Dry Carb Starting

[bob_amos]

There are a number of things that contribute to the loss of fuel in the bowls. Todays fuels defenately do evaporate at a higher rate of speed than they once did. Leaks as well as low fuel levels in the bowl to begin with all can contribute. But I have a 53 Cranbrook and a 63 Belair. While the Cranbrook would go completely dry the Belair still retained quite a bit of fuel. So, the evep rate was not my problem. Oh sure, over a number of weeks of standing they both might be dry but after a few days the Cranbrook was while the Belair was not. Here is what I found on mine. First to stop the loss of fuel at a rate that was not what it should be I located a leak at the lead plug under the bowl of the carb. These plugs are there for machining purposes when building these things. After years of use they loosen up and let the fuel slowly drip out of the bowl. Look at the intake manifold right under the carb. If it is discoloured then you might be suffering a leaky plug. I removed the old plug and installed a new one. I was sure to crush the plug properly so as to seal but not damage anything. This stopped the accelerated loos of fuel from the bowl. But I still could not let her sit for a few weeks without the fuel going bye bye. I installed an electric fuel pump (6 Volt unit) on the left frame rail. Now I just turn of the key and wait for the pumps sound level to quiet down some. I then have a full bowl. I run mine through the original pump but am thinking of gutting the original pump. Make a plate to seal it from the crankcase chamber and just use the electric pump. It'll look stock but will be much better. It will also help with vapor lock as the pump runs from 4 to 7 Lbs. pressure. That will raise the fuel volatility point and help avoid vapor lock on those hot So. cal days. It's a win/win situation. The pump really is quiet so is almost unnoticable, even to me, and I know it's there.

[NatesSedan]

I actually like my electric pump. Turn the key and wait a few seconds. You get to think about it before you start it. Anticipation factor I guess. That said, I was going to switch back to a mechanical pump in my '48 (with a 350 SBC) but the only way I was getting one in was to cut up the inner fender. Needless to stay I will stick with the electric. Tick tick tick tick tick tic ti .....voooom. One of my favorite sounds.

[55 Fargo]

Started my engine tonight, it has been over 24 hours since last started, started first crack, not sure why the other day it went dry.

I still am going to replace it ASAP, but also like the electric fuel pump idea.

[Don Coatney]

It will also help with vapor lock as the pump runs from 4 to 7 Lbs. pressure. That will raise the fuel volatility point and help avoid vapor lock on those hot So. cal days. It's a win/win situation. The pump really is quiet so is almost unnoticable, even to me, and I know it's there.

Bob;

As a shop owner what is your take on vapor lock? Can you describe exactly what vapor lock is?

[De Soto Frank]

Not trying to throw rocks at a hornet's nest, but the 1940's and '50's MoToR's Shop manuals offer a pretty down-to-earth explanation of "Vapor lock" in the "Tune-Up" section towards the very front of the book.

I certainly wouldn't mind hearing Bob Amos' take on the phenomenon...

Just about every vehicle manufactured these days has fuel-injection, and the electric pump is located in ( or near ) the fuel tank; the fuel is pushed under pressure most of the way up to the engine.

Our old mechanically-pumped MoPars (and every other old Detroit Iron) had the fuel pump up front at the engine, and "pulled" the fuel from the tank, using "suction" created by the fuel pump.

Now, increasing the pressure on a liquid in a system will raise its boiling point (the theory behind our modern pressurized cooling systems). Conversely, reducing the pressure on a liquid in a system will lower its boiling point ( At the top of Pike's Peak, or even in Denver, water boils at about 202 deg F.).

So, if our fuel-pump is is "pulling" modern gasoline a long way from the tank, and that line gets warmed by the vehicle exhaust and/or hot pavement, this increases the chances that our liquid gasoline is going to "flash" into vapor, which then acts as a "cushion", halting the action of our mechanical fuel-pump.

But, I believe that "true" vapor lock would then have to occur while the vehicle is running, and shows itself as fuel starvation (or "leaning-out") while the vehicle is being driven, perhaps only on long pulls.

The issue that many of us have encountered when we park our old sled at the grocery store on a hot summer day, and come back to it 15 minutes to a half-hour later, and crank and crank until the battery goes flat, is more likely "percolation". This results from heat from the engine "soaking" into the carburetor, causing the fuel to boil in the carb bowl, and resulting in one or both of the following: the boiling fuel creates pressure inside the float bowl, and forces liquid gas into the carb throat, flooding the carb all by itself, and/or, the boiling fuel dissipates as vapor through the anti-percolation valve or the bowl vent, depending on the carb design.

In the first case, we come back to a flooded engine; in the second case, a dry carb ( and possibly true vapor-lock between the pump and the tank); and in the case of both, we have a flooded intake and a dry float-bowl.

Chryco had reasonably managed this issue by the use of: a heat shield between the manifold and the fuel pump, a bakelite insulator between the intake and the carb, a vented float bowl, and perhaps a metal heat shield between the carb and intake.

Now enter our re-formulated, more volatile fuel-injection-tailored fuels of the 1980's to present, designed to be under pressure from tank to combustion chamber, and we have much more likelihood of both percolation and / or vapor-lock.

Citing my own experience, driving stock flat-head MoPars over the last 20-odd years, I have not had problems with the mechanical fuel pump being insufficient for the job. My cars' carbs DO go dry after sitting for several days ( and leaking-down through the soft-plugs may indeed be part of this), and I have run into percolation issues occasionally when returning to a hot car(most troublesome with my '54 Chevy pick-up).

When starting a hot carbureted vehicle, I usually depress the accelerator pedal about 1/2-way and hold it there while cranking, to help clear-out any percolated fuel during the initial cranks of the engine.

I'm still running on six-volts and mechanical fuel pumps, across the board.

For what that's all worth...

De Soto Frank

[Don Coatney]

Frank;

Thanks for your comments. With your statment "(true vapor-lock between the pump and the tank)" you have described what my long term argument has been. And like you I would like to hear Bob Amos's take on the phenomenon...

Most any liquid will turn to vapor in a vacuum. But in a pressure (read discharge fuel pump pressure) the liquid (gasoline in this case) will not turn to a vapor.

A gasoline fuel pump in our cars cannot pump with a vapor in the suction line as it requires hydraulic pressure to move the liquid. The wooden clothes pin "remedy" is snake oil. And the ammount of heat required to vaporize gasoline in the fuel line is very unlikely to occur in the fuel line before the fuel pump.

If there is a small leak in a fuel line allowing air to enter before the fuel pump then "true vapor lock" may possibly occur.

[greg g]

Oh no the dreaded vapor lock question again. I had this conversation with a number of folks at the detroit meet. Several were sefl described engineers, me, I just a farily good student with decent mechanical skills. As a result of my conversations I have refined my Vapor Lock postulate.

To wit:

In any system where fluid under pressure is being delivered to a vented recepticle, a bubble of air or an area of vaporized liquid in the delivery tube cannot stop the flow of pressurized fluid. If it could we could not bleed our brakes or get water to come out of a new hose attached to the out door spigot. It is hydrodynamicaly impossible for the bubble to resist the pressure of the fluid, pushing it toward the atmosphere.

Now after a conversation with a certain Canadian fellow who used to work as an engineer at a nuclear facility, I have ammended my view in that if perculation occurs inside the fuel pump, it could creat a situation where the fuel pump can not create enough vacuum to pull fuel from the tank, or enough pressure to move gas uphill toward the carburetor. as the air or vapor will need to be pumped throught he fluid gas before the fluid will be moved. Hence the heat shield between the exhaust manifold and the fuel pump.

That said I believe if you look at a schematic of you car, the fuel pump and the bottom of the gas tank are realitively at the same level. and since fluid seeks it own level (google the tool called a water level) the pump does't need to do a gread deal of sucking. I have a fuel filter on the frame rail near the drivers door. If I do not clamp the line while changing the filter, the gas tank will run itself nearly dry. I believe if you do the measurments the fuel pump is nearly level with the bottom of the tank. So the pump isn't really fighting an uphill battle till it gets to the carb side.

So make sure your carb isn't leaking, make sure you don't have air leaks on the tank side of the pump and use the proper starting procedure when trying to start a warm engine.

From the owners manual.

Turn the key to its far right position,

Press the accelorator pedal 1/2 half of its total travel THIS IS VERY IMPORTANT

Press the starter motor switch to start the engine.

The manual then goes into the engine does not starte immediately drill;

Are the cylinders flooded over choking or flooded as indicated by the strong odor of raw gas, Assure the choke is fully opened, the crank the engine with the accelerator pressed to the limit while oporating the starter.

Fuel not entering the combustion chambers. Disconnect the fuel line at the carb and crank the starter if gas is present fuel should be getting to the combustion chamber, unless the jets or valves are clogged with dirt,

If no gas is discharged at the carb, the difficulty may be caused by air leaking into the system through a loose connection or break in the line or an inoperative fuel pump.

The debate continues. by the way two out of three engineers agree and the other was on the fence but leaning toward agreement.

[Allan Faust]

Take the gas cap off, seal you face to the filler pipe and blow as hard as you can 3 times, then hold the pressure for the count of 15. Get in and start it. The only problem is releasing the pressure without swallowing or inhaling it. Say,,,,, maybe thats my problem.

No kidding, it does work.

Hmmm.... I can see myself, in the middle of town, kneeling and blowing into my gas tank..... it would definitely be worth the laugh anyways....

Allan

[Niel Hoback]

You don't have to kneel, just bend over.

[Niel Hoback]

You don't have to kneel, just bend over. It's not as funny looking that way.

[james curl]

Do not do that if the tank is close to full, the pressure has to go somewhere when you try to remove yourself from the fuel filler. If you create enough pressure, when you pull away you will be followed by gasoline with a big rush when the pressure is released, don't ask me how I know but a gasoline bath is not fun.

[Allan Faust]

You don't have to kneel, just bend over. It's not as funny looking that way.

I find it looks just as funny Hmmm.... Neil telling me not to kneel but bend over.... Yup, looks just as funny....

Allan

[De Soto Frank]

Do not do that if the tank is close to full, the pressure has to go somewhere when you try to remove yourself from the fuel filler. If you create enough pressure, when you pull away you will be followed by gasoline with a big rush when the pressure is released, don't ask me how I know but a gasoline bath is not fun.

Hey there, James...

I know how you know this .... experienced it a couple times myself, in my foolish youth.

I learned real quick that raw gasoline may not cause discomfort to the hands, but on any other part of the body (arms, face, etc.) it burns like H___ !

Gargling with Ethyl or a splash of Flying-A after-shave are not fun !

But, as I've gotten older, and done more thinking along with my wrenching, and taken the time to clean-out gummy fuel systems, etc, I've had to do a lot less "mouth-to-gas tank rescue breathing"...

The last culprit in this recurring crusade was my 1960 Chrysler Windsor... I couldn't do the MTGTRB, because the fuel filler is behind the the license plate, recessed into the rear valence of the trunk... only an anteater would be able to get his kisser up to the filler pipe !

De Soto Frank

[De Soto Frank]

Hey Don,

I had a couple more thoughts on this subject...

I see the clothes-pin trick most frequently on flatty Ford V-8s ( which we all know can overheat just sitting in the garage, up on blocks, with the engine shut-off )... Ford placed the mechanical fuel pump in a mecanically-convenient location: right on top of the engine, at the rear of the block, where it could be actuated by a push-rod riding the cam-shaft.

Unfortunately, that places the fuel pump at about the highest possible point, relative to the gas tank, as well as in about the hottest place in the engine bay. Add to that scenario a hot-tempered engine, and you can easily predict the rest.

AND, I usually see the clothes-pins on the section of line between the pump and the carb: the pressure line. (WTF ?)

Regarding pin-holes and other issues with the tank line - it only takes a tiny rust-perforation to wreck the "suction" on the tank line - I have this issue with my 1955 Chevy first- series dump truck: the tank line is perfed enough to allow air to infiltrate, but not enough to cause a drip; the line is rusty all-over, but it quite a bit darker in color in the most heavily pitted sections; this is where I think it is perfed.

The line is tight enough that it will provide enough fuel to start the engine, and let it idle; but if I get the big guy rolling, and have to pull any kind of a hill, the engine starts to starve for fuel half-way up... pump is new, carb is re-built and works fine on other vehicles, tank pick-up is clear... got to be that rotten fuel line.

The other culprit can be a length of rubber hose in the tank line: too long a legnth of it will tend to collapse under suction, particularly if there is a restriction between the hose and the tank. I think that's one of the reasons the factory flex-hose between the frame and the fuel pump had braided wire reinforcement inside, and was kept to a minimal legnth.

I also took a look at my desk copy of MoToR's 1935-'49 Shop manual, at the service section for the Carter BB carbs: there are several lead-plugs in the low parts of the float-bowl casting, one of which is on the front side, under the float bowl: follow a straight line down from the fuel-line connection, to a point just above the throttle-body: you'll find a lead-plug there... if that gets "loose", that could indeed be a place for gas to drain-out, and leave the float-bowl dry...

Anybody have a source for new lead-plugs for carbs ?

De Soto Frank

[Young Ed]

I've never found a need to remove one but the carb kits from Napa come with a couple of those plugs. Not sure if one is the proper one for the one you mentioned or if they are for elsewhere on the carb