Retiredvette52

I have an L-46, 1970 replacement engine in my 66 vette that I recently purchased that seems to drain gas back to the tank after sitting for a few days. I either have to crank the engine over while pumping the pedal vigorously for a bit or pull the air cleaner and add a little gas or spray a little starting fluid in the carb to get her going.The accelerator pump has no gas present after sitting , the carburetor is a rochester quadrajet. Any idea why the fuel system seems to allow the gas to drain back?

JohnZ

Fuel can't drain uphill. Sounds like simple fuel percolation during heat-soak after shutdown, which is quite common in our vintage cars with today's fuels.

PAmotorman

use a fuel filter with a check valve because as fuel evaporates from the bowl it allows the needle/seat to open and the fuel will drain out.

survivor66

...unless you consider siphoning draining uphill. I still remember the taste of gas after a botched tank siphon attempt in my youth, not that it can happen in a carb. Agree on percolation.

PAmotorman

if your Q jet has the correct needle/seat like this which is the GM one. as the float drops from the fuel evaporting it opens the needle seat and fuel flows out thru the needle seat and back thru the fuel line and into the fuel pump to tank return line. any hi perf engine with a carb should have this type needle seat so the fuel does not have to flow up passed the needle to get into the float bowl. most rebuild kits do not have this type of needle seat and you should drill a .125 dia hole thru the brass just above the seat area to allow full fuel flow

larrywalk

I have found that it improves things to stop this drain back by removing the tiny spring clip from the upper end of the needle. That way a small downward motion of the float won't pull the needle up and allow the bowl's fuel to drain back.

Also, another way to prevent this problem is to use an internal fuel filter with a check valve in the inlet of the Q-jet.

In any event, the drain-back path goes through the fuel pump through a small orifice about .060 and into the 1/4" return line and thence to the tank.

MikeM

"66 Corvettes didn't have return lines but Q-Jets will go dry.

6D2148

They leak between the base and main body

Retiredvette52

Thanks to all for the suggestions and feedback.It's a great feeling to know that help is always just a question away.

67*427

My Tri-Power BB has had this problem as long as I can remember. I'm not sure if it's the new alcohol laced fuel that does it but my carbs are typically dry when I want to fire up the car if it has been sitting for a couple of weeks or longer. I got a squeeze bottle and use that to put some fuel in the center bowl and a little squirt down the canter carb throats and it usually fires right up.

Steve

Westlotorn

We have an old Fiat 1971 124 Spider with a Webber carb, (my wife's toy ) it has done this same thing for many years. I never have worked to find the solution. I suspect a carb rebuild and maybe a replacement fuel pump is in order. Any time it sits more than a week the carb is dry and needs a prime or extended cranking to fire the engine. It is hard to get motivated to fix a car we don't drive and once it fires it runs good.

hope2

correct, 99% that is the problem, take carb apart and epoxy the two plugs, did a million of them back in the day. also, doing this makes them run better, no "leakie"

plaidside

I agree with JohnZ's statement, "Fuel can't drain uphill. Sounds like simple fuel percolation during heat-soak after shutdown, which is quite common in our vintage cars with today's fuels."
The Quadrajet has one of the smallest fuel bowls so there isn't a lot of fuel in there to start with.


As far as the needle and seat, take notice how far up the window is on the seat. It is 13/16 from the floor of the carburetor. There will still be plenty of fuel left in the carburetor even if the float dropped to allow the fuel to exit the slot.

I have rebuilt hundreds of Q-Jets and I test each of the FOUR plugs on the bottom of the carburetor body. I think I had maybe one leaker.
I have a 70 Olds 442 with a Q-jet and after sitting for a while I have to crank it more than my other cars to get it started, it is normal with today's fuels.
Joe

larrywalk

The pump shot will still go away even though there may be still fuel in the bowl. When the fuel level drops below the window, it also drops below the level needed to flow into the accel pump well. You'll get only one shot then nothing. This is unlike the Holley which has the accel pump inlet at the floor of the fuel bowl.

I agree about the leaking plugs... I've never found a leaker with any I've rebuilt.

MikeM

Why do Q Jet float bowls go empty? Read this. It'll keep you off the street for awhile.

https://www.google.com/webhp?sourcei...oat+bowl+leaks

street62

Holley's do the same thing after sitting fo two weeks. I jut put some gas in the carb.

MikeM

This recall in 1969 was for well plugs leaking.

2,966,979 GM vehicles recalled.

NHTSA ID 69-0031: 1968-69 all General Motors cars & trucks with Rochester Quadrajet carb, carburetor fires, 3-25-69

63redvette

If you keep adding fuel to your carb with the air cleaner off, do yourself a favor...put a towel over the carb before you crank it. Why? My dad was a burn surgeon for over 20 years and I heard all the stories.

JohnZ

That also spelled the instant end of the "hot-slot" design in the carb mounting pad for pre-heating the carb at cold start; I remember that day very well, when we shut the plant down (Lordstown) waiting for a chartered plane full of Q-Jets from Rochester Products while we pulled Q-Jets off of every unit on the property to return them.

donbayers

I'm having the same problem on my Q-Jet, 1970 LS-5.

Funny, it was working fine and then it all of the sudden started going dry overnight, not in a week or two but literally overnight.

Sounds like I should

1. Pull off and check these plugs and
2. Install that one-way check valve filter

Does anyone have a part number for this one-way check valve so I can pick it up at box store or order?


Thanks