Carter AVS on a 340 drains of fuel

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caper340

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I have a 1969 Swinger with the original 340\4 spd that has been completely restored over the past 3 years. The carb is the original Carter AVS and has since been rebuilt 3 times by a "Professional" carb shop. Here is my problem, when I shutdown the engine, the fuel will siphone from the carb within an hour or two. I can't find any leaks, the fuel pump has been replaced, as has: the fuel tank, all fuel lines, filter, tank vent tube, sending unit in the tank. There is nothing left to replace other than the carb from what I can tell. I wanted to avoid that as it is an original numbers matching restoration.
Any suggestion? Thanks.
 
As in NO fuel in the carb. With a see-through fuel filter installed you can watch the fuel siphon back into the tank, or wherever it is going.
 
My 69 Swinger 340 used to do the exact same thing way back in the day. But it didn't happen right away. Mine would take at least over night or if I didn't drive the car for a couple of days then I would have to crank a while to fill the bowls up to get it started. I had a couple of friends and their Darts did the same thing. The only thing I found was that on occasion if it was really hot out and the car was running a little warmer than usual, the carb would take up heat after I shut the car off and the gas would expand and run over the air horns into the intake. Also the vent on the top of the carb for the accelerator pump was open somewhat by design and this allowed for a little evaporation also. I know that it wasn't siphoning though. I clamped off the fuel hose at the filter once for a day to see if it would make a difference and it didn't. The gas simply was evaporating or boiling over. That was the only thing I ever found to be a possibilty.
 
A lot of the problem is the fuel they us today evaporates a lot worse than the fuel they used in the late 60 early 70. ( modern efi doesent need evap protection like regular carb, electric pumps ) you can try a good insulated spacer under you carb, it seems the carters are worst than holleys, or demon, ( The the fuel bowls are not directly on the hot intake like a carter.
 
That is was my first thought too when I noticed it was hard starting after sitting. However, when I evetually got to where I put a clear fiter inline, I could see it draining back. weird.
 
Is your tank vented , I have seem where it was not vented and would have a vacuum on tank. I had a demon carb on my challenger no problem, changed to a eldebrock 800 next morning no start. put a good space under and it helped a lot.
 
haplo555 said:
Is your tank vented , I have seem where it was not vented and would have a vacuum on tank. I had a demon carb on my challenger no problem, changed to a eldebrock 800 next morning no start. put a good space under and it helped a lot.
Click to expand...
Tank is vented. Even replace the vent tube with a new one. IF I was to change the carb, any recommendations Holley vs Edlebrock. I would like to go with one that would bolt right on to the stock manifold without an adapter plate if possible. By the way, thanks for all the input guys. This has been very helpful.
 
gas cap bad? the vacuum in the tank seems to be the culprit. what if you drive it, park it, take the cap off and see if it still does the siphon thing.
 
redfastback said:
gas cap bad? the vacuum in the tank seems to be the culprit. what if you drive it, park it, take the cap off and see if it still does the siphon thing.
Click to expand...
Now there is something so obvious that I didn't think to try. Thanks for the idea. The car is in storage now for the winter but I will check for that come springtime.
 
Carb could be cracked internally and dripping out the center/inside the plenum area.

Fill the carb ontop of a stand outside of the car and see if it does it again.
It may need a rebuild on the throttle shafts.
 
Like someone above said today's fuel will evaporate out after the car is shut down. I run my car with the intake manifold heat cross overs blocked off and it helps some. But no matter what I do ALL the fuel is evaporated out after the car sits a few days. Edelbrock makes a carb to intake gasket that is supposed to help with this problem:

Heat Insulator Gaskets
Designed to help prevent fuel percolation in Performer Series and Thunder Series AVS carburetors exposed to extreme underhood temperatures. Use #9265 with open plenum manifolds such as Torker II and Victors. #9266 features a divided center which is designed to seal the carburetor to dual-plane manifolds including Performer, Performer RPM and most RPM Air-Gap manifolds. All gaskets come with extended carb studs.

Square-Bore Heat Insulator Gasket (0.320" thick) #9265
Divided Square-Bore Heat Insulator Gasket (0.320" thick) #9266

http://www.edelbrock.com/automotive_new/mc/carbs_acc/access_thun_perf.shtml
 
I wouldn't think the filter would empty out from evaporation? Seems more like it is draining back somehow. Redfastback, and Rumble seem to be on the right track.
 
My money says it's the main wells leaking. Look under the carb at the soft plugs (aluminum) below the main jets. The plugs become porous, especially after a rebuild that included a soaking.
 
It might. If theres still a bit of pressure left in the line. The fuel starts to drain down in to the intake and then the carb floats drop and let the fuel in the line in to the carb. That fuel also leaks down in to the intake.

Those plugs do leak alot.

Thats your problem I bet. You replace your fuel pump so that was not it, I don't think it can suck the fuel back out of the carb anyways like that, the carb is vented.

I don't think the fuel is evaporating in 1 to 2 hours.


All you need to do is pull the carb off right after shutting it off and see if the fuel is leaking from the bottom. Were the rods go in is the vents, you can pour fuel in though there.

I have a tiny funnel and use it to the fill the float bowls up if I dont drive my car for a few weeks, you can do that each day you go start you motor until you get it repair, instead cranking it over forever.
 
I had a problem with mine hard to start warm. It ended up being the fuel pump

I installed a heat shield, and thick insulating gasket before changing the pump. Removed the pretty chrome Holley pump, went to NAPA and bought a stock 71 340 pump for $21 and problem went away.
 
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