currently running a summitt 750 carb on my 330 cu inch y block, sitting in a rolling chassis while I work on rest of vehicle.  If I don't get to start and run for .about a week, the float bowls drain completely to where I have to manually fill the float bowls.  I've worked out a simple way to do that, but my question is where is all that gas going?  Hard to believe it is ALL evaporating, but suppose its possible. running fuel out of  5 gal tank sitting on the garage floor, and line has a fuel filter.   flowing back into the 5 gal tank? Concerned it may be migrating into oil pan.  Would it hurt the motor to just let engine run out of gas as a way to eliminate this "drainback"?  Engine is equipped with a Masterlube Pre'oiling system.

Tom.  Welcome back to the site.
 
You can rule out ‘drain back’ as a reason for the fuel disappearing from the carburetor bowls.  Once the fuel bowls get low enough to expose the ‘open’ needle seats to air, the remaining fuel will not be in a position to be pulled out or siphon into the fuel line going back to the fuel pump.  I hear the term ‘drain back’ mentioned regularly on the forums but it’s physically impossible with most carburetor designs where the needle seats are positioned high in the bowl.  So that leaves some other possibilities.
 
One thing to look at is the actual ethanol content in the gasoline you are using.  I have found as high as 35% ethanol in fuel that was advertised as being no higher than 10%.  The higher the ethanol content then greater is the propensity for that fuel to evaporate.  Until you actually check the ethanol content, you don’t know what you have.  The premium in my area is typically only 3-5% ethanol from pumps saying ‘Contains up to 10% ethanol’ and that fuel is typically okay sitting for up to two weeks at a time in a carburetor before evaporating enough that starting becomes a problem  without either priming the carburetor or excessive cranking in which to refill the bowls.  Higher than normal ambient temperatures simply compounds the evaporation issues related to ethanol being in the gasoline so keeping an engine in a non-climate controlled garage simply exasperates the problem.
 
Another possibility is a pair of stopped up aniti-siphon hole on your carburetor.  There should be two of those on each end of the carburetor (one for each venturi) and those can get stopped up on a relatively new carburetor with what I’ll call fuel varnish over time depending upon the quality of fuel.  Both of the anit-siphon holes on a given side of the carburetor would have to be stopped up before becoming an issue.  I’ll add that heat or high ambient temperature is the enemy here.  Those anti-siphon holes are very small so they are easily stopped up.  When the anti-siphon holes become plugged, the fuel does siphon over into the engine thus putting it in a position to be flooded until the fuel in the intake manifold itself evaporates out.  If the engine starts easily the next day without any pumps of the foot feed or choke, then this is something to look for along with possibly a power valve issue added to that list.
 
Also a strong consideration is a leaking primary side power valve.  The Summit carb has a power valve in each end but the primary side valve is the one that would be a problem if bowl drainage is an issue after a period of time.  A leaking secondary power valve would contribute to a flooding issue when sitting still and/or fuel economy issues but I don’t see it being a cold start problem after a week of sitting.  I’ll suggest monitoring the fuel level with the engine sitting by simply looking in the clear sight glasses with a flash light.  In the grand scheme of things, I would not expect that level to drop below the lower edge of that sight glass for at least three days.  If the engine cranks up easily the next day without working the foot feed or choke, then examining and/or servicing the power valve gaskets or even a power valve replacement would be called for.
 
These are just some early morning thoughts on the subject so if any of the forum members have any other solutions to this, then by all means chime in.

Lorena, Texas (South of Waco)

Before taking anything apart, you might try buying a couple of gallons of racing gasoline (no ethanol) and see if that fixes the problem.  This could be as simple as just evaporation due to ethanol in the gasoline.

Lorena, Texas (South of Waco)

- Ethanol Manufacturing Plants Fined For VIOLATIONS: The manufacturing plants that produce ethanol from corn and grains, throughout the U.S. are dangerously increasing pollution and adversely affecting the quality of the air we breathe...Read more

Ethanol Test Kit - http://fuel-testers.com/

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Thanks Ted,   Glad to be back, and getting back to  work on the project.  You've given me some things to check, and I'll begin with the fuel quality check.  Due to our high heat and humidity here, that's most likely the problem.  I only get 2 gallons on the day I want to start it, so the fuel doesnt sit around, but  don't know the quality or how long its been sitting at the station.  Once the bowls at at the correct level, and I give it one or two blips of the throttle it fires right up and runs great.  Definitely will go with a Holley EFI  system, hopefully in the near future, and that will take care of this problem, plus being at sea level (ocassionally  UNDER)  most  future trips could only be UP in altitude.  Definitely will try to locate some race gas/ additive also.
                                                                                                                                                    Tom

Thanks KULTULZ,
  I appreciate the link to the fuel test kit, I'm on it.

Usually, engine shut-down and resultant heat soak causes the fuel bowl/feed line to pressurize and possibly lose fuel through percolation or actually being forced out of the bowl into the throttle bores. Carb designs vary.. Ethanol will dissipate more quickly with heat and of course will evaporate with open bowl vents over time.

Ethanol laced gasoline is meant for today's technology with EVAP SYSTEMS. With a vented fuel tank, the fuel is constantly drawing moisture from outside air. And of course if you are buying cheap gasoline, you have no idea where it  is from or how it was blended.

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Well, we will find out the quality of the gas I've been using.  Ordering that fuel test kit today, and supossedly my local Yacht Club has ethanol free gas, and I will test that also to be sure.  Will report the findings when it happens.  Thanks again.

To help eliminate percolating I installed 3/8” phenolic spacers under both my carbs. I live in So Cal and 2 weeks without running is still one full pump to squirt both carbs and set rear choke. 3 weeks is some cranking. I’ve never checked the fuel but will do so now 

56 Vic, B'Ville 200 MPH Club Member, So Cal.

... supossedly my local Yacht Club has ethanol free gas,

There you go, marine fuel and aviation to some extent. There are also stations that sell ethanol free but they are usually in heavily agricultural areas. Farm equipment can't run off it either.

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