Backfire through Carb

[Jeffrey S.]

I just got back from a nice drive in my '69 (freshly rebuilt SBC engine) and shut the car off on the driveway. Fifteen minutes later I went to start the car to put it away and it backfired through the carburetor. First, why did this happen? Second, did anything get damaged? Car started right up and seemed to run well afterwards. Any thoughts?
Jeff

[Domenic T.]

Re: Backfire through Carb

Jeff,
Some things are lean mixture, timing, or maybe the accelerator pump, but with the available gasoline we have that can also have something to do with it.
I wouldn't worry about damage other than it could damage a rubber valve in a Holley carb, but you would know that right away. The engine survives a back fire well.

Dom

[Jeffrey S.]

Re: Backfire through Carb

Dom,
Thanks for your reply. It is ironic that you mention gasoline in your post since I had just filled up with Sonoco 93 octane- a gas that I had not used before since the rebuild. Actually it's been about 40 years since the days of Sunoco 260 that I have used this. I wonder if there is something about this gas as opposed to BP or Citgo or Cosco which I have used with no problem.
Jeff

[Edward J.]

Re: Backfire through Carb

Jeff, Have you made sure the timing is correct? the dwell set,? Vacuum advance working and hooked to direct vacuum? Generally retarded timing will cause back fires on start-up. Or a carb dumping fuel after shut down, This generally will cause a long crank time on start-up.

[Jeffrey S.]

Re: Backfire through Carb

Edward,
The timing is correct (38* total with 8* initial) and the VAC is hooked up and operating properly (2" rule ). Car starts up with almost no cranking and shuts down immediately. I have put about 300 miles on the engine with no problems. This is the first time this has happened. Maybe it's just an anomaly. I'll take the car out again tomorrow and see what happens.
Jeff

[Edward J.]

Re: Backfire through Carb

Put a few more miles, engine break in is generally about 500 miles. And then recheck the tune up spec.s and the carburetor air mixture setting. another thing I like to do is after the drive and the engine is up to temp, let the car hot soak for 15 minutes then remove the Air cleaner lid and take a peak down the throat of the carb to make sure fuel is not dripping onto throttle blades. The ethanol fuels have a habit of raising the float levels in the carb. and dumping the overflow onto throttle blades causing long hot restarts.

[Duke W.]

Re: Backfire through Carb

For any given part of the country gasoline is blended basically the same - EPA and state (California) requirements give refiners and blenders very little leeway on chemical and physical properties, and a local distributor blender will usually service most "brand" stations. The only real variation is the additive package, like detergents, which are blended along with ethanol in the tanker that does the delivery.

Sometimes engines will backfire after a quick hot restart. As long as it's not a recurring problem, no need to worry, and backfires usually don't cause problems with Q-jets, but they can damage the power valve on Holley's.

Duke

[Duke W.]

Re: Backfire through Carb

The ethanol fuels have a habit of raising the float levels in the carb. and dumping the overflow onto throttle blades causing long hot restarts.

Can you explain in more detail?

Duke