earlwb

Are the carbs above or below the engine? If below the engine then you will have a heck of a time trying to prime the engine to start as the fuel won't go uphill into the engine easily. If above the engine then the fuel prime will more easily get into the engine for starting.
Are you able to choke the engine? Usually thecarbs have a extra choke plate on them. You close the choke, flip the prop until the engine sputtets ot coughs, then unchoke the engine and it ought to start right up then.

With Walbro style carbs that have built in pump regulators, you do not need to use muffler pressure. It is not required or needed. If the engine is running Ok once you get it started, and the carbs are under the cylinders and not above the cylinders, then you will have to get creative on how to get the engine primed to start. One way is to use a large electric starter. Maybe another way is to invert the plane, flip the prop and choke the engine to get it primed good, turn the plane back upright and then try flipping the prop to start it.

Actually using muffler pressure is not a problem with gas engines, if the carburetor is setup for it. NV_engines GX40, Evolution 10gx, and Evolution 15GX all use muffler pressure and none of them has blown up. I have been flying the heck out of a NV_Engines GX40 with muffler pressure and it has not caught fire or blown up on me. Technically to get ignition to occur inside the fuel tank you need a very specific air/fuel ratio. Normally the fuel tank is filled with gas vapors that push the air containing oxygen out as the gas evaporates quite readily. Then the exhaust gasses have little to no oxygen in them too.So it is very unlikely you'll get the contents of the fuel tank to ignite. Now I am not saying it is impossible as some very unlucky individuals managed to set their glow planes on fire before using glow fuel, So anything is possible.