bmp01

Agreed, a big flywheel can't increase the torque of the engine. But a fast spinning flywheel has more energy than the same flywheel spinning slower. So for a short period of time the output from an engine and flywheel assembly can be more than the constant speed output of the engine. ... its just that during that time the engine and flywheel will be slowing down. That might be perceived as 'more torque' by the user. A good example of this on a chainsaw is if you're unsympathatic with the saw at the start of a cut, or maybe if you hit a bit of hard wood. A saw with a big flywheel will drop its speed but if the time is short it might clear the obstacle, while a saw with a small flywheel has less stored energy to devote to the cause and bogs down. (Same as the trials car). It's the store energy that counts. Can't remember the formulae, but maybe that's just as well

Interesting point that. Purpose of a flywheel - to give a more constant crank speed, primarily useful at low engine speed and crucial for starting. Not much use above idle speed, unwanted baggage. A key benefit of the fuel injection and engine management system (which control the sparks too I'd hope) would be much better fuel/air mixture. So starting will be more consistent and generally easier, Idle will be smoother too, both especially true from cold. The knock on - Stihl engineers can afford to reduce flywheel inertia (and weight). Nice fringe benefit to fuel injection. ....

Simple answer probably about the same, no reason why the manufacturer would derate one or the other intentionally. In a bit more detail, they are both 'home owner' saws, both "clam shell" type of engines, so with out a big amount of effort there is little you can do to improve the compression ratio. There is therefore little interest in measuring their compression unless diagnosing a sick engine. If i were to hazard a guess id say the MS025/250 would have a slightly higher ratio - they both have the same spark plug so each has to accommodate that feature which is of a certain volume. That will 'hurt' the ratio of the smaller engine more but it's probably insignificant. .... HTH, BMP01

Not a recommendation as such but Lidl have a small 20 quid one on their shelves at the moment.... I know nothing about it's performance, suspect its a little small for a cylinder though. Probably pants compared to a commercial cleaners with special cleaning fluids.

Flippin' excellent ! Obviously you have a good workshop skill set to have made that. What happens when the band breaks? You're in pretty close proximity to the action, from my school woodworking days I remember there was usually quite a bang when the band broke. ....

+1 I've had mine for a couple of years, used occasionally not every day. Does what its supposed to do with a quick response and update. Please note they are secondhand from this seller though and he doesn't have them on offer all the time. Also they are sealed units so once the battery expires it's expected to be a throw away item. Don't know if it's possible to dig out the electronics/battery...

It's a stuffer then ? To move air-fuel gas around under the piston.... Piston cooling ? Fuel preparation ? Pin bore lubrication ? Or can it change the air-fuel movement in the crankcase, for better scavenging or fuel preparation? Is it solid, this bit that goes up under the piston...? Are they reducing the volume in the crankcase (if its solid) or only displacing the air-fuel volume under the piston (hollow underneath)? Got to be the result of some computer modelling I think.

Come off it mate, nobody could be that stupid. They'd get sued left, right and centre for all the injuries and deaths...... At the very least you'd have to have some sort of licencing for who'd be allowed to buy and use one, based on training and qualifications.....

Spud, No offence taken. My statement was bollox (I now realise). It needed correcting and any confusion clearing up. Not good to have doubt thrown into ones understanding of things, sorry about that. And thanks for taking the time to point out my error. Yes I'm from a racing back ground but that's not an excuse, I believed (incorrectly) all modern pistons were forged, i was wrong, simple as. I'll go and adjust the grey matter now. Forged pistons - yes the're hit with a punch but the process is a bit slower than you might imagine, there's the initial contact followed by the second stage where the punch is pressed into the material. Even though the material is heated sufficiently to make it ductile the forge is a big bit of kit, close to 100 tonnes the one I experienced, maximum of 5 inch dameter forgings. The advantage of forging over cast (generally) is to get the material grain structure aligned to the shape of the piston, for strength and durability. Getting clear part numbers on a forged piston is relatively easy. Forgings were better than machined from solid but only marginally so in the game i was involved with. With a forging there's no way to get windows in the piston side panels nor the undercut below the g'pin boss, unless they're machined afterwards. So there you go chainsaw pistons often have both those features... must be cast. Fancy split tooling as well, to achieve that, split lines are the cause of the flash lines you see.

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.