Seems every time I search on this site I get the answer, then my car behaves differently... I responded to a thread on backfiring. My 78 GTS was, I thought, popping through the carbs and seemed worse before the car was fully warmed up. I thought this might be related to the spark plugs-had those replaced twice, changed the points to electronic ignition system. My mechanic told me he had also synched the carbs. This was all before i put the car away for the winter. This Spring-when I got the car out of storage, I noticed she was actually backfiring through the exhaust- not huge bangs but a cross between pops and small bangs-under acceleration and deceleration. I'm confused as to where I should be looking- at the timing-even though I have a new ignition system, or at the exhaust for a possible leak. One condition always produces a backfire-going down a hill in gear with my foot off the gas-I'll hear pop,pop,pop, bang. Any experts out there would be greatly appreciated-....
Backfiring is usually caused by a faulty fuel/air mixture - too rich or too lean can cause it in various conditions. It is not usually ignition-system related unless the timing is quite severely advanced. It can also be caused by incorrect valve timing but that's pretty unlikely. Intake manifold gaskets/vacuum fittings/carburetor mounting leaks are the most common cause. That's where I'd look first.
Thanks Mike-so your saying check the exhaust for a crack. what's the best way to do that-seems hard to visually spot it-there's some kind of metal heat shield wrapped around it.....?
First, check timing then, when fully warmed up, check the flow on each carb at idle, then again at 3-4k. report findings back.
Thanks Geno- might as well check the timing-I think I can do that myself. To check the carbs-I remove the airbox and place a flowmeter over each hole-is that correct? You guys might be blown away with my technical terms...
"Thanks Mike-so your saying check the exhaust for a crack" No, first place to check is the INTAKE side for an air (vacuum) leak which will cause a lean mixture. It may seem counter-intuitive but a lean mixture causes backfiring more often than rich mixtures though either can do it. The end result is the same - unburned fuel/air in the exhaust. A lean mixture causes that because it doesn't correctly ignite and doesn't completely burn in the combustion chamber, passing some unburned fuel/air to ignite in the exhaust. Note that backfiring in the pre-computerized days was not unusual or even "wrong", especially in performance engines. The fuel mixture was not as well controlled with carbs as it is now so it was not possible to have the optimum mixture at every power setting. When you couple that with a more aggressive camshaft, it's quite common to have backfiring as normal at certain loads/throttle settings.
