Long Cuts = Leaning Out?

[Old2stroke]

Brings up the question of how can you tell if your high speed tune is correct. After the saw has been cutting for awhile and is all cleaned out from cold start richness and seems to be running fine, during a long cut take the load off the saw (keep throttle open) by lifting the saw up slightly in the cut and verify that the saw goes to 4-stroking right away. If it doesn't, it's too lean and will be at risk of overheating during sustained long cuts.

 

[huskihl]

More work.....because it is overheating?

The amount of work (a wooden log....bar lenght....etc) stays the same in the/troughout the process....?

Heat soak.
Under normal circumstances, a 50cc saw cuts say…12” rounds. Every 15 seconds or so the saw gets a break for 5-20 seconds. During that time the motor and its case cool down. Under continuous use, the motor and case don’t get a chance to cool down as often so they build more heat and continue to run leaner unless fuel is added to cool the motor. This is why milling is so hard on a saw and why extra fuel and oil is recommended. A saw on a mill might run 15–20 minutes straight until it runs out of fuel before it gets a break.

In my experience, as long as it has proper cooling designed into the motor and the fuel isn’t stale and is of sufficient octane, you should always be able to add enough fuel to overcome an overheat situation.

 

[bwalker]

The reason you have never had it happen is because you don't work on anything.

I have not seen it on a chainsaw. But, see it literally every week on 2 cycle blowers.

Take the plug out and blow fuel two feet in the air. You can blow air through the plug hole and blow fuel out the exhaust. See it over and over. And yes, they were locked up.

And, it's not my problem that you can't type.

Funny thing me thinking you are actually that clueless about outdoor power equipment.

I haven't seen it because it's not common.

 

[bwalker]

You can make a vacuum in an running engine yes by closing the butterfly.
A Diesel engine doesn't have a butterfly thus produces zero vacuum to get around this a vacuum pump is usually built into say the alternator like Toyota does that way ya at least have vacuum for brakes and other accessories.
This is basic naturally aspirated engine stuff is no sucking going on air is pushed in look it up It's a common misconception that naturally aspirated engines suck air in.

That's not the case.. the piston on its up stroke creates lower than atmospheric pressure in the crankcase IE a vacuum. A two cycle motor is a simple air pump after all.

 

[bwalker]

Now, this is more your speed. Babbling about something.

It must be a hell of a burden, being both old and stupid.

 

[Bob Hedgecutter]

Junk V Walker.......

 

[grizz55chev]

Junk V Walker.......

!!!

 

[rogue60]

I remember watching that video of when Stihl was developing the ms461 as a concrete saw they had it under load on some kind of dyno. There was a massive fuel tank few hundred litres hooked up to the saw they would top off when need anyway the saw was run for a few thousand hrs non stop exactly how many I don't remember or if they run it to failure.
In the end Stihl deemed the ms461 will run 100% as it was designed for 2000 hrs. Of course that was in perfect conditions for a concrete saw I'm sure they weren't introducing concrete dust via the intake.

This is it apparently it's called the Rock Boss

 

[Kenskip1]

Is the gas tank vented?

 

[grizz55chev]

Is the gas tank vented?

See post #10

 

[Maintenance supervisor]

Yep spot on, literally 1/16 of a turn richer. My question wasn’t really about tuning, I’m happy with the process and know what to do, I was just really curious to know, why does a hot engine require more fuel to run correctly. I just don’t understand the science behind it that’s all.

When i was researching supercharging for a v8 I read alot about air molecule density and heat.
A cold engine causes fuel to become droplets and its an overly rich mix, heat will vaporize fuel into a steam but since heat increases the space between the o2 molecules the fuel can have a difficult time linking up to a molecule, which is required for combustion. Note the popularity of NOS in drag racing. The sweet spot is when heat vaporizes the fuel but the incoming air is cold.
I've had several saws with symptoms like this that can range from, high speed check valve, kinked impulse line, stiff diaphragms,bad fuel filter, boiling fuel in the tank(XL12)and a blocked air filter. They are all fuel delivery problems .

 

[Vintage Engine Repairs]

When i was researching supercharging for a v8 I read alot about air molecule density and heat.
A cold engine causes fuel to become droplets and its an overly rich mix, heat will vaporize fuel into a steam but since heat increases the space between the o2 molecules the fuel can have a difficult time linking up to a molecule, which is required for combustion. Note the popularity of NOS in drag racing. The sweet spot is when heat vaporizes the fuel but the incoming air is cold.
I've had several saws with symptoms like this that can range from, high speed check valve, kinked impulse line, stiff diaphragms,bad fuel filter, boiling fuel in the tank(XL12)and a blocked air filter. They are all fuel delivery problems .

That’s fantastic info, thanks!

 

[rogue60]

That's not the case.. the piston on its up stroke creates lower than atmospheric pressure in the crankcase IE a vacuum. A two cycle motor is a simple air pump after all.

Yes that's right the lower than atmospheric pressure crankcase now gets filled buy the rushing in higher ambient air pressure. This "vacuum" is not pulling in the atmosphere it's merely making somewhere for the higher ambient atmospheric pressure to fill.
Moving on lol

 

[Maintenance supervisor]

Yes that's right the lower than atmospheric pressure crankcase now gets filled buy the rushing in higher ambient air pressure. This "vacuum" is not pulling in the atmosphere it's merely making somewhere for the higher ambient atmospheric pressure to fill.
Moving on lol

That may be true for little single cylinder engines but the long tube runners of a mutli cylinder engine can create a wave pulse that create a supercharged effect.
Similar to the way pulse of a 2stroke pipe.

 

[OM617YOTA]

That may be true for little single cylinder engines but the long tube runners of a mutli cylinder engine can create a wave pulse that create a supercharged effect.
Similar to the way pulse of a 2stroke pipe.

Properly tuned headers on a multi cylinder engine don't pressurize the intake above ambient pressure. They creat a small lower than ambient pressure pulse that the next cylinder exhausts into, so exhaust is flowing from high cylinder pressure into lower than ambient pressure, instead of from high cylinder pressure into ambient pressure. Greater pressure differential= greater flow.

Two stroke pipe tuning is wizardry, there might be some pressure wave bounce magic pushing some exhausted fuel/air mix back into the cylinder. That one's beyond my pay grade.

 

[rogue60]

That may be true for little single cylinder engines but the long tube runners of a mutli cylinder engine can create a wave pulse that create a supercharged effect.
Similar to the way pulse of a 2stroke pipe.

Yeah all kinds of clever stuff has been designed into modern naturally aspirated vehicle intake manifolds to get a little more than ambient air pressure has to offer. 2T's aswell remember the boost bottles of the 80's used to see them on Yamaha's
But if it wasn't for atmospheric air pressure non of our engines would even run, if air pressure was zero we wouldn't even be able to breathe even if the air was saturated with 100% oxygen.

 

[Maintenance supervisor]

Yeah all kinds of clever stuff has been designed into modern naturally aspirated vehicle intake manifolds to get a little more than ambient air pressure has to offer. 2T's aswell remember the boost bottles of the 80's used to see them on Yamaha's
But if it wasn't for atmospheric air pressure non of our engines would even run, if air pressure was zero we wouldn't even be able to breathe even if the air was saturated with 100% oxygen.

Wasn't there an early Jonsered that pressurized the air box with the flywheel fan?

 

[ZeroJunk]

I haven't seen it because it's not common.

Yes, it is actually. Or, too common anyway.

Probably the time of year. People starting them for the first time since fall leaves.

BR450's which is a new model are subject to it. I never see the customers usually but the ticket says something like "starter stuck". And the older flat top two cycles as well as handhelds do it.

I have had at least ten this spring. Maybe more, being old and all.

But, you pull the plug out and blow gas everywhere, and you can get it back running. But, it will just do it again unless you replace the carb. I suppose you could change the needle and metering diaphragm if it was your own and you had the time.

What makes it more complicated is that Stihl in their wisdom didn't thread the fan on these. Even the BR800X is on a taper like a chuck and if that fan spins it will ruin the taper and you have to replace the crankshaft and the fan. Did one under warranty a while back.

The BR450 is just a nut on a flat washer and believe it or not I have had several of those where they took a good pull, the fuel got up on top, locked the motor and spun the fan loose.

Then you have to take the tank screws out , the back plate off, and tighten the 15 mm nut back with whatever piston stop suits you.

So, not sure how this got started and having been at my lake house for the week end I don't remember, being old and all.

But, yeah it's real. Any of you guys ever stumble in to working on blowers be aware

 

[bwalker]

Yes that's right the lower than atmospheric pressure crankcase now gets filled buy the rushing in higher ambient air pressure. This "vacuum" is not pulling in the atmosphere it's merely making somewhere for the higher ambient atmospheric pressure to fill.
Moving on lol

The movement of the piston us very much creating a vacuum. A two cycle motor is a simple air pump, so yes its very much pulling air/ fuel into the engine.

 

[bwalker]

Yes that's right the lower than atmospheric pressure crankcase now gets filled buy the rushing in higher ambient air pressure. This "vacuum" is not pulling in the atmosphere it's merely making somewhere for the higher ambient atmospheric pressure to fill.
Moving on lol

The "work" is done by the piston and uts very much pulling.