described the good saw with duff carb only running on choke ... I think...). If it only ever ran on choke that would point to the pump circuit not able to supply fuel to the metering side of the carb (pump circuit isnt needed on choke, the intake depression from the engine sucks the fuel through).

This is from the Zama tech guide, you'll have to find the original source if this is not readable.

You say everything looks like new, I wonder if it ever ran properly ? Maybe there's aluminium where there should be a hole or the check valve or jet thats defective from new. There is a risk associated with this.... but you couId disassemble the good and bad carbs side by side to compare their inner workings 😃😃 Hope you fix it, worth nothing financially but it'll bring you a smidge of satisfaction.

Knee jerk reaction was; reject. Remembered I have a 390XP Hyway kit, unused, ready to fit. Had a look at that, its the same....or nearly, this one has obvious non-clean up on the exhaust side. It has next to no depth to the NCU and on the plus side its likely got the full plating depth. From a manufacturing perspective you can see how it happens - its impossible to hone to the top of the cylinder. To get around this the cylinder is machined tapered oversize at the top so honing tool will always run into cIearance. Where you get NCU a bit too far down the bore theyve got the clearance taper a bit big. We're talking microns tolerance here. Ideally you'd have a honed cylinder up as far as the rings run, dont think you need it any higher. If there is partial honed surface where the rings run it's likely ok mechanically, you might loose a little cylinder pressure. Be interesting to see what happens after a few hours running if you choose to run it. Probably not as bad as it seems, not the best. You could ask your supplier to have a look at other stock to see if they are all the same, take it from there.

Weird question, how many "internets" do you use... actually, don't bother, not really interested. Got nowt to do with this topic anyway, just wanted to hear what the OP concluded about his 'new' saw and his Stihl dealer, trustworth or not ? Maybe he has disappeared by now.

Blimey..... Forum banter aside.... did OP ever establish if there was any foul play at the dealer with an underhanded piston and cylinder swap before you bought the 'new' 291 ? Freebies (replacement piston and discount on new 261) suggests there was....

The spark plug threads dont form a seal on any brand of saw (or any engine for that matter). Its the washer that does the sealing. Some plugs use a tapered seat (no washer) to seal directly to the cylinder. If the treads hold the plug in place and hold it square to the sealing face then its either the sealing face damage or the sealing washer. Or the wrong plug....

Impulse line, no pulse arriving at the carb ? On choke the engine sucks fuel from tank because the jets in the carb are down stream from choke plate, so big vacuum. As soon as you open choke you loose that 'direct' suction and rely on the pump in the carb to deliver fuel from tank to metering side of carb. If you can repeat the - runs on choke, runs for a handful of seconds after choke and then dies out - it suggests the pump in the carb is not working. It may not even be connected with wrong carb ??? It might run on wide open throttle, may be enough vacuum to pull fuel directly from jets .... Apologies if this has been checked already...its a long thread.