bmp01

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.

I like Joe's approach, throws everything at the saw and backs it up with some before and after facts. The only thing missing is the transient response of the saw which can get a bit messed up with fueling. Given that he has the dyno to measure the improvements he will continue to Iearn though... Unusual power curve shape on that 201, something's a little out of tune with the rest of the saw timing.

Yeah, I'm in the 'blank the accelerator circuit' camp but using epoxy never filled me with confidence, excuse the pun. Making an interference bung and stuffing it down the drilled hole vacated by the accelerator piston does it for me.

Emmm.... whats if for then ? Obviously that transfer port extension makes the cylinder more difficult to make, more expensive, you wouldnt make it like that unless there was a reason... I've not really thought about the length of a transfer port as being critical for tuning to a particular engine speed, but I wonder if thats what its about (these modern transfer ports are a lot different to those on pre strato saws).

Well, I cant Iet this thread die with that last post and no comment, expected some sort of critism at least . So here's another picture of the 'decked' cylinder, that funny shaped bit above is an extension of the transfer port but you cant get a decent radius on it 'coz its so thin. It gets right in the way if you want to machine the cylinder to get good squish numbers too. Good or bad thing to shorten the transfer port ?

Worth noting, in theory it is possible for the engine to suck loose debris from the exhaust back into the cylinder, although even less likely if it's t'side of a catalyst. Use an air line or what every you have to blow out swarf or grinding dust from exhaust. Better safe than sorry.

It's a one way trip isn't it ? Do what ever is easiest with the tools you have. Drill and dremel work for me when opening exhaust outlets, removing dividers or spark screens etc. Pair of pliers and a small hook to remove thick wire ball from a chinese top handle exhaust, that wasnt catalyst wire but it wanted gone all the same.

Had this bit left over after playing with early MS201T 😄

That's a nice bit of workmanship ! Somewhat louder ??

Weird, that's never happened before.

Yep. Accelerator pump circuit is a good thing as new - nice throttle response. Sadly it doesnt stay that way forever and has a lot to answer for in terms of irregular or intermitant poor running. At the end of the day its a bit of unnecessary baggage to help with emissions legislations. But just deleting it doesnt always work because the rest of the carb is setup to work along side a functioning accelerator pump. Accelerator pumps didnt always exist though; it is possible to get decent throttle response with out one. As Spud says best make sure nothing else is wrong with saw before going down this rabbit hole.

14.5 psi to 1 bar 1000 psi = 69 bar in the ball park, maybe a little high.

Old tech by now, cylinder pressure measurement in high speed 4 stroke engines was done with piezo electric pressure transducers couple of decades ago. Getting a real pressure trace was possible, relating pressure trace to crank angle less convincing. Ball park peak pressures 100 -110 bar at max torque, engines had static compresion ratio 11, 12, 13, that sort of thing. I'd punt at 50-60% of that for decent brand 2 stroke engine, maybe 75% for a tuned 2 stroke engine.

Sounds like blocked exhaust to me (adw there first). But not high use .... exhaust blocked with some creatures nest or something ? Edit : could be insufficient air going in if its going rich (smoking) with open throttle but filters ok so blocked air inlet, ie before the filter.

Oil spec on current car engines is all about fuel economy and emissions reduction (so thin oils for lower viscous drag) - not necessarily whats best for engine longevity. But then again modern engines are generaly good enough to last the warantee period and then some so theres some flexibility there.