Re: Backfire through carb under load
Bigdaddy, ironic that 2 newbies are from Syracuse, but I'll have to disagree with ya about pulling the heads w/o first diagnosing the problem. There may very well be something wrong with the heads, but they can be acquitted or condemned without removal in this case.<br /><br />There was talk of camshaft issues and fuel pump volume issues earlier.<br /><br />Typically, if a SBC has a worn exhaust lobe, then the symptom will be a repeatable, rhythmic backfire under load. <br /><br />Fuel volume is not the issue here either. If there was a fuel volume problem, the resulting symptom is usually a smooth, gradual loss of power. A backfire caused by a Quadrajet's secondary air valve opening too soon is caused by a lag in fuel, not a lack of fuel. <br /><br />The float bowl has fuel in it, or the engine wouldn't be running. When you open the throttle all the way, the air enters the secondaries faster than the secondary fuel system can supply fuel. This lag causes a lean backfire. It doesn't take much fuel to prevent the lean backfire. A fuel pump volume problem will have different symptoms.<br /><br />By adding a secondary air valve that is dampened by a spring and a choke pull-off diaphram, the incoming air is slowed down enough to allow the secondary fuel system to provide fuel and prevent a lean backfire.<br /><br />There are also 2 discharge ports mounted high and in front off the secondary air valve. They act as a secondary accelerator pump, as a drop in pressure pulls a fuel shot (similar to a primary accelerator pump shot) to further aid in preventing a lean backfire.<br /><br />3 questions:<br /><br />1. Can you accelerate through the backfire? That is, after the backfire occurs, does the engine run fine at WOT?<br /><br />2. Did you check to see if the choke pull-off diaphram on the original carb holds vacuum? You can also look for the secondary "pump shot" if you have someone drive the boat for you. Be mindful about sticking your face too close to the carb...<br /><br />3. Maybe I missed it, but did you do a compression check?