bmp01

Thanks for the explanation of that strato flange, much appreciated. Not seen anything like that before, weird. Not something a manufacturer would add in unless there was some real benefit - performance or emissions. I wonder what happens if it's left out. ... Regarding piston stops - if you had to reinvent the wheel for removing clutches you'd never choose to stop the crank turning by blocking the piston movement would you ? It's only because someone tried in the dim and distant past - while working on some ancient over engineered lump - that the procedure exists today. Time for a crank stop designed in on new engines... Makes me cringe every time, imagining the forces going through pistons, bearings etc.

Pistons are forged (not cast). You need to look for the classic signs of fatigue failure to identify where the failure started - if you can be bothered. Its where a crack exists while the part are deflecting and rubbing across the crack surface, causing a distinctive pattern, often crescent shaped. This happens while the engine is running and can be hundreds of thousands of engine cycles. Bits that shatter during the last few hundred revs don't have that appearance. Trouble is most of the evidence is often destroyed by subsequent impact damage. What's a strato flange and what does it bolt to ? Barrel?

I understand a carb needs compensation to cope with clogged air filter. That's because a clogged filter causes a depression in the airbox, which adds to the depression in the carb making a bigger depression. And a carb supplies fuel in proportion to that depression. So less airflow through the engine (due to clogged filter) but a carb which is being asked to supply more fuel..... so too much fuel in the air - fuel mixture. This is improved upon by changIng the reference pressure seen by the metering diaphragm. Normally the reference pressure is ambient pressure (that's why the metering diaphram cover has a hole through it). On a compensated carb the reference pressure is taken from the airbox (or filter housing) where the small depression exists. That in turn reduces the pressure difference across the fuel circuit in the carb so less fuel is supplied to the engine. Worth remembering the engine is still derated with a dirty filter because the engine can't pull the same air (and fuel) through it compared to a clean filter situation. I don't understand why altitude affects engine fueling as it should affect air flow AND the reference pressure on the carb, so less air and less fuel. And I really don't understand why you need to adjust the carb (add fuel) for a tuned engine. Ok, so tuned engIne needs more air, but if the carb worked properly, it should add in more fuel with out manual intervention. Are carbs just operating in the constraints of size, weight and cost and are therefore not perfect devices? Did some one mention Fuel Injection.... Food for thought.

I think it's good people take the time to explain how stuff works. I think how it's presented makes a huge difference to how easy it is to understand. I'm inquisitive enough to have messed with old SU carbs in the past and more recently 2 stroke carbs. SU's were about as simple as it comes so it was an easy introduction, adapting to 2 stroke carbs was then a relatively small step. But here's the thing - I understand a 2 stroke carb and therefore I can read what other members write (or 'cocks' if you prefer - ref posts above) and decide if its correct or not. To a newbie, a carb might be a thing that mixes fuel and air ... it will be a lesson just to appreciate it has a minature pump in it too. Talk of 'metering diaphragms', 'needle valves' or any other important widget isnt necessarily going to cause a light bulb moment. I like the YouTube video posted by Peatff, it assumes little or no knowledge but does go into some detail. There's no way anyone can match that with words alone..... Ought to be a sticky IMHO.

Spud, don't be a cock, ye cocky 'ol cock ....

That's a good video, explains it well. Unfortunately the guy said 'metering diaphragm' once by accident when he meant 'pump diaphragm' , maybe that's what has caused confusion amongst respected members on here? Can't say I've seen 3 one way valves in the pump most have 2 flap valves. That aside, really good schematic, model and voice over.

Can't see how you can do that final cut without stopping the traffic ....

Wow ! Looks to me like like the cyclist was awake (before the 'event' I mean). Road position and leaning forward on the bike. Even so, there must have been a few extra heart beats ....

Stihl piston on the right hand side would be my guess ???

..... just turned blue reading that lot in one go..... ? But no sorry, no spare carb. Sounds like a fun project.

No it isnt. Depth is typically 0.025" (and not 0.25" as mentioned above). T'would be some scary contraption to rip a 1/4 inch cut per tooth.....

Yeah, we should be..... talking thousandths.....

Good point, that's a pretty awesome fact too. 72 bpm, (resting average heart rate) to 70 years old. .. ----> 2650,000,000 beats Active person might be more although at rest heart rate probably lower than the average.....

So what are you saying here ? Why didn't the fuel lines you fitted at the start of this thread not fix the problem - bad material or wrong diameter hose?

Had a similar 'project' on my mind, I had concluded the sprocket material was chuffing hard, too hard to machine with HSS tooling. But you've shown there is a way, so what machine tool material did you use ? The sprocket I wanted to machine is one of them power metallurgy things, I think.

Its mind boggling the number of revolutions an engine will do. I just did a similar finger in the air calculation for a car engine. ... 100,000 miles in the engines life 45 miles per hr average speed ---->2222 hrs ------>133,333 minutes 2000 rpm average engine speed ------> 266,666,667 cycles Pretty amazing you can have bits of metal rubbing against each other and survive that number of cycles.

I picked one of these up recently. It was a curiosity thing, 12 quid to have a look, so..... Non runner reasonable compression, and spark. 2 out of the 3 chains had been used - the guy probably got his money out of it. In this case the problem was with the fuel line, it was in two halves. Hose material was complete shite, utter, utter shite. So it works, cuts wood ok - i guess. Maybe i got lucky, it was a simple fix for a few penny's. But then again maybe not, I don't think it will be long before the quality of some other bit causes bother...

Think I'd like a middle support too. I quite like the set up. I don't there think there is a big difference between sturdy rails and a ladder. Twisting a ladder is pretty easy, the sides are not that well attached to the rungs. Steel is obviously stiffer than aluminium .... I wonder if the chap has some means of stopping the rails from travelling down the length of the trunk with gravity and the vibration ?

+ 1 With particular emphasis on getting the cord to do a lap of the piston - spread the load as much as possible.

Turning it back wards against the starter mechanism/cord ? Was it a sharp stop or did it have a bit of sponginess to it ?

Ya. That was my starting point too, then ...... way too many options. Saw looks clean, low hours I'd guess but what's compression like when its hot? Pick up the saw by the pull cord - does it drop quickly unwinding the cord or can you pick up the saw and watch the saw drop slowly as the pull cord unwinds through each compression cycle? What are you doing with the choke and fast idle when you attempt to start from hot? Get a new spark plug in it. If you are testing for spark you need the spark to jump a 3 mm gap in fresh air, do that with cold and with hot engine (heat soak the coil). If you can't come up with a scheme or tester use a known good plug and bend the outer electrode to give the 3mm gap. What happens if you drop a tiny bit or fuel down the intake and try to start it? Something to be going on with.

Uh ok.... That probably happened when you pulled the cap off, you've just finished it off stretching it to put it back on. There's always Mr Spud's repair service by mail if that saves you time and expense, I hear he likes to see a clean saw now and then.

There is a right and wrong way. ... First easy thing is to look at the throttle shaft and the throttle plate for the shadows and clean patches where there has been exposure to /air and contact between parts respectively. Looking at that throttle plate, the hole on the shaft should have left a nice tell tale circle on the shaft. Secondly, the outside 'diameter' of the plate isn't perpendicular to the plate surface, because in the closed throttle position it's slightly off 90 degrees from the port centreline. HTH

Breather just let's air into the fuel tank - so you don't end up with a vacuum in there as the engine uses the fuel. Completely blocked and you'd not restart the saw. ... partly blocked, maybe Quick and dirty test - no fear of breaking anything - run the saw up until it misbehaves, switch it off, undo the fuel cap, fuel cap back on, start it up and retest. If you can, try to note what happens as you open the fuel cap, air being sucked in or expanding fuel being blown out. ... Also worth noting a blocked (or part blocked) breather gives different symptoms with a full tank compared to a nearly empty tank. It'll be quicker for you to do the test than it took me to type this ?

Smart-arse , I thought we were clear of journalists type misquotes on here ... 'electronic raquet zapper' then just for you Yournamehere. Interesting point though, bats vs wasps, purpose vs agrivation ???