Reinstalling the pair valve on my bike and i need to know where the vacuume line reconects to ?

You should correct those exhaust issues because a exhaust leak can cause the issues you are having.

Carb vacuum lines are easy to misconnect. You have not listed your machine, or model, so I cannot determine what the details of the challenge you face actually are.
A description of your machine would help immensely.
Often a vacuum line from an intake is used to actuate a fuel valve.

If you have a petcock below the tank with a reserve switch, you should have two lines. The smaller one is a vacuum line that goes to the carbs. The other is the fuel line. The vacuum line is important, as without it the petcock will not flow fuel. The presence of a vacuum opens a pneumatically operated valve in the petcock allowing fuel to flow. There should be a nub on the middle of one of the carbs to connect the vacuum line to. The fuel line goes to a fuel rail that feeds all the carbs.

What exactly is the problem with the petcock? If it is a vacuum operated petcock and it is no longer allowing fuel to the engine check the following:

First, did you work on the bike prior to the problem occuring, and was the bike running properly before this? On bikes I've owned with vacuum petcocks, the petcock has two rubber hoses attached to it. One is the vacuum line the other is the fuel line. On one of my bikes these hoses were nearly identical in size and I managed to swap them a couple times after having the tank off the bike. Simply double check that the hoses are connected properly. If they are not, the bike will run until the fuel is run out of the float bowls then die.

Second, do you have an inline fuel filter on the fuel line between the petcock and carb? If so check it to make sure it is not clogged.

Third, remove the tank and check the entire length of the vacuum line for the petcock. Is it kinked, pinched, cracked, cut, detached, etc.? Look for anything which would prevent the line from providing vacuum to the petcock.

Fouth, test the petcock to see if it is working properly. To do this you will need a handheld vacuum pump, such as a Mityvac (you may be able to get a free loaner from some autoparts stores). remove the vacuum line from your petcock and attach the vacuum pump to it. It should only take one or two pumps to create enough vacuum to open the petcock valve. If fuel begins to flow the petcock is good, if it doesn't you could have a clogged filter screen in the tank, but most likely the petcock's vacuum diaphram is damaged. The Baha is a Chinese made bike and it's possible the rubber parts in the fuel system are not compatible with the ethenal fuel blends here in the U.S. The dealers who sell these bikes are really hit and miss when it comes to being able to provide replacement parts. Even with the internet, it may take some work to locate a correct replacement petcock for the bike. On the bright side, if you are able to get a new petcock, they are very easy to replace.

Good luck!

Lean mixtures burn very slowly, at times slowly enough to continue burning through the power and exhaust stroke, causing a backfire when the intake valve opens, and that flame gets a shot at the new mixture charge.

In normal operation, as the engine slows, the fuel delivery from the main circuit falls off, and the idle circuit is supposed to take over. If the idle circuit flows insufficiently, that becomes a transition to fuel starvation.

You can try pointing an unlit propane torch into the inlet air, and see if you can get closer to an idle while supplying a supplementary fuel source. You will need to do this in a way that gets propane to both carburetor inlets, maybe rigging a Y with vacuum hoses and electrical tape...

This started with work on the carburetors, so the fuel system would be the most suspect. That, and the fact that it will run at higher RPM would seem to rule out fuel delivery.

I was looking around at photos while developing this answer (needed to know whether this was a twin or a 4-cylinder), and one resource said the idle speed should be 1,200. I don't know if that's right, but maybe 1,000 RPM is too slow for this motor to keep it together. (I do doubt that, though.)

When I wrote that last sentence, I started to second guess myself, thinking "What if the fuel shutoff(s) is/are vacuum operated, and as the bike approaches idle there is insufficient vacuum to hold it/them open?" But the I rejected that, because there is even less vacuum at cranking speeds, yet the bike starts.

there is supposed to be a vacuum line between the transmission and the engine that tells the gearbox about load. when its disconected the gearbox will shift hard and the engine will idle lumpy. Find the hose and reconect it and your problems will go away.

I'm not quite sure I understand your English. However these bikes' fuel supply operate on vacuum and if I understand correctly you're not getting fuel to the engine. Check firstly that the vacuum pipe connected to the fuel supply valve is working correctly. 3 pipes: 1 Main fuel supply. 2 Reserve fuel supply. 3 Vacuum.
Sucking on 3 will result in fuel pouring from 1 or 2 depending on where the valve is set.(Reserve or main). If not the valve is faulty.

The line to drain the gas tank or carb? If you have auto shut off fuel valves(vacuum) set the fuel valve to "prime"(or check owners manual about fuel system)
Some bikes need a fully charged battery to start sometimes.
Will the bike fire on starting fluid?
Spark plugs OK?
Can you get a 1/4 " spark at the plug.
Contact me if you need more info.
Eric

Disconnect the fuel and vacuum lines from the pet****. With no vacuum applied, fuel should not flow. Create vacuum on vacuum line (using pump or suction) and fuel should flow. If fuel flows continually (or not at all, or from the vacuum hose) the pet**** needs replaced. Your bike's carbs probably have fuel enricheners (valves that allow additional fuel to flow through metered passages inside the carbs), not chokes in the traditional sense.

You have a vacuum leak for sure. Look and listen carefully for anything left off, maybe an egr valve or pcv hose, brake booster line?