The Stovebolt Site Forums The Shop Area Engine Shop Gas not staying in carb

This is a 51' 235 rochester 1bbl, glass bowl filter.

I have to pour gas into the carb to get it going and after sitting for a day same thing. When it is hot, restarts are fine.

Can you break it down for me where to look for the problem?

TIA

Manual or automatic choke?

Check the fuel pump one-way valve on the output side. Disconnect the fuel line at the carb inlet and deadhead it into a 1-10 PSI pressure gauge. Start the engine and run it for a few seconds on the fuel in the float bowl. The pressure gauge should read somewhere in the 3-5 PSI range. Shut the engine down.

Now- - - -

Important- - - -

The pressure should remain on the gauge for several seconds, maybe as much as 10-15, after shutdown. If it does not, you have a bad output check valve. Change the fuel pump. The inlet check valve can be checked the same way using a vacuum gauge at the fuel tank side of the pump. Vacuum should also stay up for a short time, indicating the inlet check valve is good.

H.L. Perfecto. Thanks, I'll get on it.

Hotrod, I have not made it to the parts store for a guage yet.
Volf - Manual Choke

I would suggest making sure the accelerator pump is working correct in the carb.

What does an accelerator pump have to do with starting?
Jerry

It primes the carb with a couple of pumps on the accel. pedal. I can't see all the gas evaporating out of the fuel bowl in one day.
George

Doing a flamenco dance on the gas pedal is for people too dumb to use the choke.
Jerry

I have been doing that "dance" & working chokes for 65 years. I don't think I am "dumb" about it. The accel. pump pulls fuel at or near the bottom of the fuel bowl. By "dancing" on the pedal you can prime the carb & get the engine started off the remaining fuel in the bowl before the fuel pump can fill the bowl. I'd rather pat the pedal a few times then grind away at the starter.
George

I'm not patient enough to wait for someone who knows what they are talking about (referring to myself as the one who doesn't know he is talking about), but isn't the float needle and seat at the top of the bowl? And if that is so, how can fuel "drain or be siphoned" back down stream to the fuel pump due to a leaky pump valve?
Carl

I had the same thought.

You have a serious problem if the float bowl drains (and it cant drain back through the line), except in very hot weather due to evaporation.

You are right. The fuel can't drain out of the bowl. It will evaporate though. The fuel in the line & a fuel filter in the line can drain back. Then the pump has to work several times to start filling the bowl. That is why a good accel pump is necessary to prime the carb for a quick start. All The fuel in the bowl will not evaporate overnight except very high temps. If it sets about a week or more it may be hard to start but again a good accel. pump will squirt fuel into the carb throat as soon as the bowl is partially filled by pumping the gas pedal. I feel the fuel pump & accel. pump need to be in good cond. for easy starts.

It can not drain the bottom of the bowl through the needle and seat if the seat is at the top. There has to be a rust hole or some funky siphoning going on. To truly find out if the float bowl is dry, pull the top.

I would bet the accelerator pump is not providing enough of a shot to get the engine started. In my opinion, it's standard practice to give the throttle a pump before starting a cold engine. Back when carbs had automatic chokes, you had to hit the throttle to activate the choke and at the same time shoot a little fuel into the carb.

Isn't there a slot in the bowls that feeds fuel to the the accel pump out of the bowl? If the accelerated pump is bad, some fuel could drain past the accel pump fuel into the throat, emptying a good deal of the bowl. Maybe I'm off base (again). It's been a while since I had a roch apart.

"I have been doing that "dance" & working chokes for 65 years. I don't think I am "dumb" about it."

Good morning Wrenchbender.

I also do that "dance" because I've found that a squirt of raw gas into a cold engine helps it to fire up immediately.

You are definitely not "dumb" for doing that. In fact, it is essential to depress the accelerator at least once to start a cold engine with an automatic choke on the carburetor. Unless you do, the choke can't close and the idle speed screw can't position itself properly on the fast idle cam.

I've been called way worse than "dumb" by the same person on this forum. It was so outrageous that one of the moderators sanitized that thread by totally deleting that particular post.

A few thoughts:

(1) As mentioned by others, the fuel valve is in the airhorn, so the fuel will not drain back to the tank.
(2) While less of an issue with an inline 6 than a V-8, modern fuel DOES evaporate, and evaporates MUCH MORE RAPIDLY in hot weather on the inline 6.
(3) The dual quads on my 390 CID V-8 are both bone-dry in less than 1 hour after shut down. These are Carter AFB's so no gasket below the fuel line, no fuel leak.......fuel evaporates VERY rapidly with V-8 engines that act as a heat sink. I was forced to add an electric fuel pump in order to not continually purchase batterys
(4) A couple of repetitions of accelerator pump action CAN be useful to aid in starting if, and only if, there is fuel in the bowl. As the fuel in the pump cylinder is under pressure (the pump skirt above), it is the first fuel to go. The pressure forces the fuel out of the pump discharge jet.
(5) VERY easy to test the fuel pump for fuel. Before starting when the engine has sat unstarted for a day or two, remove the air cleaner, hold the choke butterfly open, and work the throttle. No squirt of fuel.....no fuel in pump cylinder.
(6) A wee bit of testing can eliminate a lot of conjecture.

Jon.

Low float setting can "accelerate" evap. Then hard starts until, a possibly weak, fuel pump catches up. Sometimes it's more than one old tired part.

Jon, on the AFB, does the fuel evaporate through the vent tube? Can this be slowed by restricting the size of the opening at the top of the tube without affecting the operation of the carburetor?
Also, why do we pronounce it "carb-er-ater" when it is spelled to sound like "carb-you-ree-tor"?
Carl Knauer (sounds exactly like it is spelled!)

Re Dumb??? Huh??

Then call me dumb. A couple pedal pumps on a cold engine dumps raw fuel into the manifold. Both the accelerator pump and the choke enrich the air fuel mixture for the cold engine. With sufficient compression and spark, Result is usually a running engine.

Reciprocating engine aircraft do the same thing to start a cold engine....except they often use a small hand pump called a "primer". Two or three good pumps of prime enriches the air fuel mixture in the manifold and the cold engine usually starts. Must have been some dumb aircraft engineer figure that out. I tested this theory on the trusty Cessna 182 last weekend at 10 deg F., sure enough it started.

"The dual quads on my 390 CID V-8 are both bone-dry in less than 1 hour after shut down. These are Carter AFB's so no gasket below the fuel line, no fuel leak.......fuel evaporates VERY rapidly with V-8 engines that act as a heat sink. I was forced to add an electric fuel pump in order to not continually purchase batterys"

Jon,

Thank you for posting that information. Would it help any to open the hood to let some of that really hot air escape and let ambient air cool the carbs and manifold?

Ray - Opening the hood doesn't seem to help. Placing a 2 inch fiber spacer under the carbs would help, but would also significantly impact the idle quality.

Using Holleys instead of Carters would help (the bowls on the Holleys have an air gap between the bowls and the manifold), but would totally kill the fuel mileage.

The electric pump basically solves the problem as best it can with the "compromise" (engine modifications) that I have selected.

Jon.