bmp01

1 hour ago, Wonky said:

Umm.., ask me how I know that..?

 

Blimey Wonky, I bet the performance of the saw was a bit up and down. .. How long did it run for - do you think - to create that amount of damage?

And errr, was the nut a bit loose by any chance ???

BMP01

1 hour ago, Gerbutt said:

What’s ope?

Outdoor Power Equipment forum....

American forum,  mad keen on tuning amongst other stuff.  I believe the forum is moderated by / run by / owned by (delete as appropriate) a well respected tuning guy....

On 26/01/2019 at 09:49, Gerbutt said:

 

Hiya

 

I’ve been pondering about timing advance and I understand what happens when you do it, it sparks a little earlier. What I don’t understand is why this is an advantage and how come it seems to give more power? I’ve seen vids where a 200/201 had been done and it looked quite a lot better, faster in the cut and holding rpm better. Why is this?

 

Why does saw go better with a bit more advance ? Here's my go at a simple explanation. ...

1) There's an optimum igntion advance for performance. 

2) What's optimum for one engine might be too much for another dependant on engine tolerances,  engine maintenance,  bad fuels,  bad tuning., etc, etc

3) Manufacturers don't like their saws exploding,  gives them a bad reputation, sales don't do too well, so they design in a bit of safety and retard the ignition a bit.

4) Advancing the ignition a small amount is just getting back to optimum.  

I asked Matt the same question via the old messaging system. I think he was happy with it. He did some YouTube videos of it,  I don't have the link he sent me any more but going by the dates on this thread and a quick search on YouTube I think this was it. ...

h t t p s://youtu.be/l_1Jkg7AWWI  (remove the obvious gaps in https, if I put it in correctly it auto plays!)

Very difficult to judge the saws cutting performance,  wood type,  chain condition blar, blar,  but it seems pretty responsive. Saw was obviously far from new at the outset.

Matt told me he was visiting other chainsaw sites,  American, for most of his saw tuning games, don't know if he's still on here. ...

HTH.

9 hours ago, Gerbutt said:

One thing I am noticing is that the squish band isn’t flat. It’s consistent around the cylinder but is smaller beside the cylinder wall and then gets wider, 0.36 to 0.6. Is the squish band normally 3mm wide and then drop off?

If I'm understanding you correctly that sounds pretty good to me. The idea of squish band is to squish the gas mixture towards the centre of the cylinder, to promote good turbulence and rapid combustion. 

What you don't want is "end gas" stuck down the end of a narrow passageway where the combustion struggles to get to, that would be a waste of energy and won't do much for performance. You will get this if the squish band is too deep (radially).

Edit: whether 3mm is the right number is one for the experienced tuners, I think I would punt for a slightly bigger number,   say 5 or 6mm, depends somewhat on bore size and the combustion chamber shape.

56 minutes ago, openspaceman said:

It doesn't increase the toque, that's dependent on the combustion but it stores inertia so carries power over, sort of smooths out the impulses.. In the old trials cars with fiddle brakes they would have a small engine with an extra flywheel bolted on just to allow it to run slowly but still able to power over obstacles. You want the very opposite in a chainsaw.

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 

On 09/01/2019 at 21:27, spudulike said:

It must be more than the weight of a carb, they don't weigh that much. Was the flywheel a nylon one like the MS660? Always looked at that as being a decent idea, less inertia, less weight and quicker spool up time! Reckon Stihl have thrown a lot at weight saving throughout the saw!

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. 

On 16/11/2018 at 21:43, astra25 said:

Hi here's the mill I made just thought it would be of interest to you.

 

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. ....

3 hours ago, BishBashBosh said:

Husqvarna Pulse Engine Tachometer PET-302R 

 

£20 from ebay. It's a Husky badged Oppama PET-302r, easy to use and reliable.

+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. 

11 hours ago, Squaredy said:

Can you believe they fit chains like that to bars attached to petrol engines and four foot guide bars????!!!!!!!!!

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.....

2 hours ago, spudulike said:

Something nagged at me on this one. Couple of things - when I look at pistons, I see casting flash, slight unevenness in inner surfaces or casting witness marks and then machining to the critical surfaces and parts. The piston body shows obvious signs of casting - the machining will be common to all pistons.

I then remember my days on bikes and being able to fit Wiseco "Forged" pistons and then thought, if all pistons are forged, why make such a thing out of being forged.

So...basically, cast is molten metal in to a moulding cast and forged is battering solid metal using heat to soften and "forge" shape - like a blacksmith making horseshoes!

I then Googled it and most pistons are cast but the top end ones for racing or high revving engines are forged.....ah, reconciliation in my head once more 

Sorry but your statement is wrong, perhaps you have come from only using race pistons - come across them in bike and car track racing and drag racing then assumed.......

No offence meant and don't intend any ridicule but this one just "niggled" at what I understood to be fact!

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. 

12 hours ago, spudulike said:

On the MS461, there is a strange alloy bracket that bolts across the crankcase mouth and goes right up in to the piston. There has been a lot of speculation on what it does but the clever money is on dome sort of diverter creating a strato effect on a relatively standard engine design. See pic below.

I believe that the piston probably had a very fine crack in it after I tried to remove the central boss from the clutch after an employee of the saws owner didn't fix the e clip on correctly. The clutch grenaded and did the boss up super tight. I used pretty much all the force I could muster to shift it and ground it off in the end with a diamond burr after I couldn't shift it - it lasted a few months before letting loose. The stop I used was the Stihl one followed by rope when the damn thing didn't shift.

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.

37 minutes ago, spudulike said:

Now he is calling us members - isn't a member another name for a blokes appendage.....i.e. cock 

 

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

23 hours ago, peatff said:

 

I over simplified it but in my understanding he asked about what the impulse hose does and it does all the pumping and causes the metering which is the part that is affecting the fact that his saw won't run. Apart from it being Chinese.

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.

26 minutes ago, donnk said:

jesus.

 

HSE will have a field day.

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. 

9 minutes ago, EricBradley said:

Still wont make any difference but it is actually 30 thou difference not 3 thou

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.....