AVS is sinking floats

69 RR 383 with 4615S Carter AVS. I have had two brass floats fill with fuel a couple years apart. Both times the top gasket shows signs of fire and one float will have dark marks all over it. The carb was restored by a respected person for concours use when the car was completed 5 years ago. The car only uses non ethanol fuel from a fuel distributor. If it sits for more than a couple days the bowls are empty and it has to crank for 15-30 seconds before firing. Less than that, it fires immediately but is cold-natured.

My take is that after sitting when I try to start the engine it "pops" back through the carb and starts a flash fire in one of the bowls. This melts the solder and then the float fills and then the fire is out. I then get clouds of black smoke and running on 4 cylinders.
Is this possible?
What allows it to do this?
How do I stop it?

Install an electric pump. Use it to fill the carb. Turn that pump off. Then use the key to crank over and start the car, while using the mechanical pump for the start and run. Check the timing.

Carter bowl vents are notorious for allowing evaporation from the float bowls. Every AVS carbed motor I own does it. Electric fuel pump is one solution. As for a 'fire in the carb'? You'd be losing more than floats if that were true - you could be melting the castings and burning the car to the ground. Black smoke and running on 4 cylinders sounds like it's flooded. I'd chase the source of any 'backfiring' down first, like timing, crossed or bad plug wires, valves lashed too tight or burnt valves. And then buy new floats.

I have had this problem also. Yes it backfired and the flash was hot enough to melt the solder opening a small hole in the seam of the float halves. It alowed enough fuel into the foat to sink it resulting in raw fuel dumping into the motor.Didn't find until I took the float out and shook it

We timed it using the "total timing" method and it runs fine when it is used regular but when it just does the car shows I get the dry bowls. No misses, no pops, no bogs, it runs perfect except for this little punishment when I leave it in the garage to long. The car is assembly line correct and I don't want to deviate from that. I suppose I could fill it from the vent tubes before firing it up when it is dry, but I would like to know how it is blowing flame back into the float area. I'm glad it is not just me experiencing this. I will take some pictures tomorrow and post them so you can understand the issue. I don't take tools on trips and I prefer to not start.

On mine it was going thru the hole on the driver side where the rubber flapper valve is that opens and closes with the accelerator pump

I don't see how the flame could go in from the top. This has to be in a power valve type of circuit. Maybe up through the jet, around the power rod when it is lifted up. It then flashes when there is very little fuel in the bowl and goes back out.

The issue is not the carb. It is the availible fuel. Today's fuel evaporates quick. Add an electric pump, problem solved. I don't believe anyone has had a fire in the float bowl. There would be significant damage. When I has this explosion it melted the accelerator pumps. Did nothing to the rest of the carbs.
Doug

It happened long ago with real gas. 69 440 HP DD Ya it had a few miles on the motor. It did back fire and started running rich. I took the carb apart and found the float sitting on the bottom of the bowel. When I removed the float and shook it there was liquid in it but did not leak. There was a small sooty black mark around a bright spot in the solder on the float. This was just under the opening on the driver side where the accelerator pump is located. Some solders melt at fairly low temps. Apparently the solder melted long enough to allow some fuel inside and as it cooled resealed the float. Gas in an open container doesn't explode it burns. So I guess with an open source of fuel a little oxygen and a ignition source it can't happen. Aerodynamically Bumble Bees can't fly either

Here is a pic of the latest episode. The first time actually burned a big hole in the gasket above the float. This time is was just a little spot on the gasket and the float is all black in spots, while the other side is pristine.Time in between incidents is about 2 years and less than 1000 miles. Gasket was replaced with a new one.
Interesting note repeating from an above comment. The float does not leak but is about a quarter full of fuel. Both times!

Look on the soldered seam you should find where the solder melted should be about where the burn mark is on the gasket

Well I have to say I stand corrected. I wouldn't have thought any fire in there possible w/o major damage.
Doug

Best blame so far has been ignition. Wouldn't have happened back when the car was younger and the gas did not evaporate. The best explanation I have gotten so far is that carb is empty, and when the fuel pump get first hint of gas into the carb it is vapor. The engine mis-starts and backfires up through one of the jets, around the step-up rods and lights at the needle and seat. It melts the solder and by that time I am trying to restart the engine and the fuel pump finally gets liquid in the bowl and it fills the float, because the hot float creates a suction as it cools, then it cools the solder and seals the float back up, because apparently liquid gasoline does not burn, just the vapor. Now the float is bad on one side and the engine only runs right on 4 cylinders because of the dual plane intake. Unless someone has a better Crystal Ball than the outside people I have quizzed about this.
Best workaround without modifying the car is going to be filling the vent tubes with fuel before starting when the car has been sitting for a while. Or dynamite.

If you don’t want to put an electric pump on it, just crank it over without pumping the gas pedal or setting the choke until after it’s had enough time to fill the bowls.

The pump it a few times and it should take off.

Today’s pump gas is lacking some of the additives that used to be there for vapor lock, etc.

IMO, an electric pump in the rear is a good idea for anyone these days.

The car I drive most often (3-5 times a week) acts like you are describing. If I leave it sit over night after use it will fire right up. If it sits for two days you have to crank, pump crank, pump crank, pump and maybe one more crank and pump to get it to fire especially during winter months. This issue is completely resolved if I run 50% or more 110 race gas mixed with any variety of pump fuel. It's a stock 383 so it runs on 87-92 and personally I'm not so sure the ethanol free fuel is that good either. On straight pump fuel (any of the above) the fuel will percolate in the float bowls at shutdown if the engine is heat-soaked.

The best remedy is to toss five gallons of 110 in the tank now and again with the unleaded swill. Unfortunately that is fallacy of the "pump gas" craze in engine building...you build this killer engine with compression that will work with pump gas but then you are afraid to feed you new engine crap lol.

Personally I don't like cheap electric fuel pumps for reliability/QC reasons and don't want a quality pump buzzing away over the oem mufflers.

appears to be black on the choke blade and air valve. indicator of lean condition or late timing; maybe both. I did a 4615 for a friend last summer who had a float issue. I think maybe those old floats may have seen there days. anyhow, I replaced the float (edelbrock floats will work), went to a .093" primary jet, and richened up the idle circuit using the adjusting needles at the base of the carb. works very well. the fuel evaporation issue is the nature of todays gas and engine heat. both my cars use carter carbs and I fill the float bowls thru the vents before starting if the car hasn't been ran regularly.

I had one of the floats sink on my AVS last fall. When I removed it you could hear and feel the gas sloshing around inside of it. The strange thing is that I was unable to shake any gas out of where the leak was.
I drilled a tiny hole in the float and drained out the fuel then soldered the hole back up and reinstalled the float. Problem solved for now, but I'm wondering if it is only temporary.

It will leak again.Wipe the float dry,lay it down in various positions,eventually you will see where it is leaking,you can re solder it but just as easy to replace it.Just went through that a week ago on a friends Sport Fury.

Here is the first flame out I had a couple years ago. Float still has fuel in it.

It’s been said a bunch of times above - you’ve got some sort of timing issue if it backfires. Make sure your timing marks are correct on the damper, use a positive stop on piston #1.

I’m lazy, and decided not to install an electric fuel pump. What I do when my dart sits and the gas in the avs evaporates - is I take a small amount of gas (about a 1/4 cup) and pour it into the fuel bowls through the bowl vents. Then it starts immediately !

You may not have any problem other than the float bowls being dry. A lean air / fuel mixture can cause an engine to back fire through the intake. (A rich mixture will be through the exhaust.) With the float bowl being dry, the mixture is likely to be lean while the float bowl is filling.

If it was my car, I would very carefully check out the ignition system to make certain there is not an ignition problem as well. It is entirely possible the car has two problems. Gasoline burning inside the carburetor is not good!!

The only vent on a AVS that I know of is the one that is on the driver's side. It is fairly easy to add fuel to that side, but how do you add fuel to the passenger side bowl? Can it be done?

The holes right next to the metering covers.
Doug

Back in the 70's & 80's, the only carbs that would go dry after just a few days were GM Rochesters in my experience. You could only start GTO's after priming the carbs - it was maddening. NEVER had a Mopar with an AFB, AVS, or Thermoquad go dry - even after sitting for a month. My '71 Hemi Charger could sit all winter & it would fire in the spring after pulling the choke, a few pumps to the floor & a little cranking.

The Rochester's had the same problem as the TQ's,leaking wells under main jets.

Doug, Is this the spot to fill the passenger side bowl?

Check the underside of the AVS for main well plug leak. Too much time in the carburetor dip can damage the aluminum main well plug on the bottom allowing the fuel to drip out/into the manifold. If it's difficult to drive when cold then there's a miss-adjustment on the choke valve or the thermal bimetallic choke thermostat rod on the passenger side (pictured).

I'm pretty sure the factory correct carburetor base gasket will cover up the main well, but I'm not sure. Old age memory.

A stuck exhaust manifold flap valve (I forget the correct title) located in the passenger side manifold can cause the intake manifold to run too hot and boil the fuel. Make sure the spring loaded valve can open fully and move freely.

BTW, I too have a 1969 RR 383 with a 4615 AVS. Best of luck and we need more pics.