I recently bought a enduro 175 that had a stuck piston and missing carb but found a used carb and got the engine checked and rebuilt by a professional mechanic. After getting the bike and engine together we did the essential wiring and got the bike running.But after the first few seconds the bike revs up super high and unless we  keep a hand behind the carburetor it would not start . If anybody has a solution for this problem please help

Welcome. Not easy to give a definite answer but sounds like either a major air leak or the throttle isn't shutting right down. So first thing check is that the slide is hitting the bottom, & be sure to back out the slide stop screw so it 'can' go to the bottom. So, ok lets assume its shutting off. Therefore where can it get air!!, first thought is the left side crank seal behind the mag. But it'd need to be almost right out of the case but if it was completely out it wouldn't run at all. If the other side crank sea as out it might try to run but need choke [or hand choke like you described] but would smoke so bad that your neighbors would call the fire brigade. Be helpful to know what year & a few pics too, or even a video. But--does sound big airleak to me but could be wrong.
Would it be a good idea to get your professional mechanic involve or has it already cost you enough dollars.

Thanks for the help. But my slide is going down properly. I'm also suspecting the crank seal.but some people are saying it's the rubber boot from the engine to the carb is it true?

If it was 'that' bad with the rubber inlet manifold you'd be able to spray something around it & watch it get sucked in or bring the idle down momentarily. You say the slide is dropping, is that with the slide stopped screw backed out?. Not to be confused with the mixture screw. If you had the carb in your hand & looked through to the light--can you see under the slide when its shut down. Sorry to go on--& on. Inlet manifold is what the carb pokes into, not the boot from the air filter to carb!!, so we're on the same page.

Yes I adjusted the idle screw all the way out but still the bike is running and has a very high idle. As you mentioned I will check the rubber boot by spraying it with something.Thanks for the info. 

I hesitate to comment because if a "professional mechanic" that knew what they were doing "checked and rebuilt" seized engine there would be no question of air leaks or bad seals...  an air leak is a very likely reason it might have seized in the first place. 

I probably shouldn't say anything but I guess it bugs me that so many mechanics these days don't have a clue on how to work on these simple and reliable machines and the owner has to pay the bill and maybe get discouraged. 

If carb slide is "thunking" closed, my first guess is ignition side crank seal as well...  I'd use the better 77-79 IT 175 seal when replacing it. 

Good luck and I hope you get it up and running soon!

(I'm also a big fan of pressure/vacuum testing engine as it eliminates all guesses and usually saves time.)

Yep my thoughts too about the proffesional mechanic, but just needed someone else to say it [too chicken].

I' don't think it was a air leak that caused the engine to sieze this bike has been sitting for at least 2-3 years without a crab and the last owner had not covered the inlet port properly so I guess water or the humidity caused the piston to sieze inside the cylinder with the lack of fuel and 2 stroke oil. The mechanic that rebult the engine is checking the engine all over again.thanks for the info and the advise.

First place water would go is into crankcase which means crank plus main and rod bearings may have been damaged by corrosion.  Anyway "rebuilt engine" to me means going through the whole thing...  which involves splitting cases to make sure bearings didn't get damaged and means new crank seals and no air leaks.

Also don't know what you were charged...  if it was $50 to unstick piston and hope for the best otherwise that's one thing.  If it was hundreds to "rebuild engine" that's another. 

I don't know what mechanic did or what was charged or what experience he/she has...  I'm just jaded by experiences friends and members here have had with shops who charge for work that isn't done or done right...  mostly because they don't know what they are doing and then give it back and blame the old bike.  

That might not be the case here at all.  One other thing is if you used a cheap Chinese carb that can be a crap shoot.  Some work great, some don't work at all.  If slide is closing solidly and all the way with stop screw backed out, but engine is still revving high, then it has to be getting air from someplace. 

Good luck and don't give up.  Hope it's running like new soon! 

First thing I'd do is a leak down test on it.

 Hope it gets figured out.