Carb backfire with flame

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Carb backfire with flame

June 22, 2009, 04:43:07 PM

Help me get my head around something.
Floor it, the engine bogs down and backfires out the carb.
What ignites the gas vapor in the intake manifold?
I can understand the bogging down being an acceleration pump problem and it's going way lean.
But I can't see how a plug can be firing with it's intake valve open.

Can it be that the combustion chamber is still hot enough to ignite the fuel when the intake valve opens if the mixture is too lean to cool it down?

Carb backfire with flame

Reply #1 – June 22, 2009, 10:50:09 PM

gas puddle in the runners will back flash when the plug gets spark.

this is usually a sign of timing being off in conjunction with flooding.

Although it may be running lean, it being difficult to start may allow fuel to puddle. that fuel vapor is just waiting for an opening to the spark.

i hate the noise it makes and the deadly sounding blech followed by a pow.

I was foolin around with the holley 750 and the car had been off for atleast 4 seconds and it still ignited,, crazy going on in there.

if you talking about the backfire happening even when your not cranking,, thats a mystery to me and a good question. I think its something to do with the vapor lock in combo with the surrounding temps of the exhaust manifold and having an exhaust valve and intake open at the same time at the perfect moment. with 6 or 8 cylinders,, this isnt all that rare to have both valves paritally open or atleast an intake partially open allowing the fuel to backfire upward.

its screwy and sucks.

Carb backfire with flame

Reply #2 – June 23, 2009, 01:17:00 PM

if you have 130k +, a timing chain may help,... i know a lot of the time a bad chain will cause backfire,..... thats how i got my BB for so cheap,... the guy was like yeah it started to backfire real bad thats why its off th road,....... when i cheked it, there was about a 3/4 in of play,...

Carb backfire with flame

Reply #3 – June 23, 2009, 03:53:47 PM

I had this issue and it was the death of my motor. It would seem alright at idle but when I hit the throttle it would backfire like a SOB.

Long story short I tried everything and diagnosed it down to a problem in the valve train. I had a lifter stick and that rounded a cam lobe.

I'm not 100 percent sure on the procedure but hook up a vacuum gauge and watch the needle. If it bounces around 4 degrees then you have a valve train problem. (I could be wrong on this).

I ended up rebuilding the carb twice. Changing all the duraspark system stuff, plugs, wires, cap, gaskets, etc. Basically I wasted alot of money.

Also all of my plugs except 1 were dirty. The one was the cylinder with the bad lifter. It was completely clean.

Carb backfire with flame

Reply #4 – June 23, 2009, 04:05:12 PM

recheck your plug wires and make sure that you dont have a wire crossed. also if your still using vacuum advance make sure that its hooked up and working properly.

Carb backfire with flame

Reply #5 – June 23, 2009, 08:21:15 PM

I had a Trans Am that did that. It was pretty funny to watch, actually, since the (functional) shaker scoop pointed at the windshield :hick: