Carburetor bowl leak down

Hi all, I am wondering if the fuel in the carb bowls should be gone after sitting overnight? I have a Quick Fuel 735 with see through sites on front and back bowls. I drove it yesterday and there is no gas visible in either one today.

No, unless it was hot enough to evaporate the fuel out of the bowls. It may have just evaporated enough so it's no longer visible in the sights.

On my S/A 670, it will for sure evaporate some overnight, but it will start after a couple of cranks. If it sits for a week, then all the gas will have evaporated. I often wonder if the bowl vent tubes are too big and allows the gas to vent off too quickly.

Going back to when I got the car, it had a 670 on it, but it ran so badly, I just bought a custom 650 Holley. While the car did run waaay better, I still had the issue of fuel evaporation especially when hot. It would be hard to start after just a short time sitting. That was when I found out about blocking the exhaust heat cross over passage on the stock manifold and adding a thick insulator under the carb. I don't have ram air, so height is not too much of a worry. This all helped, but yes, the gas does still evaporate overnight. 

Maybe I should design a valve that only vents when the engine is running!! Now there's a thought!

Well that’s what I thought. It wasn’t hot out and my engine was running at normal temp. I am wondering if I my have not seated the power valve gasket correctly when I changed it. Thanks, Don I better check it out.

Heat may be an issue. Also if the tank is full of winter blend gas and the weather has gotten warmer, the fuel will evaporate faster than a summer blend fuel. I have 3 carbureted cars in a 60 degree winter temperature garage. Any more than 3 days between starts and it is crank and crank some more. It is worse in the wintertime. Chuck  https://www.hsoil.com/seasonal-fuel-blends-explained.html

Heat may be an issue. Also if the tank is full of winter blend gas and the weather has gotten warmer, the fuel will evaporate faster than a summer blend fuel. I have 3 carbureted cars in a 60 degree winter temperature garage. Any more than 3 days between starts and it is crank and crank some more. It is worse in the wintertime. Chuck  https://www.hsoil.com/seasonal-fuel-blends-explained.html

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Ya Chuck, I could see if it was 3-4 days since I had run it, but this was overnight. I keep my at a 60 minimum temp too. Filled my tank that day, but I’m not sure if they have switched to the summer blend yet. That’s a good point about the winter vs summer blends, I never thought of that, but sure does make sense. Thanks Chuck!

Well that’s what I thought. It wasn’t hot out and my engine was running at normal temp. I am wondering if I my have not seated the power valve gasket correctly when  I changed it. Thanks, Don I better check it out.

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If the engine is not running the power valve is open. There is no pressure differential to draw fuel out of the boosters, the float level is lower than booster feed so, the gasket not being seated would not cause the fuel to "disappear" overnight. Look for a leak or the heat (percolation) possibility. I have seen porosity problems in the castings cause internal and external slow leaks. Let us know what you find. Chuck

Well that’s what I thought. It wasn’t hot out and my engine was running at normal temp. I am wondering if I my have not seated the power valve gasket correctly when  I changed it. Thanks, Don I better check it out.

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If the engine is not running the power valve is open. There is no pressure differential to draw fuel out of the boosters, the float level is lower than booster feed so, the gasket not being seated would not cause the fuel to "disappear" overnight. Look for a leak or the heat (percolation) possibility. I have seen porosity problems in the castings cause internal and external slow leaks. Let us know what you find. Chuck

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Ok, thanks Chuck.

Same thing with the Holley DP on my '71 429 CJ-R. Some prior owner replaced the stock Quadrajet. If I start it the next day after running it, it fires right up. If it sits for weeks or over a month in winter where I'm just starting it in the garage occasionally, it will crank and crank before starting. Rather than going through all of that extra cranking I have been adding a little fuel to the carb, buttoning down the air cleaner lid again and then it will fire up quickly. I keep the battery on a Battery Tender but I never liked all of that extra cranking. I never felt the need to investigate any further because it runs great after it has started. i think the car is trying to tell me to take it out more often. lol

Took the front bowl off and there was hardly any fuel in the bowl and no in the line. Everything seems fine, so I guess the gas just evaporates quicker than I thought. Learn something more everyday. Thanks for the replies!

Took the front bowl off and there was hardly any fuel in the bowl and no in the line. Everything seems fine, so I guess the gas just evaporates quicker than I thought. Learn something more everyday. Thanks for the replies!

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 Hey John, if you did not use a new gasket on the bowl, I'd check under it tomorrow or after its been running just to make sure it's not leaking. Sometimes you can get away with not replacing it, but I'd just check anyway. Maybe a need to snug the bolts again as well. Just a thought that I'm sure you're already aware of.

Do you have an insulated gasket under your carb? If not, maybe get one at least 1/4" thick. If I remember, you have a non-stock manifold, but is there an exhaust heat cross-over. If that's the case you might need one thicker. I've also seen heat deflector plates under a carb to keep heat off the carb, but I'm not sure if they fit Fords.

As I said earlier, on my stock manifold, I not only blocked off the cross-over, I also use a 1" thick fiber spacer that is supposed to help with torque as a bonus, but I still get evaporation after a while.

Took the front bowl off and there was hardly any fuel in the bowl and no in the line. Everything seems fine, so I guess the gas just evaporates quicker than I thought. Learn something more everyday. Thanks for the replies!

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 Hey John, if you did not use a new gasket on the bowl, I'd check under it tomorrow or after its been running just to make sure it's not leaking. Sometimes you can get away with not replacing it, but I'd just check anyway. Maybe a need to snug the bolts again as well. Just a thought that I'm sure you're already aware of.

Do you have an insulated gasket under your carb? If not, maybe get one at least 1/4" thick. If I remember, you have a non-stock manifold, but is there an exhaust heat cross-over. If that's the case you might need one thicker. I've also seen heat deflector plates under a carb to keep heat off the carb, but I'm not sure if they fit Fords.

As I said earlier, on my stock manifold, I not only blocked off the cross-over, I also use a 1" thick fiber spacer that is supposed to help with torque as a bonus, but I still get evaporation after a while.

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Carb bowl gaskets are fine, no leaks.

 I have a Blue Thunder intake and they have no heat cross over and it does have a bit of hi-rise to it, so I don’t think I have any problems with heat. It’s running pretty good actually, I just never thought all that fuel would evaporate overnight, but like Chuck said maybe it’s still the winter blend gas.

Took the front bowl off and there was hardly any fuel in the bowl and no in the line. Everything seems fine, so I guess the gas just evaporates quicker than I thought. Learn something more everyday. Thanks for the replies!

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 Hey John, if you did not use a new gasket on the bowl, I'd check under it tomorrow or after its been running just to make sure it's not leaking. Sometimes you can get away with not replacing it, but I'd just check anyway. Maybe a need to snug the bolts again as well. Just a thought that I'm sure you're already aware of.

Do you have an insulated gasket under your carb? If not, maybe get one at least 1/4" thick. If I remember, you have a non-stock manifold, but is there an exhaust heat cross-over. If that's the case you might need one thicker. I've also seen heat deflector plates under a carb to keep heat off the carb, but I'm not sure if they fit Fords.

As I said earlier, on my stock manifold, I not only blocked off the cross-over, I also use a 1" thick fiber spacer that is supposed to help with torque as a bonus, but I still get evaporation after a while.

Click to expand...

Carb bowl gaskets are fine, no leaks.

 I have a Blue Thunder intake and they have no heat cross over and it does have a bit of hi-rise to it, so I don’t think I have any problems with heat. It’s running pretty good actually, I just never thought all that fuel would evaporate overnight, but like Chuck said maybe it’s still the winter blend gas.

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 Good to know your all good. 

How do you like the Blue Thunder manifold? I think they are pretty similar looking to the stock one. Do you find it makes a difference enough to warrant buying one?

I thought it made a big difference compared to the stock or performer intakes. My engine revs to the 6k range very easily. It’s a great intake very similar in performance as the air gap, but looks like a stock aluminum one. It’s actually supposed to be a lot like one on the b351. But just like everything for our cars, kinda pricey but I think it’s worth it.