HELP! Carb Fire!

Hi all,

I went out to start my Suburban, it's a '76 with a crate GM motor that has an Edelbrock Performer carb and manifold kit. It was about 20F outside and it started right up (as always) and was running in the fast idle.. everything just fine. I switched the heater control over to defrost to get the frost off the windows, and suddenly the thing died and wouldn't re-start. I thought maybe it was flooded, so I put the peddle down and held it while I turned it over. That's when the carb caught fire. It wasn't too bad and the flames quickly went out, but I don't know what's going on. It almost seems like a timing issue. I get some backfire through the carb and the exhaust, but no real firing sequence. I'm wondering if it could be vacuum related since it occurred when I moved the heater control over to defrost.

I was hoping that someone here might be able to help me focus on some of the quick and easy things to check before I drag the beast down to the local garage... Any good ideas? The things that I'm considering are:

1) Broken vacuum line

2) burned distributor rotor

3) something in the carb?

4) timing chain? (please tell me that the GM crate motors don't use nylon timing gears...)

Thanks all!

Turns out that it was the distributor rotor. It had come off somehow. I replace the rotor and distributor cap and now I'm good to go.