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Can fuel drain back from the carburetor and leave the carb empty, making it difficult to start and necessitating an electric pump? I have read this numerous times in these forums and decided to analyze the alleged problem.

1. Fuel enters the carburetor float bowl through the needle valve. The valve is always at the top edge of the float bowl, therefore if there is any drainback it cannot empty the float bowl at all.

2. Fuel pumps have an inlet valve and an outlet valve. Both of these valves are one way only and allow fuel to flow into the pump from the tank through the inlet valve and out of the pump to the carburetor. Fuel cannot flow in the other direction, that is back from the carb and into the pump or tank.

3. There is rare exception if BOTH of the valves are leaking, but in that case the pump would have very low pressure and need replacement. What would empty would be the line form the fuel pump to the carb. An electric pump would help, but a new mechanical pump is what is needed. The bowl would stay full and the car would start.

Answer, in a word, NO.

I've noticed that there is a usually a standard 3-4psi still in the line between pump and carb after letting the car sit...if there was a leaky needle/seat valve or float issue, could the residual pressure not over fill the bowl and let some run down into the manifold as it relieves the line? (wouldn't think much, maybe 1-2 tbl spoonfulls?)

Of course the bowl would still be full when the pressure evened out, and i'd imaging what leaked wouldn't be enough to notice when it came to restarting. Maybe you'd have a strong fuel smell a bit after shutting it off?

I don't think we've seen that happen. Food for thought.

After my 2bbl was cleaned, a small crack that the gunk in the carb bowl was sealing was discovered. It drained the fuel out of the carb bowl slowly over a couple hours, leaving the manifold stained and wet, outside the carb.

Rochester 4GCs are vented to the atmosphere. If my 400 sat for two weeks or longer I had to crank it a good while and keep pumping the accelerator to get it started. I checked more than once and the bowl was empty. I didn't think it drained back - it just evaporated.

I have also noticed the gas in the carb bowl evaporating after sitting for a couple of months on my 34.

Tom

RANDY AND 34 ROADSTER

I started this topic in response to the myth/misunderstanding often posted on packardinfo that gas drains back from the carburetor and empties the bowl. I ma running a 30 day experiment on evaporation right now, results will be posted.

I ma running a 30 day experiment on evaporation right now, results will be posted.

That will be an interesting post. I'm also of the same thinking that evaporation is a problem.

Another question or two would be climate, location and formulation differences. Does it evaporate at same rate in places with 90 degrees and 90% like Eastern states as it does at 90 degrees and 20% like Western states. Also, how much formulations come into play. Clean air state formulas vs rest of country. I know ours and a few other states are supposed to have formulas reducing volatility to the atmosphere but if that formula is any thing to go by and actually works, I'd hate to see one without.

NJ is a claen air state with fume capture on gas pumps, so we woiuld have relatively low volatility gas I presume. The carbs are outside under a piece of sheet metal like a hood to simulate utside storage with tempeature variations form aobut 50 to 95 degrees.

As far a replication the study in deifferent areas of the ocuntry with different formulations and temperatures, I haven't quite yet perfected being in two three or four places at the same time. But I am running experiments of ocurse.

I ma running a 30 day experiment on evaporation right now, results will be posted.

From my own experiences. Both my car's carburetors are vented to the atmosphere; the 34 (Stromberg EE-23) directly to the underhood space, and the 56 (dual Rochester 4GCs) thru the air filter canopy.

If I attempt to start either car within a week of the last use, they will start nearly instantaneously, perhaps with about 1/2 turn of the starter motor following a single stroke of the accelerator. After 2 weeks the 34 will require cranking for perhaps 2 seconds; the 56 perhaps twice as much. After a month the 34 still requires only about 3-4 revolutions of the engine to start but the 56 often requires at least 10 seconds of cranking which isn't surprising, a lot more fuel is required to fill both carbs.

Dave,

Allow me to put some words in your mouth. You mean to tell me that your 78 year old sedan starts on just a few turns without an electric fuel pump and electronic ignition!!
It still stars and runs like a Packard, unbelievable!

And your '56 with multiple carbs takes a few seconds more!!
Also unbelievable.

Thanks for the testimonial to the guys on East Grand

fred

Here in the desert southwest just about everything evaporates! This includes premium gasoline which (Chevron) I put in my '55 Packard routinely. BTW, I took her for an early morning drive today (after 2 weeks of sitting inside my garage) and I'm glad I had a e-fuel pump. After a few cranks and a single pump of the 4GC she fired. My fuel pump doesn't need rebuilding. It is brand new (Carter) with less than 500 miles on it. Simply the fuel in the bowls evaporates. Modern fuels and their compositions are markedly different than they were in 1955.