bmp01

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. 

4 hours ago, tenner said:

Blocking the carb's accelerator pump: this does work but not for long in my case and you can't un-epoxy it to rebuild the pump afterwards, no? I got another 10 tanks or so out of my old carb, but performance was noticeably reduced and eventually the un-tuneable and airleaky symptoms returned so I had to spend more time/money working on it anyway. Got a brand new OEM carb, night and day difference.
 

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.

3 hours ago, Crocky said:

I cut it off when I’m cutting the base on them too. It does no harm. Crank case still feeds the transfers the same. 

 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. 

2 hours ago, sandspider said:

I'd guess mine is rev limited, it won't rev above a certain level either in wood or out of it. Can I just drill a few holes in the spark mesh? I assumed I could take it out and put it back in if necessary, but apparently not.

Hefty cat in there too, but that I will leave alone.

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 ??

3 hours ago, spudulike said:

I think the OP has pissed off in to the distance!

Weird, that's never happened before. 

16 hours ago, openspaceman said:

I had a similar sort of problem with a ms181C which @bmp01 diagnosed as an air  leak past the accelerator pump (as I recall), he plugged the thing as a least cost repair and the saw was returned to its owner, still running last I saw it.

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.

9 hours ago, spudulike said:

Good question but can't answer it. I have often wondered the same but on the top end. The compression must be phenomenal compared to the start up pull over at 14,000 rpm.  

I guess if you tried to measure it, all you would get is an average pressure or vacuum as it all happens so quickly.

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.

On 25/05/2023 at 22:17, Trailoftears said:

A thought provoking topic.I'm no oil expert.But if you look at cars we have moved from mineral to semi synth,now fully synth oils.No modern car's service schedule would spec mineral oils I assume?Again,I assume fully synth oils flow/warm quicker due to their lower viscosity?So minimise start up/cold engine wear due to better initial warm-up/ early movement thru the engine.Which sounds logical....

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.

I've edited your pictures with red circle to show where the impulse signal is going through to the carb.  So the impulse line needs to connect to engine side of the orange plastic, which is what Chris stated 2 posts back....

As per adw's question a few posts above how does the impulse signal connect to the carb on the MS261 ?

You need an impulse pressure signal to make the pump side of the carb work. The pressure pulses causes the membrane to move in and out ---> fuel is pumped through the one way flapper valves.

Worth noting:

-Engine will run on choke without the impulse pressure signal because the closed choke causes sufficient vacuum to suck fuel directly from tank.

-Engine will often run at wide open throttle without the impulse signal because the high engine speed often causes  sufficuent vacuum in the carb ventui to suck fuel directly from tank.

-Most other scenarios,  no impulse pressure signal usually means engine wont run.

1 hour ago, Stubby said:

There was someone on here a while back that ported a " Farmertec " saw . It was all sorta electric blue color and badged as Farmertec . I think it was a copy of a 372XP . Apparently it ran well .

I think the blue ones, which are fairly recent, are already assembled ?

Each 'new' model they bring out is supposely better than the previous, but old models were pretty dire so might not be saying much. I've not touched one so that's now third hand reporting 😃

25 minutes ago, chopchop said:

No that's fair enough and I know that deep down. Might just park it in the garage. The work, hassle and investment I'd be happy with if it was genuine. But I'm looking at trying to fix a saw I have little confidence in working no matter what I do. Different with a stihl as you know they will work if you work through the diagnostic process. 

If you get to parking it I'd be interested in buying it, I'd like to see the 'naff' quality of a 'Farmertec' kit saw for myself.

Reasonably confident it will run by the time I've finished with it, might even run quite well.

PM me if this of interest.

Going back to why saw won't fire even when you've added fuel; 

Have you swapped in a good spark plug ?

Have you tried getting the ignition system to jump a big plug gap ? It's sometimes the case the ignition appears to work with the standard plug out of the engine but once plug is under pressure in the cylinder it falls over.

WRT timing - what matters is piston position when flywheel is passing the coil. There will be variations in coil design and in the advance curves within the electronics of the coil but you'll get some idea if things are about right if you compare this one with your 260.

Good luck.

Well this is a fun thread.

There's only one rule - you have to let us know when you get it running and report on your findings.

Personally I've never seen any trouble with bearings from running wrong oil/petrol mix. Plenty of piston problems of course but even those saws that seize pistons, more often than not bearings are ok after a good clean.

First thing to go with wrong oil mix is the piston - be interesting with this one to pull the exhaust off and take a look at piston skirt and piston rings.

That last bit might undo your plan....not sure a 6T rim sprocket exists.

Dont forget the outside diameter of the rim sprocket is proportional to number of teeth - that's to say a 6 T rim might be too small to function on the splined clutch drum.

Happy to be proved wrong.....

On 21/02/2023 at 00:46, Macpherson said:

 

Aye, from my useage of the stuff the most important thing to take onboard while getting it to bond is just how quickly many alloys form an oxidised surface layer and the utmost importance of using a stainless wire brush to avoid ferrous particles depositing in the repair and cleaning the area of all oils including finger prints.. Isopropyl alc or Acetone do that. 

 

And if you get a bit 'tacked' and go to set about the next bit... just give it another brush to keep the surface exposed and fresh while remembering all the time that all you've already done will run away like watery  solder if you over heat it.

 

You'll be surprised just how hard any excess material that you have to dress afterwards is compared to an  Alloy casting and I'm also sure that if you manage to get the hang of it you'll find many instances in life where this is the best repair option... Cheers. 

 

BTW. you will likely use more rods than you would think to do a small repair but any spillage can be saved for the next time and reused.

Thanks for the tips. 

I can feel some repair work coming on. I've got a busted pole saw head in need of fixing too.

I agree with Ramsey's experience, toolless tensioner is simply inadequate, I've dug into it in the past to look for a solution .... failed.

The standard single bolt / nut is just fine though.

Good news is you can sell the secondhand toolless tensioner parts on ebay  😃

Cranks not rotating though when youre pulling the chain round by hand.

Points to eccentricity in the clutch housing or rim sprocket. Or the interface between the two - the rim sprocket is a loose fit on the clutch so it will 'walk' around on the drive teeth as the chain is pulled round by hand (probably locks up in one position when the engine is turning it over at speed).