1957 312 drivability

[1957Bird]

Have a 1957 312 D code with aprox 100,000 miles that after driving for hour plus will pop back through the carb when slowed down to make a tight turn then accelerated again after the turn??
Temp is good, compression is acceptable, head gaskets recently replaced, fairly new Holly carb, Petronix ignition w/coil, plugs new, thats about it??
seems to run fairly well other than this.
Anybody's ideas would be greatly appreciated.
Thanks
Roger

 

[Douglemmo]

Hi,
Backfiring, in my experience is associated with leaking valves. Since it only happens under the conditions you mention, could it be a combination of a slightly leaking valve (have you done a leak down check in addition to a compression check) and a slightly richer fuel mixture upon acceleration out of a corner (float level and accelerator pump setting). I have also heard of a cracked distributor cap causing a backfire, but not an intermittent backfire. You are correct in focusing on this problem. A backfire can lead to an engine fire. Make sure your fire extinguisher is readily at hand.

Good Luck,

 

[Lorne]

Have a 1957 312 D code with aprox 100,000 miles that after driving for hour plus will pop back through the carb when slowed down to make a tight turn then accelerated again after the turn??
Temp is good, compression is acceptable, head gaskets recently replaced, fairly new Holly carb, Petronix ignition w/coil, plugs new, thats about it??
seems to run fairly well other than this.
Anybody's ideas would be greatly appreciated.
Thanks
Roger

Sounds like you need a valve lash adjustment. Y blocks are common for this

 

[1957Bird]

leak down test was on my to do list for when the car comes out of winter storage. Valves were fairly recently adjusted, could possibly have one that is not set correctly???

 

[CSPIDY]

If you are having an after fire, unburned gas hitting the hot muffler. You may have an ignition problem.
Take a good look at your plugs, you could have one that is not firing correctly.
What condition are your wires?
Take a good look at the rotor and cap as well.
A backfire is usually caused by burnt or highly carbonized valves. The fuel will burn before entering the cylinder.
Incorrect timing can also cause this, but you said it runs well.
Would not hurt to look at the timing. Make sure the mechanical and vacuum advance are working proper.
As said above a leak down test will check your valves.
Remember this is all theory, you need to do physical examination to find the problem.

 

[1957Bird]

Thanks much CSPIDY
Cap, rotor,wires,plugs all new
timing set to spec
Mechanical and vac advance seem to be functioning as designed. Other than one blown head gasket, (due to someone placing long head bolt in wrong hole)
heads and valves looked real good when I did head gaskets last winter.
Seems like leak down test needs to be pretty high on my list when my N.H. based car comes out of hibernation in mid April
Thanks again
Roger

 

[donnyonee]

borrow compression tester and check, then oil cylinders and recheck.
small jump cranking , good rings,
then fast leak down = valves
imo?
(under the tree out back)

 

[CSPIDY]

Should do the check with the engine warm

 

[donnyonee]

I don't think it matters hot or cold
idk