Installed new carb-saw will idle fine,but will not rev.Idle scew has no effect on r.p.m.As soon as you hit the throttle,saw dies.Almost act's like the chain brake is on but it is dissengaged.Both air-fuel mixture screws are adjusted correctly.Saw is a poulanpp4620avx.Saw will not rev fast enough to turn chain. Chain turns freely on bar.HELP PLEASE!!!!!!!

Pumping the primer bubble while the engine is running should cause the engine to over fuel and stall (if the tuning is correct). Sounds like its not fueling enough, pumping the primer is forcing fuel through the carb causing the engine to pick up rev's, but this is soon exhausted which is why it wont rev on the throttle.
* Check that the fuel line is not damaged and drawing in air. (if this is intact try the next step)
* Strip and clean the carb. Replace any diaphrams if needed.
* Check gaskets seal between carb, air filter and cylinder.

Just out of curiosity what standard carb settings did you use.?

my 50cc scooter will idle at first and rev very well but wont idle what can this be

There is not an actual rev limiter. The top end is controlled by the carb. You actually want it to sound a bit rich at full throttle so that it has enough fuel when cutting under a load. What causes me some alarm is the revving up part. If it revs too fast that is a lean condition and can quickly ruin the piston and cylinder much like running straight gas. A torn intake boot can cause this. The high speed spec for the TS400 is 9,600 rpm + OR - 500 RPM. Your saw may not have a needle to adjust the high side as some have only the low speed adjuster. Also when you disassemble and clean the carb always use a new kit with the gaskets and diaphragm. Any Stihl dealer can furnish the kits and they aren't terribly expensive. Often between $10-20.

Adjusting carbs on chainsaws is not to be taken on lightly, there will normally be three screws, an L screw, an H screw, and an idle scre, the L screw will increase the fuel mixture from idle to half throttle ( they are fuel screws not air screws ) the H screw will adjust from half throttle to maximum revs, start with a base setting of between one turn and one and a half turns out, start the saw and warm through, allow it to idle, if need be increase the idle screw to get an idle, turn the L screw in slowley, the revs will increase as the mixture leans off, it will reach a point where it starts to die, now back it off a quarter of a turn, blip the throttle, the saw should pick up clean, if it has a flat spot increase the L screw slightley, now set the idle so it idles without the chain moving on the bar, now screw the H screw in to get full revs, be aware this should be set with an engine rev counter, do not exceed the manufacturersmax revs, hobby saws normally run at about 12500 rpm with an idle speed of 3000 rpm.

Ok first the plug gap is around .030". Next you have to have compression. With the sparkplug in you should be able to feel the "hard" spots while pulling the starter rope. The rope should not just pull out smoothly. Now for the carburetor. Start by screwing the hi and lo jets in as far as they will go. Gently screw them in, don't force them or you will damage the needle and seat. Now turn them out 11/2 turns each. The idle speed will be adjusted later. Make sure that the fuel is fresh and has the right amount of oil in it. Now start it up and let it warm up. You may have to use the throttle to keep it running. Adust the idle screw so that it will run on it's own. Now adjust the lo jet until you get the highest rpm, adjust the idle to keep it at a good idle. Now adjust the hi jet, you do this at full throttle until you get the hight rpm. Let it idle, now open the throttle, if it hesitates turn the lo jet out about 1/8-1/4 turns and try it again.

This sound like a fuel issue. Have you replaced the pump?
You may also check the fuel regulator valve

check to see if the choke is stuck or the carb could be dirty (gummed up)

unleaded fuel is notorious for going stale,and it can leave a resinous coating in the carb-try running the saw on half choke with new gas and less oil eg 50 to 1 for say ten minutes,this should clear the gunk out-if this does not work,you have two choices-strip and clean the carb,or adjust the H (high) scew on the carb anti-clockwise by a quarter turn a time till it revs out full bore-good idea to remove the chain and wear ear defenders when adjusting-and always use a chainsaw at its max rpm,or you run the risk of "kickback"

vacuum leak.....get some carb cleaner an when you can get the bike to idle somewhat normal spray around hoses and intake manifolds...if idle increases that where the problem is.

Your carb may be the issue but your ignition coil could be the culprit. Once they start going bad, they can act up intermittently or give various symptoms from day to day. The fact that your saw dies at full load or under load, probably means that when your saw gets hot or requires more spark, your coil is not able to provide it. Your carb is easy enough to clean out, you can take it all apart and clean all screens, etc....

I just replaced my coil due to the exact same symptoms and now my saw runs great. $60 off ebay and you can put it on in 15 minutes yourself. Check your carb first though