This is a 80 Buick 350 with 96,000 miles. In neutral the motor revs fine but idles a little rough, when I put it in gear and push the gas it starts to rev up then at about 1500 rpm it sputters and shoots flames out the carb with a loud poping sound. I just dropped this motor back in, and I had no problems with this before (even on my 5,000 mile drive to were I'm working for the summer). I pulled it to change the rear main seal and I only loosened the rear main, I torqued it back up properly and thats the only thing I messed with inside the motor. This is a bone stock motor and I've triple checked the initial at 12 deg., idle speed is 800 rpm, all wires are flashing on the timing light, all plugs are clean, idle mixtures screws (q jet) are set at 2.5 turns out pulling 12 psi of vacuum, all ground straps are hooked up. I have no idea whats going on here, any ideas? I've been tinkering with this for a few days now. Sean.
12" of vacuum is nothing to write home about on a stock engine, unless you're taking the reading when the trans is in Drive. Are the mixture screws at 2 1/2 turns because that's where it idles best? Does it idle better if you spray some aerosol carb cleaner into the primary venturis? If it does, it's lean. Might be an adjustment, or it might be a vacuum leak. What is the compression in each cylinder, and when you disconnect each plug wire in turn, does the rpm and vacuum drop the same amount for each cylinder? Popping out the carb can mean intake valves that aren't sealing, or exhaust valves that aren't opening.l
You have a vacuum leak somewhere. 12" of vacuum is LOW for a stock motor, should be more like 20". If you push the engine, you get a lean backfire through the carb. Check the carb to intake gasket. Spray some carb cleaner around the gasket with the engine running and the vacuum gauge hooked up. When you spray the leak, the engine and gauge will respond. Check all your vacuum hoses, you may have cracked one when you removed the engine, they get hard and brittle with age.
It turns out that I had the 12v wire to the dist plugged into the tach output (hei) and thats why it was backfiring. I replaced the module, cap, rotor, plugs and checked the wires and conected the wire corectly but now it runs really rough, smells like gas and has surges when accelerating. I need to feather the gas to make it run ok and it goes bad then good depending on the load and the amount of gas I give it. Opinions?
Nope, got to be something else. The engine would not run with the feed wire connected to the tach output. Sounds like a carb problem. Vacuum leak, float level low/high, fuel pump problem.
