Backfire 101

What was the sequence of events during restart? Did it backfire while cranking (how long cranking?), or after firing-up (how long after?). What were you doing with the throttle when starting? How was the previous shut-down (clean or stumbling?). Did you rev it when shutting-off? Questions, questions ... (sorry, no answers yet!)

 

Backfire on start up is VERY unusual. It's almost always on shut down. I have no idea what it could be....but I'd do the 'troubleshooting 101' thing. Start with electrical. Timing okay? Plugs the right color? Wires, rotors okay? Then check for fuel and vacuum leaks around the carbs. You know, just an overall check of the systems.

Whatever is causing it, look for something out of the ordinary that might be a symptom from whatever the cause is. I would even check the bolts holding the headers on; last night I found one of mine was finger loose and causing popping that at first I took to be detonation.

Good luck and let us know if you uncover any more symptoms.

Ken

 

My '77 308 did the exact thing. I took it to a mechanic that was well versed in float adjustments and the problem went away.

 

My 308 did that and found it to be that a couple of the cylinders were not firing.

 

I have a strong running carb 78 308. It's always had minor backfiring issues until fairly recently. when I say minor i mean a tiny thump-thump after engine is turned off. I was now getting spitting, popping on deceleration-say down a hill in low gear with my foot off the throttle. My mechanic decided it was an exhaust issue and proceeded to tear out the exhaust to find a leak. This was after a leakdown on the cilynders-ruled out the engine. He also did something, replaced the jets into the carbs. the probelm is the car is still backfiring. the spitting/popping has diminished but now i'm getting fewer but louder 'Bang" type backfiring. does anyone have any thoughts on this?

 

Backfire is fuel igniting in the pipe. Typically caused by rich condition(s). I believe over-advanced ignition will also cause this. If one cylinder is mis-firing and then combusts unburned fuel in the pipe next time through you'll get the symptoms you describe.

Here's what I'd do. Run car, let it idle, pull and read the plugs (looking for evidence of rich condition on one or more cylinders). While it is running, a pyrometer aimed at the base of the header pipes will also tell you if/which cylinder is missing. Second, check the ignition timing with a timing light at idle, at 2000 rpm and on overrun. The curves are in the manual.

 

Too rich at idle. Turn mix screws in 1/2 turn on all. If that doesn't cure it, turn them in a further 1/2 screw. If still a problem, pull the tops off, measure the floats/check the jets.

 

Um, actually a lean condition will cause this. Running too lean is running too hot and the exhaust heat explodes unburnt gas. If you have one cylinder (or pair) too rich and another too lean, you will frequently get a backfire. The way to test this is let the car idle 30 seconds and shut off. If it doesn't backfire, it's likely a lean condition that caused it.

I had an exhaust leak that caused what sounded like detonation, but it wasn't a full fledged backfire. It would occur when you open the throttle, NOT on deceleration or when the car was shut off or switched on.

Ken

 

Not my experience, Ken. Lean usually means they spit back through the carbs. Rich pops through the muffler.

 

I see...I've never had a backfire through the carbs...or have I? Can you tell the difference by the tone? When I had my lean backfire I assumed it was at the tailpipe. Going a tad richer eliminated the problem...

Ken

 

Ken
Come round some time (or come to the track) and I'll start my P6 cam'd 308. I run it slightly lean at idle (for another set of reasons). You can hear it chuffing away until it is fully warmed up.
Philip
P.S., I live in Winnetka