chevy 350 timing problem

[c man350]

I recently bought a Chevy 350 truck. When I try to time the ignition it backfires through the carburetor and has no power but if I advance the timing mark so that I can no longer see it the back firing stops and the engine has power again.

Any idea what my problem is?

[abg]

You don't say what year the truck is. On the older models you have to disconnect the vacuum advance hose from the distributor when timing the engine. On the newer ones you have to disconnect a wire.