bmp01

Think I'm with adw on this, the metal bits that are made hard in manufacture by heat treatment have most likely been re-heat treated - meaning they will now be softer than they should be and not last. The scortched paint tells where it got hot. The thing to note; its not just hot where it was pinched .... I'd say the (nose) sprocket itself got hot and subsequenly transferred the heat to the near by bar, hence the circular pattern of scorched paint. For the little sprocket to put that amount of heat into the relatively bIg bar I'd say it got very hot. And if the sprocket got hot so did the needle rollers, these are (were ?) both hardened material parts. If you take the chain off and clean / degrease the nose spocket can you see if it has gone blue ? Edit: heat pattern is inboard of the nose sprocket, looks like it matches with where the rolling bearing is....

Ah, thanks for feedback good to have some run time on it. T'is the final confirmation the gremlins have been driven away.

So swap the old 'top half' on to the new non purge bulb 'bottom half' ? That'd work I think and not much to go wrong with 'top half'. Think I'd ditch the purge bulb if new was the only option. But just fix the old carb, such a waste and expense to bin it.

A nice tell-tale, if you are prepared to do a bit of disassembly, is to drip some Redex into the accelerator piston hole behind the throttle shaft and see if you can suck it into the hi speed circuit. I tend to blank off the drillings in the metering chamber with blutack, remove hi speed screw and suck through there via soft tube. Any time you see red coming through the carb you know the accelerator piston seal is leaking.

No worries, pleased to be able to help

And so it starts......

Thanks for posting your findings and well done in doing the comparison between good and bad carbs, you have some certainty there. So, these valves can go wrong in a range from the working parts 'just get stuck' through to parts completely missing. The latter is often the result of high pressure air being blasted through the adjuster screw holes. There's a chance you can resurrect a 'just stuck' valve with carb cleaner, petrol, etc or ultra-sonic cleaning - basically you are just freeing up a tiny disk that 'floats' in the valve. Remove the low speed screw to access.... start with low pressures and progress, not much to loose at this point 😈 😨

Ok, I hadnt realised that, apologies. The point about the purge bulb pulling in air is still valid - it shouldnt do that. As stated before either hose fit on the purge bulb fittings is loose or the bulb itself is leaking. Always worth fixing things like this before they create a problem or symptoms you don't understand. ...

I'm confused by your experiment and statement that its not sucking fuel. I see fuel and air bubbles in the return line. So it must be sucking some fuel through the carb, no ? I don't see any air bubbles in the line between carb and purge bulb, is that all fuel? Edit: Looks like 100% fuel in the line between carb and purge bulb. I agree with Openspaceman, you have an air leak around the purge bulb, might just be a loose fitting hose on purge bulb, or a cracked leg on the purge bulb....

So you've got 2 fuel circuits in the carb, a low speed and a high speed. Low speed circuit is fed via the bung in your blue circle and there is a little flap check valve on t'side. High speed circuit is fed via the small hole below the main jet / check valve assy. If you have a piece of small dia hose, preferably soft silicone, you can hold one end over the entry hole and blow / suck to confirm the check valve operation. Adjustment screws need to be NOT bottomed out. Low speed circuit will flow a lot less than the high circuit....

...this was a running saw... Not a nice running saw but still a runner.

No it isn't. I didn't spot this from the pull cord compression test, held its own for over a minute. Compression test was 135 psi - low for sure but I passed it off in my mind because I had to use 10mm adapter for the gauge. Aw well, you win some, just not this one.

I was just going off this. ... post 1

Sorry for the delay, that 201 (up the page somewhere) isn't going to run right for a little time yet. Needs a piston that measures a bit more than 39.70mm across the skirt. Probably explains why the idle screw had to be wound in some to get it to run. Cylinder has cleaned up surpisingly well.

Yeah, block them both off if you have concerns about the bulb. It's not necessary to run the saw. If the one way valve in the bulb isn't working 100% then the carb sees tank pressure which definitely upsets the apple cart. Also, just check that release of tank pressure you mentioned in post 1 was positive pressure and not vacuum.... hard to tell difference in sound between gas coming out or going in. Of course, if the fuel cap popped out ....

....time to f'kin learn then... 😈 It isn't hard or any sort of magic by the way. I understand there are plenty of videos on that well known video site.

Why would you not adjust the carb if the saw needs it ? You might be missing out on some of the potential performance gain... (And avoid seizing the thing....)

T'is a risk. Care to share details of std seal, is there a part no ? I've looked on 200t parts list but no joy there.

Yeah, I agree regular sizes would be preferable.... the original oring is a little over 1.0mm section I think (hard to be sure it's degraded and changed shape etc). Wouldn't be hard to machine up a new piston to take 1.0mm section Oring but at that stage might as well just buy new from Stihl.

Thank you ! Ever tried replacing just the O ring ? Piston groove: Dia = 3.17, Width = 1.20 Bore in carb 5.0 O ring with ID 3.0, Section 1.1 Viton material would be about right.... probably unobtanium.

MS201 carb in bits. Air leak on the hi circuit. Hi jet (check valve) and accel piston removed. Hi jet is fine, accel piston seal is smaller than the piston (see piccy 2) and looking a bit shabby. That'll be it then. Apparently you cant buy a replacement accel piston assy from Stihl though..... Just needs a new O ring seal really.

Ah ok hadnt related throttle posn, thought it was all wide open throttle ... So are you saying it will it hold high rpm on part throttle for a good length of time but as soon as you go wide open throttle it falls over? That would be consistant with your blocked strato port expt. Suggests ignition is ok but not 100% certain... The accel pump is just a small spring loaded piston which gets pushed by a notch on the throttle shaft to squeeze extra fuel into the high speed circuit as throttle is opened. The piston is sealed in its bore by an o ring. The o ring wears, splits or degrades with bad fuel, piston and bore also wear in bad cases. So the problems arise because there is then an (small) air leak path across the accel piston into the high speed fuel circuit. Poor or inconsistant throttle response are the usual. Not sure it would do what you are seeing but then again who knows... Not a million miles from Wellingborough. If your mate travels up the M1 I'm a mile from M1 Jn 16 if that helps. Can sort this out via PM if you want. Reward is in helping fix the bugger !

Might be me but didn't the saw rev up, then start misfiring and drop revs ??? Agree with Spud that sounded rich at idle and the initial rev up didn't sound clear or particularly high revving. Was that from cold, any different warm? I wouldn't rule out spark plug / ignition trouble for the misfiring from high speed. Broken plug I had on a MS211 made saw impossible to start though, plug was arcing internally through the ceramic. Again as Spud says if blocking the strato port allows it to rev cleanly then its not ignition. Got a 181 here if you want me to test carb, coil on it. Would need posting, Northants. Last thing, this range of saws are pretty mediocre on compression from new, I've found high comp engines are more tolerant to fueling being a little out of wack. Have you checked compression?

Yep that's the sort of thing. I've got a bit of a thing against purge valves this week. ... Small 26cc top handle (chinese) was running ok'ish, put it down for 10 seconds and struggled to start it. When it did finally stumble into life, barely clearing itself, it was pig rich. This was boringly repeatable. Found nothing untoward in the carb - bit of a surprise. The light bulb moment was realising the purge bulb, while basically doing its job - for the first instant of pressure on the bulb flowed in the wrong direction (this was during diagnosis, not hot restarts before someone says..). Turns out the one way valve inside the bulb was perished. Not until there was significant pressure on the purge bulb did its internal valve seal. So, in this case the metering chamber in the carb (that oh so critical control parameter) was more or less connected directly to the fuel tank..... and seeing fuel tank pressure. Further, as the primer bulb fuel return line dipped below the fuel level in a brimmed tank, when engine was switched off the tank pressure (from heat soak) was back flowing fuel into the metering chamber, flooding the engine. I guess I had previously considered the purge lines being 'after' the carb - they couldn't really upset the carbs operation. This week I've changed my mind. A few extra pulls on the start cord and the simplicity of a carb without extra gubbins definitely trumps the 'easy start' sales pitch.