MS251 Troubles

[Jpreston]

I've owned this saw for about 12 years. Never really had any trouble with it. I tried doing a muffler mod on it, and removed the limiter caps from the carb high and low jet screws. I adjusted the carb to what i thought sounded good. But when i went to cut some logs up it doesn't want to cut right. It starts bogging down half way into a log, Sometimes it bogs so bad that the chain comes to a stop. So i readjust the carb a few times to see if i could get it cutting any better and it doesn't seem to be running any better no matter what i do. I looked in the book and it says max rpm's is 13,000 but when i put a tach on it all i can get is 10,500 roughly. The saw has good compression. I tried a new carb, air filter , fuel filter, and spark plug. So far nothing has changed still only getting around 10,500 on the rpm's. when cutting limbs and small size stuff it seems to be cutting normal but cutting anything bigger it just doesn't want to cut right. Any ideas on what i could try? I also got a new muffler and took off the mod muffler to see if that helped and no go still running like crap.