Ms 391 help

[stihlman27]

Ok guys been a while since I posted anything lately but I'm having issues with my 391. When I as cutting yesterday I started off by topping off the fuel and bar oil. Choked and decompressed the saw started right up with in a few pulls. I then let it warm up for a minute or two till I myself got ready to cut. After the saw was warmed up and me cutting wide open on about a 24-26 inch diameter oak I topped let the saw idle for a little shut it off and topped it off with fuel and oil again. I was always told to keep things topped. After that I tried starting the saw again and it started after a few pulls once again but after it started trying to go wide open it would want to bog down and shut off so after easing into it a while I finally got it wide open so I started back on the oak log again. After a little while running wide open I notice the chain was going dull so I let it idle to cool down shut if off and changed saw chains. After that tried starting the saw and it would not start what so ever even choking it it would start but then shut off right away.
A little info on the saw its a Stihl ms 391 25 inch bar muffler modded and tuned. New NGK spark plug just put in and spark screen taken out.
My question is why does it want to bog down and why won't it start? Now that it's been sitting since yesterday if I go to start it then it would start so it seems like it's getting vapor locked or getting overheated. Does it bog down cause of fuel? The fuel was new with a fresh Bottle of echo 2 cycle oil. I'm just at a loss right now

 

[stihlman27]

Come to think of it thinking the gas was a little old but would that still keep it from not starting? If I'd let it sit overnight and try to start it the next day it would start

 

[backhoelover]

sould like either the coil is breaking down or the tank vent is bad. need to check spark when it shuts off and you need to do a vacuum test on the gas tank to check the tank vent

 

[backhoelover]

yes. i dont let my gas in the shop get over 2 weeks old and i go to the gas start that i know is used alot as the gas get gone faster and not setting in the tanks in the ground