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Keeping gas in a Marvel updraft carburetor. Where does it go?


cjmarzoli
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I have a 1931 Buick model 8-67.  Has the marvel updraft carburetor.  If the car sits for more than a day or two, the  bowl becomes virtually empty which makes it difficult to start as the carb bowl must be filled by cranking  and cranking before the engine gets any gas.  I have a loop in my fuel line to the carb and installed a backflow preventer just behind the carb so where is the fuel going?  I don’t think it could possibly all evaporate that quickly.  I don’t see any evidence of it leaking out or running out the front.     Anyone else had this issue?   Does anyone have a good method for pre priming the bowl when it does empty?  There is a very small vent hole on the top, perhaps there is a way to inject fuel through there.  You cannot just dribble it down the top like you can on a modern downdraft unit.  Thanks!

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With the float filling most of the bowl, It does not take much gas to fill to operating level.  Many people put the electric pumps in just for  this initial fill.  I know that some people only put two screws in the caps so they only have to remove  and loosen the other to swivel the cap out of the way.  I don't think the top vent hole will work well as there would be no direction for the air to escape.  Small fuel bowl, open system and a hot engine bay will cause evaporation quickly.

 

Bob Engle

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Today's fuel evaporates at room temperature. A few days of sitting and carburetor bowls are empty. As Bob says, it doesn't hold much so they go dry pretty quickly. An electric pump is no longer a crutch or a band-aid solution but rather a necessary part of making an old car live happily in today's world with modern fuels. Run it for a few seconds to fill the bowl and the car will fire right up.

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You should be good for several days if not a week with modern gas if your carburetor is working properly.  If I let the Buick sit for 2 weeks, I can tell that it is a little starved for gasoline at the start, but I can even go for a month and it will start.  Just not on the first try always.  Pull the choke when cold and that will increase the mixture during the start and increase the amount of vacuum to the vacuum tank which will bring gas more quickly to the vacuum tank.  

 

Here are some thoughts based on my 1925 Buick / Marvel.  I have a vacuum tank only.  No fuel pump. 

Lets start with where has the fuel vaporized  to.  

1) place an aluminum pie pan under the carburetor.  Once you shut off the engine it should stay dry.  Check it every couple of hours for liquid after turning the engine off.  You may have fuel in the pan, and then it is evaporating.  I assume the fuel is leaking out where the mixture packing is, and then it is evaporating off the ground.  Buy some 3/32 graphite packing ($4 on Ebay) or $3 at your hardware store).    Repack under the nut.  You may get away with just tightening the nut.  Note.  You are not supposed to adjust the mixture with the nut tight.  Loosen the packing nut, adjust the mixture, tighten the packing nut.  Otherwise you could break the wheel off the adjuster shaft.  The packing nut may just need to be tightened.  If you decide to repack, mark the mixture wheel, count the number of 1/4 turns until gently seated,  so that you can put it right back where it was after repacking.  

2) With the car outside, you can remove the three bowl cover screws to observe the float level.  Remove the bowl cover metal plate and look at the gas level in the carburetor.  On my car, you can start the car with the bowl cover off.  You should be able to observe how quickly the fuel level is being lost.   

3) The photo show the carburetor and the low speed jet.  The fuel level should be just below the low speed jet or fuel will leak out thru the air intake when the car is shut off.  The level should still only drop to the level of the low speed jet.  The packing nut is the next culprit to allow the fuel to drip out of the carburetor.  You could also have a fuel leak on one of the other 2 gaskets under the bowl - like the inlet banjo and there is also a plug on the bottom of the carburetor as well.  

 

One more thought on starting after 3 weeks.  The vacuum tank holds about a quart of fuel.  As the level in the float bowl drops, the float should drop and the needle will raise off the seat and refill.  It should continually do this until the vacuum tank is empty.  So you are losing more than just the amount in the carburetor.     Hugh

 

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Ben, we've all been there.  Not in that exact situation, but doing "bonehead" things, then scratching our heads later!  thanks.   Hubert, thanks for the excellent writeup.  I will check my float level.  On the '31 the mixture adjustment is on the top rear of the carb so I don't think I have that packing nut on the bottom but i will verify.    There is a bolt on the bottom that covers a jet port I think but I don't believe it is leaking.   The '31 does not have a vacuum tank either, it uses a mechanical fuel pump on the side of the engine so that is not a factor in my case.  

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With no vacuum tank, that should be in your favor.  I would install a low pressure electric fuel pump.  When you turn the key on, it will fill the float bowl before starting if you wait a brief moment before starting to ensure the bowl is full.   

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You might want to put an on/off valve somewhere. My carb leaks and drips very slowly, if it sits for a few days or a week, between dripping and drying the carb empties and the vacuum tank empties into it, and I have to add gas to the vacuum tank to start the car. To prevent this, I close off the valve at the bottom of the vacuum tank when I turn the car off, so when I want to start it again, I open the valve and whatever is left in the vacuum tank fills the carb back up again. Since you have no vacuum tank, you might want to rig some other sort of on/off valve at the carb and reservoir of some sort above it. Maybe just a foot or two of fuel line with an air valve above it to let air in while you drain it.

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With the fuel pump, you don't have the Molotov Cocktail in the engine bay, but you also don't have the slow leak INTO the carb to replace evaporation.

 

The is very little fuel in the bowl and it evaporates quickly.

 

The easy way out is a 1~1.5 psi electric pump. If you cannot find a pump in this pressure range, then a regulator MUST be used with higher pressure. When buying the regulator, do NOT allow the thickness of your wallet to influence your decision. You need a regulator with an internal bypass, not one of the inline dial-types that regulate pressure by regulating volume. They are too much even if free! And there best use is if you have a rabbit problem in your garden, and a strong and accurate arm! ;)

 

But to answer your other question: acquire a spray tube from a can like WD-40, and a plastic squeeze ketchup or mustard container. Insert one end of the tube into the end of the squeeze part of the K/M container, and you can fill the bowl through that small vent.

 

Jon.

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Thank you all.  I ran the car today.  It had been 3 days since I drove it and the bowl was empty before I started it up!    My smart idea was to cap the vent on the carb lid after I got done driving the car to prevent some evaporation.  I found a small rubber cap that fit perfect over the vent dome.  Well, it didn’t work.  I placed it on after my excursion and immediately heard a hissing noise.  Then gas started oozing out of the top bowl gasket under the cover and dripping out of the front air horn between carb and air cleaner.   I quickly removed my cap but it kept dripping slowly out the air horn for at least another 10 mins.  I’m not sure if it would have dripped out the air horn (front of carb) had I not capped it, but if it does, that is where some of my gas is going.  Not sure why the bowl was pressurized after I shut the engine off enough to pump gas out after I capped the vent port on the bowl cover.  Temperature Expansion?  ================  I love the WD-40 stick and mustard bottle idea, I will give that a try!

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The following is copied from my website troubleshooting section, and was written for downdraft carburetors; but the cause is the same. Since the throttle shaft in an updraft is above the fuel level in the bowl, the fuel finds a different spot to leak:

 

"

FUEL LEAK BY THROTTLE SHAFT(S)

 

A common complaint today is fuel dripping out of the throttle body by the throttle shaft AFTER the engine is switched off. While a number of issues may cause this problem, by far the most common issue is the volatility of modern fuel. Mechanical fuel pumps have a check valve which prevents fuel from moving back to the fuel tank. The problem is as follows:

 

(1)  After the engine is switched off, heat from the engine heats the fuel in the fuel line.

(2)  The expanding fuel (increased volatility) creates pressure in the fuel line from the pump to the carburetor.

(3)  The check valve prevents the fuel backing up through the fuel pump.

(4)  The pressure increases to a point the float/fuel valve combination in the carburetor cannot withstand the pressure.

(5)  An amount of fuel (usually from a teaspoon to a couple of tablespoons) flows into the fuel bowl of the carburetor.

(6)  This raises the fuel level in the bowl above the main discharge nozzle(s).

(7)  Fuel flows through the main discharge nozzle(s) and drips onto the throttle plate(s) which is/are closed, and exits out beside the throttle shaft(s) dripping onto the intake.

 

Possible solutions:

 

(1)  IF POSSIBLE, AVOID ETHANOL LACED FUEL! Sometimes you can buy real gasoline at a marina

(2)  Buy the lowest octane name-brand fuel that does not ping or detonate in your engine (the higher grades often have more ethanol)

(3)  Install a “vapor return line” (take a look at return lines used on many factory air-conditioned cars)

(4)  Learn to live with the issue.

 

HARD STARTING, COLD

 

Difficult starting a vehicle that has been allowed to sit for a number of days (that will then start well the rest of the day) is often caused by modern fuel. Modern fuel begins to vaporize (evaporate) at a much lower temperature than fuel before the 1970’s. Once the engine is shut off, the fuel in the carburetor bowl begins to evaporate through the bowl vent. If there is no fuel in the carburetor, the engine will not start. Pumping the footfeed during this time simply prolongs the agony, as the accelerator pump will pump the fuel into the engine, but in amounts insufficient for starting. If you have this problem, try priming the carburetor by using an eyedropper and filling the carburetor bowl through the bowl vent prior to cranking the engine. If you do not wish to prime the engine, crank the engine for 15 to 20 seconds WITHOUT PUMPING. Then stop cranking, pump the footfeed 3 or 4 times, release it, and then reattempt to start the engine. Priming eliminates excessive wear on the starter. Another possible solution is the installation of an electric fuel pump. If an electric pump is installed, check local, state, and federal laws about wiring; and pick a pump with pressure not exceeding that of the original pump."

 

Jon.

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Start with re-building your mechanical fuel pump, with a good working pump it should fill the fuel bowl fair quickly and start easily if all else is in good working order. Otherwise, any 6/12volt electric fuel pump mounted near the fuel tank activated by a momentary push button switch will do the job, you wont need a pressure regulator.

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The advantage of the electric fuel pump is that it turns on when the ignition switch is in the on position.  This will fill the float bowl prior to cranking the engine.  My Jaguar has an SU fuel pump that clicks until it builds pressure.  It builds pressure as the needle and seat begin to close as the fuel level rises in the float bowl.  The clicks are rapid at first, and then more time between the clicks, then it stops clicking when the bowl is full.  I listen to the clicks and when they stop I crank the car.  With a mechanical fuel pump you will still be cranking with a no fuel situation until the bowl fills.   If you are not worried about where the fuel evaporated to, just install an electric fuel pump.  

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My 29 McL-Buick has various ways to get gas into the carb after the car has sat for a while. 
 

It has a Dole Primer Pump as standard equipment. You pull it out and push it in a couple of times while you are rolling the engine over and it sprays raw gas into the cylinders - starting the engine immediately. 
 

Another way to fill carb bowl is to roll the engine over with ignition off - this also gets the oil moving through the engine without much friction. 
 

And, an auxiliary low pressure electric fuel pump helps, too. 
 

Bill McLaughlin

1929 Silver Anniversary Buick Club

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Thank you Jon for posting the Fuel Leak document.  When I go to start my 1937 after it has been sitting for weeks I usually fill the bowl on the carb and give a bit of gas down the carb throat. It usually starts very quickly. Another issue with modern fuel in older open systems is the amount of fuel evaporating in the tank just sitting.

 Over the winter several gallons can disappear. 

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Putting an electric pump on one of these older cars has a big advantage for preventing vapor lock and filling an empty float bowl so you don't have to beat up the starter to get the mechanical pump to fill the bowl to get the engine started.  Having done this on antique boats there are a couple of considerations.  If you wire the pump to run while the ignition is on you could be creating an oil dilution issue if the fuel pump diaphragm leaks through and the car will run happily because the electric pump can keep up with both the combustion fuel demands AND the leakage, I.E. the first you know there is a fuel pump diaphragm issue may be to the detriment to all that rotating machinery in the basement of the engine, not good.  The other concern is if the float valve can't contain the pressure from the electric pump you have a flooding and fire hazard issue.

 

If you wire with a momentary switch that will minimize the issues stated above but may not eliminate them.  Mechanical fuel pumps often still run the car while leaking fuel into the oil and if the float valve can't close appropriately when the electric pump is being operated by the momentary switch it can still result in flooding.  The other problem with the momentary switch arrangement is the electric pump is still in the fuel line back at the gas tank when you shut it off and it has some flow restriction that may result in fuel starvation at some engine loads even if it will run the car at curb idle or low speeds. 

 

I installed a Carter electric pump on my 1965 Chris Craft Sea Skiff which had it's original Chris Craft converted solid lifter Chevrolet 327 and the stock Carter AFB carburetor.  I went the momentary route on the wiring and always started the engine with engine hatch open so I could hear the Carter pup start to labor and slow when the carburetor bowl filled and the float valve closed dead-heading the electric pump.  I went from not knowing from one launch to the next whether the battery and starter would last long enough to get me started at the launch ramp to 15 seconds with the electric pump before the starter was engaged, then VRROOOM.   It was all good up to about 2800rpm with the Carter pump switched off but if I wanted to go fast I had to switch it back on because the engine was stumbling for lack of fuel.

 

This link is a Woody Boater article on the boat I restored and sold to the then president of the Antique Classic Boat Society.  He drove from NJ to fetch it home from my place in Port Huron, MI.  The ACBS was at the time pretty exclusively wood boats only but the clubs began to realize a demographic shift from the old guys like me who grew up around wood boats to a newer generation that grew up with fiberglass and wanted in on the fun.  The Sea Skiff I restored was hull #12 of 80 1965-67 Chris Craft Sea Skiffs made of fiberglass in the old Cortland NY Thompson Boat factory when Chris Craft was making the switch to fiberglass. 

 

https://www.woodyboater.com/blog/2014/09/26/12-makes-statement-skaneateles-ny/

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On 5/19/2020 at 2:48 PM, Ben P. said:

It’s been eluded to, but to go along with what Morgan just pointed out, and of course this doesn’t apply to the ‘31 being discussed here, but for the older carbs supplied by vacuum tank it was STRONGLY suggested (suggested kind of like it’s suggested to put on a parachute before jumping out of an airplane) to me by several people to never shut the engine off via the ignition switch. Rather, close the fuel valve after the vac-tank and let the carb run dry and let the engine quit. The vac-tank has more than enough fuel in it to fill the bowl instantly at next start up and will then refill itself within moments of the engine starting.

Just thought worth mentioning since you never know who might come across this thread down the road and I at least couldn’t have been more clueless with cars this age. They may obviously have foot pedal starters but for more reasons than that they are in no way ‘turn key’ cars. The older carbs all leak whether you can see a drip or not and it simply isn’t safe to leave any fuel in the carb of a sitting car at all, not to mention the full volume of the vac-tank which will replenish the leaky carb. Not safe at all.

 

Ben P. - AKA HardWay


The limited occasions I drive our 27/27 Buick, this is exactly how I “shut the car down”, I leave the dash switch in the on/run position and then turn the fuel off at the bottom of the SW Vacuum Fuel System canister right at the sediment bowl and proceed to let the car run out of fuel or just about run out of fuel.

 

When the car starts to sputter then I turn the dash switch to the off position.  I do this after a suggestion from a few members here as I was very nervous shutting the car down that’s got so much wood in it and fuel that would drip at times after shutting the car down (before I started this shutting the fuel off first before the dash switch) and it seemed like it would be the perfect conditions to ignite!!

 

Now I still worry but I don’t worry as much about fire!!

 

Then when I drive the car the next time I don’t have to worry about fuel evaporating out of the carb float bowl, I reverse the process returning plenty of fuel to the bowl and tap the “tickler” fuel enrichment button on top the bowl and start the car on the first two to three cranks every time!!  
 

Hope this works for you @cjmarzoli

Edited by Crazyfamily (see edit history)
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