wesleyshawn

fresh 496 stroker, timing 12 degrees btdc, holley 850 vacuum secondary, float levels set properly, engine starts and runs great, but maybe one time out of twenty when I nail it from a dead stop or low rpm it pops through the carb before it takes off. Doesn't seem to be dependent on engine temp or outside temp, I might drive it hard all day and nail it almost every time I take off and it does fine, then out of nowhere everything else seems the same and I get the pop. I would think that a backfire through the carb would come from ignition when an intake valve is open, but if that's the case why would it be so intermittent?

wesleyshawn

28 views and no input, now I don't feel so bad that I couldn't figure it out yet either

jetnoise

Crossfire on plug wires?
intermitten bad ground?

Drizzle

funny, I am having the same problem with my 70 350/350. At 12degrees btdc it pops 1 out of 15 times I mash it (exactly the same as yours). at 14 degrees btdc, it never pops, but I have trouble getting it started once I kill it (the compression is too high with the piston further up). Today, I set it a little higher to 12 1/2 and it popped once out of 40 or so tries (I kept trying ). So my fix is to advance the timing a little so that the backfire goes away, but not so far advanced that it wont crank up.

I AM NO MECHANIC. I THINK WHAT I AM SAYING IS TRUE, BUT IF SOMEONE KNOWS MORE, PLEASE CORRECT ME. No HARD FEELINGS.

But that worked for me, so far....

--Donnie

...Roger...

Backfire through the carb is a lean condition.

stingr69

It is a lean backfire. Check the accelerator pump linkage for any slack.

-Mark.

wesleyshawn

Plugs show it running a tad rich. I'll check the accelerator pump, but I'm curious why it would only do it about 1 out of 20 times if that was it. Is there a carb issue that would be very intermittent itself that I could look for?

I'll try advancing the timing a bit also to see what kind of difference it makes.

...Roger...

Would like to add one thing to consider.Your vacuum advance can.I have seen cans with too much advance built in (usually adjustable ones)that cause a cough just off idle.To be honest I'm not sure why exactly.My guess would be the extra timing at idle is helping to lean out the mixture.Brings us back to a lean condition being the cause of a backfire.Of course there other reasons that can cause lean conditions.

dannyman

Check that your fuel evap. control system is working correctly. In particular; check that the charcoal canister isn't full of fuel.

bojangies46

I would advance timing 2 degrees, check fuel pressure, and vacuum advance. My 2ct--

wesleyshawn

My car hasn't seen one of those in years.

wesleyshawn

I'm running a Holley blue electric pump with the regulator set at 7psi. I'll try the timing next and report back, but I tore it apart yesterday putting a Mark VIII electric fan and shroud in place of my little electric fan, so it will be a couple days before I drive it again.

Drizzle

Getting ready for cruisin the coast?

76 VETTE

Moisture in gas tank maybe.

wesleyshawn

exactly
I've been spending every evening after work and every weekend for months getting ready. It's ugly right now, been prepping for paint among other things, but I'm taking it anyway.

Drizzle

If you see a dark blue 70 convertible, come say hi.

--Donnie

wesleyshawn

I'll watch for you, keep an eye open for me as well. Mine is a '79 with the 80-82 style front and rear, 6" cowl hood, various paint finishes (primer, flat black, gloss black, bondo, etc) moon disk wheel covers, and stainless hooker sidepipes. Not much to look at yet but now that it's running right I'll get back to the pretty stuff.

Follow up on my problem, finished installation of my Mark VIII fan, advanced the timing to 14 degrees before tdc, and after 3 days not the first backfire through the carb.
Thanks to all for the help and input, much appreciated.