Well, y'all. I haven't checked anything. I reconnected the vac advance after a couple days to see if it would duplicate and it hasn't popped since. I'll be doing a big check-up before Columbus, so I'm sure if anything is amiss, I will find it.


Lawrenceville, GA

Turning the idle screw in all the way and seeing if the engine speed slows is indeed a good check for a leaky power valve providing that particular check also worked before the backfire.  If the carb is pulling in additional fuel from the transition or main circuits to begin with, then the idle screw check may not be a good indicator.  If the engine doesn’t smell any richer out the exhaust than before, the power valve is likely okay.  On my end, I have a power valve checker that eliminates the need for all the other less than definitive checks that can be performed.

Power valve protection in the carb helps but doesn’t guarantee that a backfire will still not damage the power valve.

You mention that you are running live (direct) vacuum to the distributor so this rules out the ported vacuum signal being premature and causing the popping.  This also eliminates the fast idle on the choke causing the problem.  But the popping itself can be explained by the live vacuum to the distributor and especially if the ignition timing at idle was set with the live vacuum unplugged.  Why the popping is happening now and not before assuming nothing else changed is beyond me unless the engine before the distributor was removed was originally timed with the live vacuum in place and not unplugged.  But running the ported vacuum to the distributor should eliminate the popping if it’s vacuum signal related.  At dead idle, the ported vacuum signal will be zero simply due to the port in the carburetor being above the throttle blades.  As soon as the throttle blades are opened enough to expose the ported vacuum hole, then a vacuum will be supplied to the distributor.

Lorena, Texas (South of Waco)

 make sure the vacuum canister is holding vacuum and functioning properly, and rule out any vacuum leaks.[/quote]

      Likely cause, especially if disconnecting stops the problem.

Frank/Rebop

Bristol, In ( by Elkhart) 

I'm running live vacuum. I don't think it has anything to do with the way I have it tuned. I didn't change anything. I put the static timing back where it was. The popping started after the backfire through the exhaust. It did not do it at all before.



I'll check the power valve, air bleeds, make sure the vacuum canister is holding vacuum and functioning properly, and rule out any vacuum leaks.


Lawrenceville, GA

Charile are you running your outfit with "live" manifold vacuum at idle? Its a quick change to swap over to the "ported" and vice versa. I would expect a reduction in advance to make it more rich though. Live vacuum will lean it at idle and raise vacuum - ported will richen it and increase water heating. 

Carbs tend to get tuned to your spark advance curve - if you've got a quick one, that goes up to say 36°/38°, the plugs will look lean until you deliberately fatten the carb to match. Generally (and this is just my own habit) - I try to tune my engines to run clean (edge of rich) with no vacuum advance at all. Then the vacuum pot gets used to give it cruise economy (lean out) without knocking. If you've got a lot of mechanical advance built into the distributor, then the vacuum pot has to be adjusted to cut back the gross amount at maximum.

 
 Steve Metzger       Tucson, Arizona

I pulled and plugged the vacuum advance line before leaving work yesterday. No popping while warming up before leaving work. No popping this morning during warm up. I don't know why the vacuum advance would have changed. I didn't do anything to it. But it could indicate that a change in the mixture took place.



In my Holley manual it says to check for a bad power valve by turning one of the idle screws all the way in. If the engine dies, the valve is good. Is that not so reliable? The carb has the power valve blow out protection.


Lawrenceville, GA

If you’ve ruled out an intake or carb gasket being blown loose during the backfire, then a ruptured power valve in the carburetor comes to mind.  An easy check for the power valve without removing it is to check the float level immediately after killing the engine and then checking it a couple of hours later.  If the float level drops, then a faulty power valve is the likely candidate.

 

Another thing to try is just spray some carb cleaner in the eight air bleeds located around the venturies.  There’s always the chance that some debris found its way into one of the metering block channels when the engine originally backfired.

Lorena, Texas (South of Waco)

Charlie,

             No clue as to the intermittent poping, but the boom was caused by unburnt gas (fumes) in the exhaust system when you were cranking it w/o the coil hooked up. Might look and see if you fluffed a plug for the miss.r

Frank/Rebop

Bristol, In ( by Elkhart) 

None of the others are reversed. I did not remove any wires from the distributor cap and I have all wires numbered on both ends. I made sure they were all right after discovering that I had swapped 5 and 6.


Lawrenceville, GA

Charlie, I had 7 and 8 reversed once and it would make a loud pop out of that bank.Can you isolate which side it's comming from, or does the h pipe make that impossible?

  Dennis in Lititz PA