Reducing weather sensitivity of Carb setting?

[maowwg]

Yesterday, I cut for 2.5 hours with the muffler modified Poulan 2550, late 1990's early 2000 Type 7? It's my junk saw and I was cutting muddy logs that had been on the ground for 2 plus years. This saw is from the time when they added 3 springs and a bypass handle to the standard boxy Poulan saw as anti vibe. It's head and shoulders above where it cut pre MM, or any of the several Poulan strato saws I've run. This saw 4 strokes out of the wood and leans up and pulls wood when it goes under load and it maintains speed with it's 18 inch bar fully buried in 18 inches of wood....which it never did pre MM.

However, the low screw seems to be a little bit temperature / humidity sensitive. It's been worked hard several days at different temperatures and humidity. It often requires a Low screw adjustment to maintain its idle. Recently in colder weather, it required more gas to idle. Yesterday was warmer and it loaded up and puked when I let off the trigger. 1/8 to 1/4 turn in on the Low screw and it ran perfectly the remainder of the day as it did with all the other daily adjustments. So I keep a screwdriver on hand... no big deal.

QUESTIONS:
Is there a simple solution I'm missing?

Should I turn the Low Screw out even richer to just a little bit past where it pukes on a lower throttle setting and increase the throttle until it runs, then adjust for temperature or humidity with the throttle vs adjusting the low screw? Is one better than the other? There's a spot on the low side where these Poulans will puke, but if you turn out in the idle screw, they run (offsetting the rich air fuel mix with more air VS less fuel). Anyone know which approach leads to less weather sensitivity?

 

[John Stryker]

Better to be too rich than too lean. Tune it on a high barometer day and you should be safe, in my opinion, although I'm not really familiar with your saw model.

 

[Campin' Cutter]

I don't have anything to add to solve your problem, but I have never had a problem with my saws tuning going from down to 17 degrees to around 120 degrees (AZ has extreme temperature range depending on where you are at what time of year). I find it sort of odd that humidity and temperature have such a pronounced effect on your saw to the point it won't idle.

 

[maowwg]

I don't have anything to add to solve your problem, but I have never had a problem with my saws tuning going from down to 17 degrees to around 120 degrees (AZ has extreme temperature range depending on where you are at what time of year). I find it sort of odd that humidity and temperature have such a pronounced effect on your saw to the point it won't idle.

“I find it sort of odd that humidity and temperature have such a pronounced effect on your saw to the point it won't idle.”
Me too! That’s I’m looking for more info. Cutting this weekend again. I’ll richen it up and see what comes of it.

 

[ShermanC]

“I find it sort of odd that humidity and temperature have such a pronounced effect on your saw to the point it won't idle.”
Me too! That’s I’m looking for more info. Cutting this weekend again. I’ll richen it up and see what comes of it.

Both my STIHL MS460 Magnum and 066 Magnum have the seasonal slide door port in the air filter Cover. I change them twice a year because the Manual directs it. I don‘t know if they would be harder to start and run if I failed to change them.

 

[J D]

I'd be looking for other factors influencing the mixture at varying temperatures... Marginal fuel lines or degradation of the primer may be more apparent at a certain temperature, if the metering valve is getting marginal then tank pressure may be overcoming it on warmer days....

 

[maowwg]

Fuel lines and primer were replaced with new materials...albeit cheap stuff. I didn’t touch the metering valve. That could be leaking at higher pressures. But would they be consistently leaking so it runs consistently after the adjustments?

 

[Stihl-Pioneer]

Are you just adjusting the needle or are you also adjusting the throttle screw? If just the needle, you may need to screw in the throttle screw a hair to help.


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[J D]

Fuel lines and primer were replaced with new materials...albeit cheap stuff. I didn’t touch the metering valve. That could be leaking at higher pressures. But would they be consistently leaking so it runs consistently after the adjustments?

It would leak more or less depending on pressure which may depend on tank pressure which would depend on temperature

 

[oldfortyfive]

My MS193 tends to favor cool weather. I have to give it an 1/8 turn in warm weather or I have trouble restarting.

 

[maowwg]

1/8 turn richer or leaner? I assume leaner given warm air is less dense

 

[RED-85-Z51]

Both my STIHL MS460 Magnum and 066 Magnum have the seasonal slide door port in the air filter Cover. I change them twice a year because the Manual directs it. I don‘t know if they would be harder to start and run if I failed to change them.

That is to direct hot air over the carb and filter to prevent filter/carb icing in severely cold temp...on vehicles and water cooled engines they divert hot water through the carb or throttle body

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[funky sawman]

That's what a muffler mod does, unfortunately. At idle the combustion temps are too low, and with a unrestricted outlet, the fuel charge circuits out of the cylinder leaving unstable especially when it's cold. Some carbs are just not as sensitive. Check your pop off pressure, might need to lower it a bit due to weak impulse

 

[J D]

Check your pop off pressure, might need to lower it a bit due to weak impulse

Pop off pressure is not something you want to lower

 

[bwalker]

It's impossible to have a carbed saw perfectly tuned at hot and cold temps.

 

[Huskybill]

Around sept, oct there’s a cold weather setting around then with the low speed screw. Do not touch the high speed if it’s set correctly. This needs to be done in the spring and fall.

I warm the saw up, take the low speed screw and turn it in slowly till the rpm drops and the saw almost stalls. Then back the low speed screw out 1/2 turn, stop. Adjust your idle rpm. It may take more than once to get it exactly right, Done.works on every older saw with a carb..

 

[funky sawman]

Pop off pressure is not something you want to lower

If it's 25 psi you can. Alot of these newer China carbs I getting excessive popoff readings

 

[SteveSr]

If it's 25 psi you can. Alot of these newer China carbs I getting excessive popoff readings

Pop-off pressure over 7psi is pretty much irrelevant. All the needle has to do is to hold of the fuel pump pressure. The needle is opened by movement of the metering diaphragm and not pressure from the other (fuel pump) side.

 

[funky sawman]

Pop-off pressure over 7psi is pretty much irrelevant. All the needle has to do is to hold of the fuel pump pressure. The needle is opened by movement of the metering diaphragm and not pressure from the other (fuel pump) side.

Yep, and how does the metering diaphragm move?

 

[funky sawman]

Whatever I'm not going to argue with you know it all, I know what has been working for me and some customers for years, I'm out.