sbc guys I need your help

[rockmad]

I have a question for you motor gurus. I have a new 350 with vortec heads, flat tops with a mild RV type cam. The carb is the stock Q-jet that has been rebuilt and all the vacum emissions stuff removed.

I can't seam to dial it in. It runs good, but it back fires when I short shift it. I took out some timming and it still seams to do it. The shop that removed the vacum lines checked the tail pipe and said I was not lean. What else could cause the back firing?

The shop also set the timming at 12Deg. I think it sounds a little too advanced. I have a 190* thermostat in it and it runs at 220*. I took out some timming but have not driven it to see if that cools it off.

 

[rockmad]

I forgot to mention that the back fire is after I short shift it and let off the gas at lower RPM's. I get it in the exhaust not through the carb. It aslo does it a little when I down shift.

 

[4X4HIGH]

Your description is indicative of a small exhaust leak somewhere in the system which brings in cool air when you let off the throttle and that's when the backfire will occur.

 

[rockmad]

That could be, this is my first exhaust system I built myself. Do you think it would be near the headders, or further back? I will look tonight and see if I can see any leaking areas. Could this be caused by the air injection system being connected but no vacuum lines atached? I have the pump and hoses still atached, but the vacuum lines have been taken off.

 

[4X4HIGH]

Has nothing to do with the AIR system. The leak could be anywhere in the exhaust system but more probable to be closer to the engine.

 

[rockmad]

Thanks, I will look tonight.

 

[K5dreamer]

google - afterfire.... backfire goes through the carb, afterfire is the crackle pop boom from the tail pipes, and as stated, is caused by an exhaust leak.

my iroc does it really bad after installing the headers, but i never did install that ""hard to reach" header bolt...... so im guessing thats my problem

 

[rockmad]

I just looked for a leak and could not find one. I took some timming out and drove it. Almost all the after fire was gone. I also turned over the lit of my air cleaner. It is the stock one and after that it felt like I doubled my HP.

 

[4X4HIGH]

I just looked for a leak and could not find one. I took some timming out and drove it. Almost all the after fire was gone. I also turned over the lit of my air cleaner. It is the stock one and after that it felt like I doubled my HP.

Flipping the lid allows hot air rather than cool air (assuming you have the tube through the radiator support) and cool air will make more HP than hot air. Try finding the tall air cleaner lid and use the appropiate filter and use the lid as it should be and make sure you have a fresh air tube up to the radiator support.