251 Stihl , vapor locking , or ??

My MS 251 has been acting up this summer . The last couple years it has been cold blooded. But this summer it started refusing to spool up after being started in the hot weather .
Anyone have any knowledge about how to remedy this problem .
Believe it or not . This saw has been nothing bit a long term money maker f
Thanks in advance.or me . Over the more than decade I've had it . It's cut around 75 cord and been up many trees . Toping and blocking down .
I hope to get whatever this is fixed so it's back to being dependable.

 

If it still has the exhaust screen make sure it’s not plugged then do the fuel system. If it’s became harder to start I’d look at new fuel filter and carb kit. Also may not hurt to vacuum test fuel line for pin holes or collapsing because it’s gotten soft. Then if that isn’t it put a tank vent on and vacuum test the saw may have leaky crank seals. If the saw changes idle as you roll it in different positions it’s a possibility the seals are leaking.

 

Does it run ok in the cold?

 

Have you tried to tune it? Has it been ran on ethanol fuel its whole life? Is the exhaust screen plugged?

 

Replace the tank vent.

 

Yes . It starts and stays running when cool or cold . After the 1st or 2nd tank it will get so it will start on choke but not advance above idle or high idle .
I put it back in the truck and ran my 565 last time in the woods . Burned a tank then thought ide try the 251 again .
It started good and ran great . So it seems to be a heat thing .

 

I thought about that.

 

Am more referring to cold ambient temps. Wondering if the ambient air density is a factor.

 

This saw has gone out of tune a couple times since new in 15 . I've retuned it . But really haven't done any wrenching on it since new .
Could be it needs a carbon kit .

 

It runs real good at 40 below and colder .

 

But it also runs good when it's 80° F , for the 1st tank .

 

But at -40 will it run more tanks with no acting up? If so I’m leaning more towards the carb and the needing a rebuild than a tank vent. High temperature should cause more tank pressure which shouldn’t make the vent starve the saw any easier than in cold weather. Could be vapor lock in real high temperatures depending how much fuel the station you buy gas gos through. Some stations sell 91 e-free that’s left from winter because they don’t sell enough. Winter fuel has different additives to vaporize in cold weather and will vaporize to easy in warm vapor locking saw. Try some canned fuel in it on the real hot days see if it still acts up. All fuel will eventually do it but canned will take the most heat to cause issues. Personally being it’s 10 years old just rebuild the carb and put a filter in because tuning it will just compensate for the stretched diaphragms in carb. Rebuild and it will probably automatically run way better off the bat, some of the tree service saws come in after a year or year in a half with an unstable idles and hard starts. I change filter rebuild carb and clean the air filter and exhaust screen and they go back to normal. Sometimes the factory tune gos back to running perfectly after the carb has fresh diaphragms. Could also be a lean bog from crank seals leaking some after saw heats up. If you need choke to start after warm that’s a lean condition could be fixed by the rebuilt carb and fuel system but may be bad seals also. Needs the vacuum and pressure test to see that. Yes it still can be the vent also and you may just want to put a vent, carb kit, fuel line and filter with cleaned exhaust screen to rule that all out. Needs it anyway after 10 years

 

Thank you . I'll get a carb kit , filter and vent on the way .
Some may think the 251 is too lack luster. But it will put out $450 a day worth of wood . Day in day out .
Considering I bought it new for $339. That's not bad at all .